School Science Lessons
(UNPh28)
2024-08-18
Light rays, geometrical optics
Contents
12.0 Light (Primary)
28.4.0 Light sources
28.11.0 Optical devices
28.3.0 Reflection, curved surfaces
28.2.0 Reflection, flat surfaces
28.1.1 Light, speed of light, nature of light, shadows
28.1.2 Stroboscope
28.6.0 Total internal reflection, critical angle
28.4.0 Light sources, producing light
28.103 Sources of light
28.117 Absorption spectrum of sodium
28.101 Candoluminescence
28.115 Emission spectrum
28.121 Focal length of a convex lens
28.116 Incandescent lamp
28.135 Infrared rays source
28.102 Low voltage light source
28.165 Light bulb (incandescent filament lamp)
28.120 Light rays through lenses
28.105 Light travels in straight lines, pinhole magnifier
28.102 Low voltage light source
28.104 Luminance and illuminance
28.106 Reflection of beams of light
28.107 Smoke box to study light rays
28.11.0 Optical devices (low-cost)
Experiments
28.11.6 Microscope as a micro projector
28.11.3 Model refracting telescope
28.131 Optical bench to study lenses
28.11.4 Projector for filmstrips or slides
28.110 Ray box for beams of light
28.107 Smoke box to study light rays
28.11.1 Water concave lens, ice lens
28.11.2 Water drop magnifier
28.3.0 Reflection of light at curved surfaces, curved mirrors
28.3.10 Amusement park mirrors
28.3.2 Concave and convex mirrors
28.3.6 Light a match with reflectors
28.3.9 Parabolic mirrors
28.90 Reflection at a curved barrier
28.3.4 Reflection from a concave mirror, (converging mirror)
28.112 Reflection from a concave mirror with a ray box
28.113 Reflection from a convex surface
28.3.5 Reflection from a convex mirror, (diverging mirror)
28.3.1 Spherical mirrors
28.3.7 Variable curved mirrors
28.2.0 Reflection of light at flat surfaces, plane mirrors
28.2.07 Image equation for both lenses and mirrors
28.2.02 Laws of reflection
28.2.01 Mirror equation, shadows
28.2.05 Plane mirror images
28.2.04 Real images and virtual images
28.2.02 Reflection, Laws of reflection
Experiments
28.2.3 Angle of incidence and angle of reflection
28.2.19 Corner cube, corner reflector, retrodirective mirrors
28.2.16 Diffuse reflection
28.2.10 Image position in a plane mirror, pin parallax method to locate image
28.2.6 Kaleidoscope
28.111 Laws of reflection using a ray box
28.2.8 Laws of reflection using an electric torch
28.2.9 Mirror image needs a flat smooth surface
28.109 Mirror images (inversion, lateral inversion)
28.2.20 Mirrors at an angle with a candle
28.2.21 Parallel mirrors with a candle
28.2.5 Periscope
28.110 Ray box for beams of light
28.107 Make a smoke box to study light rays
28.89 Reflection at a straight barrier
28.2.18 Reflection by grazing
28.106 Reflecting beams of light
28.2.13 Reflection of clock face in two mirrors
28.2.2 Reflection of light by a plane mirror
28.108 Reflection with a smoke box
28.2.11 Reversed writing, identify a person's handwriting, lateral inversion
28.2.17 Scattering of light using aluminium foil
28.2.24 Unreversed image of your face
28.6.0 Total internal reflection, critical angle
28.6.01 Total internal reflection
28.4.04 Critical angle and total internal reflection
28.6.1 Total internal reflection, fibre optics cable
28.6.2 Critical angle in ripple tank, refraction tank, aquarium
28.127 Critical angle, total internal reflection, "pouring" light
28.6.2 Critical angle in ripple tank, refraction tank, aquarium
28.6.4 Diamonds
28.6.6 Optical disc with prism semicircle
28.4.05 Prism as a mirror
28.6.3 "Pour" light from a beverage can
28.6.5 Right angle prism inverter
28.165 Make a model electric light bulb (incandescent filament lamp)
See diagram 4.65: Heat and light from electricity.
1. Push the ends of two pieces of copper wire through a cork in a small bottle.
Connect the ends of the copper wire inside the bottle with a stand of steel wool.
Connect this model electric lamp model in a circuit with one or more dry cells, or lead cell accumulators, and a switch.
Close the switch until the fine wire filament begins to glow.
At first the heated iron wire produces light, but soon the iron combines with the oxygen of the air inside the bottle and burns.
2. Examine a manufactured electric light bulb.
It contains argon, but no oxygen.
It has a tungsten carbide wire filament that glows without burning when heated to a high temperature.
The argon restrains the blackening of the inside of the bulb by deposition of tungsten vapour.
Fluorescent lamps containing mercury vapour or neon gas are much more energy-efficient than incandescent lamps.
28.89 Reflection at a straight barrier
Note ripples hitting a straight barrier or the wall of the ripple tank:
1. circular pulses.
2.straight pulses hitting the wall at an angle of incidence smaller and greater than 45o.
28.90 Reflection at a curved barrier
Note ripples hitting a circular barrier:
1. on the outside.
2. on the inside.
Repeat the experiment with lens-shaped barriers.
28.101 Candoluminescence
Candoluminescence is the light from heating substances with a flame to a high temperature so that some wavelengths are more than expected by black body emission at that temperature.
This occurs in some transition metals and rare earth metal oxides, e.g. zinc oxide, cerium oxide, thorium dioxide.
Carl Auer, later Freiherr von Welsbach, 1858-1929, Austria, invented the Welsbach gas mantle containing thorium nitrate and cerium nitrate to increase light from gas lamps.
Gas mantles are pieces of fabric soaked in metal oxides.
They were used for gas lighting in homes, but nowadays are used only for gas pressure lamps for outdoors work and camping.
28.102 Low voltage light source
See diagram 28.199: Low voltage light source.
Make a compact light source from any small, high intensity electric light bulb that has a short, straight filament, e.g, light bulbs used in car tail lamps.
Use a small light source to make very sharp shadows with the light bulb filament end on.
Cover the light source with a small beverage can.
Darken the room.
Punch 2 mm diameter holes in the beverage can on all sides.
Blow smoke around the can to make the emerging rays visible.
Make enough holes so that you can see clearly where the light comes from and in what direction it travels.
28.103 Sources of light
See diagram 28.199: Low voltage light source.
Make a compact light source from any small, high intensity electric light bulb that has a short, straight filament, e.g. light bulbs used in car tail lamps.
Use a small light source to make very sharp shadows with the light bulb filament end on.
Cover the light source with a small beverage can.
Darken the room.
Punch 2 mm diameter holes in the beverage can on all sides.
Blow smoke around the can to make the emerging rays visible.
Make enough holes so that you can see clearly where the light comes from and in what direction it travels.
28.104 Luminance and illuminance
Luminous intensity, C, is a measure of the brightness of a light source, i.e. how much light emitted per second, and is measured in the candela, cd, formerly candle power.
Luminance, L, measures the brightness of a surface in candela per square metre.
A source of light measuring one candela emits one lumen of light, 1 lm.
Illuminance, or illumination, I, is a measure of the quantity of light falling on a surface at a distance from the light source, and is measured in lux lx.
Illuminance is directly proportional to luminous intensity, C, and inversely proportional to the square of the distance, d, from the light source, I = C / d2.
One lux is the illumination of one lumen per square metre.
One lux is the brightness at one metre from 1 candela light source.
Light meters, exposure meters, used in photography, measure illuminance in the unit lux.
Experiment
See diagram 28.10.11: Projected filament with lens.
Turn on the light bulb.
Move the light bulb to focus the image on the side wall.
The focal lengths are marked on the lenses.
Show the effect of aperture size on the sharpness on the focus by placing different sized stops in front of the lens.
28.105 Light travels in straight lines
See diagram 28.105.1: Light travels in straight lines.
See diagram 28.105.2: Pinhole camera.
1. Make a pinhole magnifier.
Cut a very small hole through a piece of cardboard with a pin.
Hold the cardboard very close to the eye in good light and look through the hole at some small print.
The print appears larger and clearer, because light rays pass through the small hole then spread out.
The small hole functions like a camera shutter keeping out the extra light that would make the image blurred.
2. Look down on a tightly closed fist.
Open the fist very slightly to let the smallest amount of light pass through.
Look at some fine print through the fist.
Move the fist up and down to get the best magnification.
3. Pierce a hole with the pin in the centre of a piece of cardboard.
Hold it 10 cm in front of one eye.
Hold the pin between the card and the eye.
See an upside down image of the pin will be observed.
4. Make a pinhole in a sheet of aluminium foil.
Hold the aluminium foil between a lighted candle and the wall.
See the inverted image of the candle flame on the wall.
5. Hold the hole in the cardboard 3 cm from the eye.
Keep the eyelid almost closed.
See inverted images of the eyelashes.
All objects will cast an upside down image on the retina when the eye is focussed on them.
The brain interprets the upside down image as right side up.
6. Make a pinhole in the middle of one end of a rectangular box, e.g. a shoe box.
Cut a window in the other end of the box and use adhesive tape to attach over it a screen made of greaseproof paper, lunch wrap paper, baking paper.
Draw the letter T on a piece of thin white paper, or greaseproof paper using a marker pen.
Attach the paper with the T drawn on it to the front of a light source.
In a dark room, direct light from the light source towards the pinhole and, at the other end of the box, look at the image on the screen.
The image of the T is inverted.
28.106 Reflection of beams of light
See diagram 28.106.1: Reflection.
See diagram 28.106.2: Law of reflection.
A Light source, B Comb, C Mirro, D Reflection in a plane mirror, E Eye, F Angle of incidence, G Angle of reflection
Laws of reflection: 1. The incident ray, reflected ray and the perpendicular normal, N, at the point of incidence all lie in the same plane.
The angle of incidence, i = the angle of reflection, r.
Experiment
Hold a comb so that the sun's rays shine through the teeth and fall on a piece of white cardboard laid flat on a table.
Tilt the cardboard so that the beams of light are several centimetres long.
Place a mirror held upright diagonally in the path.
Note that the beams which strike the mirror reflect at the same angle.
Turn the mirror and note the direction of reflected beams.
28.107 Smoke box to study light rays
See diagram 4.107: Smoke box to study light rays: A Smoke box, B Electric torch, about 1 metre from the smoke box, C Smoke D Light beams, E White card.
Make a wooden box 30 cm wide and 60 cm in length.
Fit clear plastic or glass in the top and front of the box.
Leave the back open and cover with a black cloth curtain.
Hang this curtain in two sections, with a 10 cm overlap at the centre of the box.
Paint the inside the box with black paint.
Cut a window 10 cm high and 5 cm wide midway between the top and bottom of one end and 10 cm from the glass front.
This window lets in light rays.
You can cover the window with different kinds of openings cut from cardboard and fastened with drawing pins.
Fix a piece of black cardboard with a 5 mm diameter hole over the window.
Fill the box with smoke from smouldering paper.
Remove the smoke source and allow the apparatus to stand for 5 minutes to clear the smoke box of the heavier particles.
The interior of the smoke box appears clear, but still contains enough fine smoke particles to produce visible scattering of light rays.
Set up an electric torch or a projector 1 metre from the window.
Focus the light down to a parallel beam and direct it at the holes in the window.
The smoke makes the light rays in the box visible.
Also, a laser may be mounted on a labjack and raised or lowered as required, or the light may be passed through a series of slides to produce multiple beams.
Use the smoke box for ray tracing through optical elements e.g. lenses, mirrors and prisms.
28.108 Reflection with a smoke box
See diagram 28.203: Reflection with a smoke box A Reflected beam of light, B Mirror, C Light beam.
Fill the smoke box with smoke.
Shine the torch beam on the hole in the window.
Hold a plane mirror inside the box and note the clearly defined rays after reflection from the mirror.
The light rays reflect without scattering.
Move the mirror to change the angle of reflection.
28.109 Mirror images (inversion, lateral inversion)
See diagram 28.204: Inversion.
See diagram 28.109.1: Lateral inversion.
See diagram 28.109.2: Window-faced envelopes.
Experiments
1. Write your name, e.g. JOHN, on a sheet of white duplicating paper with a black pencil.
Hold the paper up to the light with the writing away from you.
Your name seen through the paper is laterally inverted.
Look at it with a mirror.
It is still laterally inverted.
2. Write a name on a piece of carbon paper, carbon side up.
Then read the underside of the sheet of paper.
Look at it with a mirror.
3. Use a felt pen to write ABC on the window of a window-faced envelope.
Hold the envelope in front of you while standing in front of a large mirror.
The reflection of the ABC is not laterally inverted.
4. Wear a heavily-printed T-shirt inside out.
Look at yourself in the mirror.
5. Write a name on a piece of paper, but look at what you are writing on the paper only through a mirror.
Some people can write in mirror images without using a mirror.
6. Look at the letters b, d, p, in a mirror, at the side of the letters, above or below the letters.
What do the letters now read?
Write a secret message in mirror writing.
7. Place a photograph of your face on the bench.
Imagine a line that cuts the image of your face from top to bottom and exactly in half.
Place a mirror vertically on the photograph with the back of the mirror on the imaginary line and the front of the mirror facing to the left.
Move your head slightly to the left so that you can see your whole face.
half the face from the photograph and half the face reflected in the mirror.
Note whether you face is symmetrical and whether this composite image is the same as in the photograph.
8. Make an artificial mirror image.
Fold a sheet of paper in half and paint a shape on one half.
Fold the other half over the painted half and press down.
Open the folded paper to see the mirror images.
9. Paint a design on the right side of your face.
Look in a vertical mirror and notice that the design is on the left side of the face in the mirror.
Place another vertical mirror so that its edge is touching the first mirror at an angle of about 120o.
Position yourself so that you can see half your face in the first one mirror and the other half of your face in the second in the other mirror.
The painted design is now on the right side of your face in the mirror.
10. Measure your face in a mirror.
Use a ruler to measure the distance from the point of one ear to the other, or, if you wear glasses measure the width of the frame.
Look at yourself in a plane mirror and mark the distance from ear to ear or the width of the glasses frame.
Or used a streamed mirror and wipe off the condensed steam the distance from ear to ear.
The image of your face on the mirror is half the size of your real face.
A virtual image of your face formed "behind" the mirror is at the same distance as your real face is in front of the mirror, so the mirror is located halfway between your real face and your virtual face.
11. Walk towards a large plane mirror.
Your image appears to be approaching your at twice your walking speed.
Hold a ruler in your right hand and stand before a large mirror.
In the mirror image, the ruler appear to be in your left hand, because of the left-right reversal of the image.
12. Draw a 6-pointed star on a square piece of paper, so that the points of the star almost touch the edges of the paper.
Draw a second star 2 cm inside the first star.
The area between the two stars is your star path.
Place a barrier, e.g. a book, between you and the sheet of paper so that you cannot see the star path on the paper, but you can look over the book.
Place a vertical mirror on the other side of the paper so that you can see the star path in the mirror.
Hold a pencil vertically down on the star path.
Move the pencil around the star path until you come back to where you started.
Note how long it takes you to move your pencil around the star path without running off it.
28.110 Ray box for beams of light
See diagram 28.110: Ray box: A Ray box, B Lamp, C Lens, D Screens.
This apparatus consists of two sides of an oblong box 22 × 6 cm with the lens placed at one end of the box.
The box has no bottom, and in use rests on paper pinned to cardboard.
The light source is a 12 V 24 watts, W, motor car lamp.
The lamp holder has a sleeve of brass tubing just fitting into a hole in a wooden slide, which forms the top of the box.
The groove in front of the lens is for screens and filters.
A piece of card with a slit in it provides narrow rays, and a hair comb will give a bundle of rays.
Adjust the position of the slider to form convergent, parallel or divergent beams.
Do experiments with light rays using plane mirrors, glass blocks and prisms.
A curved piece of tin will show a caustic curve.
In experiments with lenses and in refraction, push down the lamp, so that the light does not pass over the top of the obstacle.
For optical experiments, in front of the lens use a card with a hole and cross wires.
28.111 Laws of reflection using a ray box
See diagram 28.206: Laws of reflection with a ray box.
Cut a vertical groove in a cork and fix a plane mirror in it by cutting a groove in the cork.
Stand the mirror on the table.
Place a piece of drawing paper in front of the mirror.
Insert a board with a vertical slit in a ray box to make light rays travel along the paper surface and reach the mirror.
Shine beams of light from the ray box along the paper and mark the path of the incident ray and the reflected ray with crosses.
Join the crosses and continue the lines to the mirror.
Remove the mirror.
Draw the normal line at the intersection of the above two lines.
Measure the angle of incidence and the angle of reflection to see whether they equal.
28.112 Reflection from a concave mirror with a ray box
See diagram 28.207: Reflection from a concave mirror.
Make a concave mirror from a fruit tin cut in half or a part of a metal ring.
Measure the focal length of the mirror by directing a parallel beam of light on to it.
28.113 Reflection from a convex surface
See diagram 28.208: Reflection from a convex surface.
Use a convex mirror, e.g. a motor car wing mirror, side mirror, with the ray box and note the reflected rays of light.
Compare its reflection with the reflection from a plane mirror and a concave mirror.
In motor vehicles in Australia, Canada, India and USA, "Objects in the mirror are closer than they appear" is on the passenger side mirror, because these convex mirrors makes objects appear smaller, e.g. another car behind in an adjacent lane.
So the message is a warning against changing lane without warning.
28.114 Dispersion, spectrum with a ray box
See diagram 28.114: Dispersion with a triangular prism.
Dispersion occurs when light of different wavelengths is spread out by a prism into a spectrum.
Experiments
1. Use a glass prism to produce a spectrum from a parallel beam of light.
Place a card with a narrow slit in front of the lens of a ray box.
Use colour filters to suppress certain colours, e.g. use a transparent purple filter so that you see only red and blue lines on the screen.
2. Study light rays through a prism.
Hold a glass prism in a parallel beam of light and note how the beam refracts.
Rotate the prism on its axis.
When white light splits into the colours of the spectrum, i.e. disperses the violet light end of the spectrum refracts more than the red light.
The refractive index of violet light is greater than the refractive index of red light.
However, monochromatic light has only one colour and does not disperse.
28.115 Emission spectrum
If individual atoms of an element receive enough energy, they produce a characteristic line emission spectrum.
Each element emits characteristic lines of radiation with specific wavelengths.
Compounds contain more than one kind of atom, so they produce a band emission spectrum.
28.116 Incandescent lamp
Light may be directed into a room by using an abatjour skylight or lamp.
Hot solids or liquids emit wavelengths of radiation depending on the temperature as a continuous spectrum.
At lower temperatures they emit red wavelengths, so the metal appears to be "red hot".
At higher temperatures, they emit the full visible spectrum as white light, so the metal appears to be "white hot" or "incandescent".
The incandescent filament in an electric light globe, a filament lamp, is "white hot".
28.117 Absorption spectrum of sodium
1. When white light passes through a vapour of atoms, they absorb their characteristic wavelengths of light and reduce these wavelengths in the continuous spectrum emitted to produce a line absorption spectrum.
White light from the sun travels through cooler elements surrounding it that absorb their characteristic wavelengths.
The dark absorption lines in this line absorption spectrum, i.e, solar spectrum, identifies these elements, e.g. Helium.
2. Heat a wire coated in sodium chloride in a Bunsen burner flame and placed in front of a sodium light source.
The sodium vapour from the heated wire appears as a black mist, because of its absorption of the characteristic wavelengths of sodium.
28.120 Light rays through lenses
See diagram 28.120: Ray diagrams for lenses.
Ray diagrams to show the formation of images by lenses A Real, inverted, diminished image, B Real, inverted, same size image, C Real, inverted, magnified image, D Virtual, erect magnified image, E Concave lens, virtual, erect, diminished image Parallel rays of light that pass through a convex lens, converging lens, all pass through the principle focus, F.
Parallel rays of light that pass through a concave, diverging lens, diverge as if coming from the principle focus, F.
In the diagram, 1 to 4 are convex lenses that form real images when the object is more than one focal length from the lens.
Light rays come from a distant object.
The object is twice the focal length from the lens.
The object is between the focal length and twice the focal length from the lens.
The object is less than the focal length from the lens.
A concave always produces the same kind of image.
28.121 Focal length of a convex lens
Experiment
Attach a sheet of white paper on a wall opposite a bright window with the sun not visible, because it is behind an outside object, e.g. a tree.
The light rays passing through the window from the distant sun will be almost parallel.
Hold a convex lens vertically about 5 cm from the paper the move it in a straight line towards the window until a clear image of the window appears on the white paper at a distance of the focal length of the lens.
28.127 Critical angle and total internal reflection
Critical angle is the angle of incidence in a more dense medium, which produces an angle of refraction of 90o in a less dense medium.
Total internal reflection occurs when the critical angle of incidence is exceeded.
Triangular prisms can change direction of light by total internal reflection if the angle of incidence > critical angle.
When a parallel beam of light from the lamp is aimed vertically upwards from beneath the cylindrical clear plastic trough containing water the beam is not deviated.
It passes straight up.
If the beam is aimed up at a small angle of incidence, i, then both an internally reflected beam, R, and a refracted beam, T, can be seen.
The beam, T, is passing from water to air, from a medium of higher optical density to lower optical density.
So the refraction is away from the normal, angle r > angle i.
As the angle of incidence, i, is increased, r increases until the direction of the transmitted beam, T, approaches the direction of the surface of the water.
When i reaches the critical angle the refracted ray just grazes the surface of the water, so angle of refraction becomes 90o.
When i > critical angle there is no refracted beam, because all the light is reflected as total internal reflection.
There is a sudden increase in the intensity of the reflected beam as the angle of incidence increases beyond the critical angle.
The critical angle is an angle of incidence in an optically more dense medium, which produces an angle of refraction of 90o in a less dense medium.
When the critical angle of incidence is exceeded, there is no refracted light at all, instead, all the light is totally internally reflected.
Experiments
1. See diagram 28.127.4: Semicircular Plexiglas
Rotate a semicircular slab of Plexiglas with the light ray entering and exiting through the curved surface.
Rotate the semicircular slab until the critical angle is reached and total internal reflection is obtained.
2. See diagram 28.127.1: Candle behind fish tank.
Put a short lighted candle behind a glass or plastic rectangular fish tank.
Fill the fish tank with water to a level just above the wick.
Look at right angles to the fish tank so that you can see the lighted candle directly opposite.
Raise and lower the level of your eye above and below the level of the water.
The top of the candle flame and the bottom of the candle flame around the wick are in one line.
Move your head to the left parallel to the front glass of the fish tank.
When your eye is above the water level, the top of the flame appears to move to the left.
When your eye is below the water level, the bottom of the flame appears to move to the left.
The angle between a line from the candle at right angles to the fish tank, the normal, and your line of sight, the incident ray, is increasing.
For most glass, when this angle reaches about 43o, the critical angle, the incident ray cannot pass into the water, so the image disappears.
3. Return to the first position where you first looked at the candle directly opposite you.
Lower your eye to the level of the bottom of the fish tank and look up at the bottom of the water surface.
See the reflection of the lower part of the candle that you saw when your eye was just below the level of the water.
Light from the candle up to the surface of the water is at an angle greater than the critical angle is reflected at the water surface, total internal reflection.
4. See diagram 28.127.2: Spoon in glass of water.
Stand a spoon in a glass of water at the edge of the table.
Look up from just below the table surface at the spoon pointing down towards you.
The surface of the water acts like a mirror and so you see the reflection of the lower part of the spoon that is under water.
However, you cannot see the upper part of the spoon above water.
5. Right angle prism
See diagram 28.127.3: Right angle prism.
Shine a light into one of the two sides of a right angle reflecting prism.
The light reflects off the hypotenuse and passes out through the other side.
The light reflects, because the angle of incidence at the hypotenuse is greater that the critical angle for crown glass, 43o.
Reflecting prisms are used in binoculars, prismatic compasses and periscopes.
Prisms allow you to see around corners!
28.131 Optical bench to study lenses
See diagram 28.219: Optical bench for studying lenses. An optical bench allows you to hold mirrors and lenses in position and to measure distances accurately with a metre scale.
Experiments
Use wooden or plastic blocks with grooves that just fit over the metre scale.
Stick a pin into the centre of each block.
Use strips of tin screwed to the side of the blocks to make lens holders.
Attach a torch bulb to a block as a light source.
28.135 Infrared rays source
See diagram 28.223: Infrared rays: A Heat lamp, B Visible light, C Iodine solution, D Infrared rays, E Burning black paper.
See diagram 28.135.1: Infrared spectrum pic 1. (University of Melbourne).
See diagram 28.135.2: Infrared, spectrum pic 2. (University of Melbourne).
Experiments
1. Iodine dissolved in alcohol gives a filter transmitting in the IR, but absorbing in the visible.
To produce infrared radiation, use a heat lamp for treating muscular ailments.
Fix the infrared lamp on the table so that it shines horizontally on the bulb of a large flask of water.
The flask acts as a lens.
Hold your hand between the lamp and the flask to feel the heat.
Move a piece of black paper on the other side of the flask to find the focal point.
Add iodine solution to the water and shake the flask to make the iodine solution uniform.
Place the flask back at the original position.
Hold a piece of cotton wool soaked in methylated spirit at the focal point.
It starts to burn.
Iodine solution stops visible light, but allows the longer infrared wavelengths to pass through.
Infrared radiation is invisible electromagnetic radiation of wavelength between about 0.7 micrometers (0.7 m).
and 1 millimetre (1 mm), i.e, between the limit of the red end of the visible spectrum and the shortest microwaves.
All objects above 0 0K, including humans, absorb and radiate infrared radiation.
Infrared radiation is used in medical photography and treatment, in astronomy, and in photography in fog.
Infrared radiation can be detected by a Golay cell detector that contains xenon gas.
2. Show that electromagnetic radiation extends beyond the visible into the infrared and its equivalence with heat radiation.
A normal colour spectrum is produced with the aid of the slit and slide projector and the prism.
Rotating the prism will bring different sections of the spectrum into the entrance pupil of the thermopile.
Maximum reading is obtained just passed the red end of the spectrum.
This experiment requires that the infrared filter is removed from the slide projector.
Plastic slides will melt.
3. Set up a slide projector to display a normal spectrum on the screen.
Remove the IR filter and place a 2-3 mm slit in the slide carriage.
Focus a digital movie camera on the image and compare the images in normal mode and night vision mode.
The CCD elements are sensitive to the infra red and normally an IR filter is used to block the IR.
In night vision mode this filter is swung out of the way, allowing the infra red to be displayed.
Experiment
Take the lenses from an old pair of spectacles or used optical instruments, or purchase reading glass lenses and hand magnifiers.
Cover the window of a smoke box with a piece of black cardboard with three holes punched in a vertical line.
The holes should be the same distance apart, but the distance between the two outside holes should be a little less than the diameter of the lens.
Arrange a torch supply parallel to light rays.
Fill the box with smoke and hold a double convex lens in the path of the three beams of light so that the middle beam strikes the centre of the lens.
Note the beams on the opposite side of the lens from the source of light.
Repeat the experiment using a double concave lens.
28.2.01 Mirror equation, shadows
1. A general mirror equation relating: object distance u, image distance v, and focal length f.
1 / v + 1 / u = 1 / f
28.1.1 Light, speed of light, nature of light, shadows
See: 6.2.34 Speed of light, C, Einstein was right, e = mc2
1. Speed of light = 3 x 108 m/s
See diagram 28.1.1.2: Speed of light.
In transparent materials, the speed of light is less than it is in a vacuum.
Léon Foucault, France, 1819 - 1868, first used rotating and fixed mirrors to measure the speed of light.
Albert Michelson, USA, 1852-1931, improved on this method to get a more accurate measurement.
Speed of light in a vacuum (universal constant) c = 2.99792458 × 108 m / sec, to three significant figures: c = 3.00 × 108 m / sec, (6.71 X 108 miles per hour, USA).
2. Waveform
Light as a pair of transverse waves, electric field and magnetic field.
See diagram 28.1.1: Electric and magnetic fields.
All electromagnetic radiation, including light can be polarized, so light must have a transverse waveform.
An electromagnetic wave arises from accelerating charges and travels only at 3 × 108 m / sec in a vacuum.
The wave has a changing electric field and a changing magnetic field at 90o to each other.
3. Dual nature of light
* Light behaves as a line of particles, so light travels in straight lines as rays, e.g. reflection.
See diagram 28.106.1: Reflection.
* Light behaves as a wave, so light has wavefronts, e.g. refraction and diffraction.
See diagram 25.04: Wavefronts.
4. Visible light
See diagram 28.133: Electromagnetic spectrum.
Visible light is part of the electromagnetic radiation spectrum made up of (increasing frequencies, decreasing wavelengths) radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and γ-rays.
5. Production of light
Light is a form of electromagnetic radiation that a physical change can produce, such as the heating of an object, or a chemical change, e.g. burning magnesium.
6. Shadows
Light travels in straight lines and causes shadows when an object is placed in the path of the light.
See diagram 28.105.3: Shadows.
Light travels in straight lines causing shadows when barriers much larger than the wavelength of light are placed in the path of the light.
A full shadow is called the umbra and partial shadow is called the penumbra.
Experiment
On a sunny day observe the shadow of a tall vertical pole, e.g. a flag pole.
At its base the shadow has sharp edges, because it is umbra and no penumbra.
The shadow of further up the pole has less and less sharp edges with more and more penumbra and less umbra.
7. Reflection
Diffuse reflection of light occurs at surfaces that are not flat.
Specular reflection occurs at polished surfaces.
28.1.2 Stroboscope
See diagram 28.1.1.1: Stroboscope.
A stroboscope, "strobe" uses flashing light to make a spinning object appear stationary.
It can be used to examine the detail of a spinning object without having to stop it and also the check the speed of spin, e.g. turning speed of a gramophone record (disc).
It may be a light flashing on and off a certain number of times per minute or it may be based on the same frequency as alternating current.
A rotating disc with slits can have the same effect if the speed of spin of the disc and the spinning object is the same.
Strobe lighting is a form of electronic flash for still photography of moving objects and for use in cinematography to provide a short exposure period when photographing fast moving objects.
The high intensity blue-white flash of light from the wing tips of aircraft is a form of strobe lighting.
28.2.02 Reflection, Laws of reflection
See diagram 28.3.0: Laws of reflection.
The incident ray, reflected ray, and normal (perpendicular) at the point of incidence on the reflecting surface, all lie in the same plane.
The angle of incidence, i, is equal to the angle of reflection, r.
These laws apply to both plane and curved reflecting surfaces.
Reversibility of light
If light can travel along one path via reflections, travelling along the reverse path must be possible for it.
Reflection values of different substances
Reflection values for 300 nm radiation: (1 = complete reflection) snow 0.85.dry sand 0.17.water 0.05.grass 0.025
Surfaces that are not flat cause diffuse reflection of light.
Polished surfaces cause specular reflection.
28.2.04 Real images and virtual images
Image is a picture or appearance of a real object, formed by light that passes through a lens or is reflected from a mirror.
A real image is one through which rays of light actually pass and the image can form on a screen, e.g. images produced by a camera or projector lens can be projected onto a screen.
An image than cannot be projected onto a screen such as that seen in a flat mirror is called a virtual image.
A virtual image is an image from which diverging rays of light appear to come.
28.2.05 Plane mirror images
1. A mirror forms an image by the law of reflection.
Images in a plane mirror are always virtual, upright, and the same size as the object, (not magnified).
Any image in a plane mirror is as far behind the mirror as the object is in front of it.
So any point on the image, e.g. A', appears to be the same distance behind the mirror surface as the corresponding point on the object, A, is in front of the mirror surface.
28.2.06 Ray diagrams for lenses
See diagram 28.120: Ray diagrams for lenses.
For the lens, the image forming properties depends on refraction through the lens material thus the focal length will depend both on geometric factors and the index of refraction of material of the lens.
28.2.07 Image equation
For both lenses and mirrors, the image and object distances are related by the same equation: 1 / v + 1 / u = 1 / f, where u is the distance from the object to the optical centre of the lens or to the mirror pole, object distance, v is the distance from the image to the optical centre of the lens or to the mirror pole, image distance, f is the focal length of the lens, which is positive for converging lenses and negative for diverging lenses.
28.2.08 Do mirrors "flip" horizontally?
Instead of labelling an image as left and right sided, try labelling it using compass points.
Stand facing north with a mirror in front of you.
In front of you is north, behind you is south, top/bottom are where they should be, and left becomes west and right becomes east.
Look at the mirror and hold up your right hand. It is on the east side. and the hand of the image is also on the east side.
28.2.2 Reflection of light by a plane mirror
See diagram 28.2.2: Reflection in a plane mirror.
Experiment
Put a piece of white paper on a table in the sunlight.
Hold a black plastic hair comb teeth down on a table.
Let light shine through the spaces between the teeth of the comb then onto the table to form a shadow of black and white stripes.
Incline the paper to make the stripes on the paper become longer.
Hold a vertically mirror on the table at an angle to the comb.
The white stripes are reflected beams of light after going through the space between the teeth.
Measure the angle between the incident beams of light and reflected beams of light.
Turn the mirror and observe how the reflected beams of light turn.
28.2.3 Angle of incidence and angle of reflection
Experiment
Push the pins into two rectangles of cardboard to make two optical sights as shown.
Put a sheet of paper under the mirror.
Put one sight, in front of the mirror, on the left side pointing in towards the centre.
On the right side, line up the other sight so that the images of the four pins are all in line.
Before you remove the mirror, trace along its back edge in contact with the paper.
Press the optical sights down gently so that the ends of the pins leave a mark on the paper.
Remove mirror and optical sights.
Draw a line through the marks in the paper from the first sight, onto the mirror, and a line from the mirror through the second sight.
Where these two lines meet on the surface of the mirror draw a line at right angles to the surface of the mirror, called the normal.
Use a protractor to measure the angle of incidence and the angle of reflection.
28.2.5 Periscope
See diagram 28.2.5: Periscope.
Toy plastic periscopes
Experiment
Use a strong tube or make a long box.
Paint the inside black.
Insert two small mirrors, parallel and at 45o angle to the tube.
Note the upright image when you look ahead, but inverted image when you look over your shoulder.
28.2.6 Kaleidoscope
See diagram 28.2.6: Kaleidoscope (during construction).
The kaleidoscope was invented by Sir David Brewster (1781-1868).
Experiments
1. Cut out a 16 × 16 cm piece of cardboard.
Use a pencil to divide the cardboard into 4 strips each 4 cm wide.
Use a sharp knife to score along the pencil lines so that the cardboard can be easily folded into 4 strips.
Use paste to attach aluminium foil over 2 of the strips at one end.
Colour the next strip black.
Fold the strips to make a tube with a triangular cross-section.
Two of the inside walls of the triangular tube have aluminium foil on them and the inside of the third wall is black.
Use adhesive tape to keep the folded cardboard in shape.
Attach clear plastic to each end of the triangular tube.
Put coloured beads or small coloured shapes on the clear plastic at one end.
Make a lid out of tracing paper to cover the beads or coloured shapes.
The lid must be high enough to let the beads or shapes them move about.
Look through the clear plastic at the other end of the triangular tube, which is now a kaleidoscope.
Turn the kaleidoscope while looking through it a see the changing patterns formed by light bouncing off the aluminium foil.
2. Make a better kaleidoscope using three identical small mirrors instead of aluminium foil, e.g. cut from a 5X7 dime store mirror.
Use rubber bands to hold the three pieces of glass together to form a solid triangle and to attach translucent grease paper to one end.
Put little pieces of cut soda straw inside.
Hold the kaleidoscope horizontally and look through it while turning it slowly.
28.2.8 Laws of reflection using an electric torch
See diagram 28.2.8: Reflection with a torch.
Experiments
Use a 23 cm × 42 cm, flat, hard piece of cardboard.
At 2.5 cm from the longer side of the cardboard draw a semicircle of radius 200 mm and equally divide the central angle.
Bisect the semicircle at 90o line into two 1/4 circles.
At the bottom of one 1/4 circle remove the remained 2.5 cm wide strip.
At the bottom of the other 1/4 circle bend the 20 mm level line ahead.
To bend it into right angle cut it to the half deep beforehand.
Glue the two 1/4 circles behind them with adhesive tape along the 90o line and joining the 0o line and 180o line leaving a very narrow slit at the connecting to rotate the 1/4 circle removed the 2.5 cm wide strip free.
On the 20 mm wide strip place small plane mirror under the centre to make this 1 / 4 circle stand vertically in front of the mirror.
Rotate another one backward then let it stay at some place.
Cover the glass of a torch flashlight with a piece of black paper with a small hole in the centre.
Let the light rays from the electric torch shine along the line with an arrow and reach the mirror at the centre.
Rotate the back 1 / 4 circle to look for the reflected ray and measure the angle of incidence and the angle of reflection.
28.2.9 Mirror image needs a flat smooth surface
Experiments
Put a piece of black paper on the table.
On it put a flat bottom glass container, e.g, a fish tank, containing water.
Hold a pencil above the surface of the water.
Note the image of the pencil.
Put your finger into the water and move it up and down rapidly to form waves on the surface of water.
Now you cannot see the image of the pencil.
When surface of water becomes stationary and smooth again, the image reappears.
Slightly crumple aluminium foil to produce an irregular surface.
You cannot see the image of the pencil on that surface.
Remove the black paper below the glass and observe whether the image of the white holder or black holder is more clear.
A clear image is produced by a smooth surface that can fully reflect the light rays.
A rough surface cannot be used as a mirror.
28.2.10 Image position in a plane mirror, pin parallax method to locate image
See diagram 28.2.10: Pin parallax method.
Experiments
Cover a large white paper on the table, Stand up a plane mirror in centre of the paper.
Draw a black point S on the paper in left front of the mirror by a pen.
Put a projector on the paper in right front of the mirror, point at the black point in the mirror in two different directions in turn, after point it at each time press the pin to leave traces A1, B1, A2, B2, which meet at point S'.
Connect SS' and original position line of the mirror MN to point 0.
Measure the length of SO and S'O and the angle between SS' and MN.
Finally conclude the law of the image in the plane mirror.
28.2.11 Reversed writing.
Identify a person's handwriting, lateral inversion
See diagram 28.109.1: Lateral inversion.
See diagram 28.204: Inversion.
Place a piece of carbon paper with two sides on a table and a piece of white paper on it.
Vertically stand a plane mirror near some outside of the white paper.
Ask other person to write a familiar word then you read the word through the mirror without seeing the word on the paper directly.
You may fell very difficult to read and even not tell what the word is.
Invert the paper to make the back of the paper upwards then you read the word duplicated.
You may feel difficult similarly.
Move the mirror to the next side of the paper then you read the word through the mirror.
You will identify the word instantly.
28.2.13 Reflection of clock face in two mirrors
See diagram 28.2.13: Clock face in two mirrors.
Stand two square mirrors vertically on the table with their edges touching at right angles.
Stand a clock vertically in front of the left mirror.
Observe the numbers 3 and 9 in the image and on which side from you.
Stand a clock vertically in front of the right mirror.
Observe the numbers 3 and 9 in the image and on which side from you.
Stand the clock so that a vertical line joining 12 and 6 is in line with the join in the mirrors.
Observe the image of the clock face again.
Repeat the experiment with a mark on your face.
28.2.16 Diffuse reflection
Experiment
Hold frosted glass at various angles in a beam of light focussed on the wall.
28.2.17 Scattering of light using aluminium foil Experiment
Reflect light off a sheet of aluminium foil then crumple and flatten it to create many facets.
28.2.18 Reflection by grazing
Experiment
Place a lantern and piece of clear glass midway between two walls and show the difference between reflecting by grazing on one wall and normal reflection on the other.
28.2.19 Corner cube, corner reflector, retrodirective mirrors
See diagram 28.2.19: Three mirrors
Experiments
1. Three reflectors are placed on the inside corner of a box corner reflector.
Two mirrors at 90 degrees or three mirrors mutually perpendicular.
Look at your image in a corner cube.
2. The arrangement of three mirrors with three mutually perpendicular faces is retrodirective; i.e, this arrangement will reflect all incoming rays back along their original directions.
When viewed from any angle, the image of the observer always appears at the vertex of the three mirrors.
A laser beam directed to the reflector comes straight back.
Use a cloud of chalk dust makes the light beams visible.
28.2.20 Mirrors at an angle with a candle
A candle placed between angled mirrors forms multiple images.
Two hinged front-surface mirrors show multiple images of an object placed between them.
Experiments
Place a light between two mirrors hinged together and standing vertically.
Place a sheet of clear glass between the mirrors forming an isosceles triangle.
28.2.21 Parallel mirrors with a candle
Experiments
Form an infinite number of images using a candle between parallel mirrors.
28.2.23 Image of candle in a glass of water
Experiments
1. Place a sheet of glass between a burning candle and a glass of water, so the image of the candle appears in the glass.
2. Place a candle in front of a sheet of glass using a glass of water an equal distance behind the glass.
Observe the image of the candle in the glass of water behind.
You can do the experiment with the entire apparatus on a rotating table.
28.2.24 Unreversed image of your face
Experiments
See yourself as others see you!
Use two small identical mirrors without frames.
Hold the mirrors vertically in front of you at right angles to each other to see a reflection of your face.
Close the right eye and see the image close the right eye in the reflection of a reflection of your face.
28.3.1 Spherical mirrors
The distance to the centre of curvature is twice the distance to the principal focus.
Spherical mirror formula: If u = object distance from mirror, v = image distance from mirror and f = focal length of mirror, then 1 / f = 1 / u + 1 / v.
Magnification = height image / height object = v / u.
Concave mirrors (converging mirrors)
All rays parallel to the principal axis meet at the principal focus on the principal axis.
A real image forms when the object is further than one focal length.
Convex mirrors (diverging mirrors)
All rays parallel to the principal axis through the mirror appear to come from the principal focus behind the mirror.
The images that are always virtual, upright, diminished, and less than one focal length.
28.3.2 Concave and convex mirrors
Experiments
Shine parallel beams at convex and concave mirrors.
Use a thread screen for display.
Mount either concave or convex mirrors in an optical.
28.3.4 Reflection from a concave mirror (converging mirror)
See diagram 28.3.4.0: Real image.
See diagram 28.3.4.1: Virtual image.
See 2.0.5: Conic sections, parabola.
See 2.0.6: Parabola equation.
See diagram 28.3.4.2: Real image positions.
For most spherical and parabolic mirrors, all rays parallel to the principal axis through the mirror either meet at a point on the principal axis called the principal focus, F, for converging or concave mirrors, or appear to come from such a point behind the mirror for diverging or convex mirrors.
The centre of curvature, C, for a spherical mirror is twice the distance of the principal focus, F, from the mirror.
Use the Cartesian co-ordinate sign convention: 1 / v + 1 / u =1 / f.
Real images from a concave mirror forms when the object is further than one focal length (distance f) from the mirror.
Where u is the object distance from the mirror, v is the image distance from the mirror, So is the object distance from the principal focus, Si is the image distance from the principal focus, f is the focal length of the mirror, Ho is the height of the object, Hi is the height of the image.
Virtual images occur when the object is less than one focal f length from the concave mirror.
Experiments
1. Observe the reflection and convergence of a parallel sided beam, by using a large metallic ring with plating as a concave mirror.
2. Place a light globe at the radius of curvature of a very large concave mirror.
Adjust the height of the mirror to ensure the light globe is placed correctly in the vertical plane.
3. Use a concave mirror.
Draw a vertical line on the side of a cylinder made up of hard cardboard then draw another opposite line; bisect the cylinder along the two lines by using a paper knife, paste the inside of one half to make it tidy.
Insert a flash board with many slits into the ray box made at experiment 28.1.1 and use a cardboard to shade the light rays emitting from bottom so that a parallel sided beam is obtained.
Observe the reflection and convergence of the parallel sided beam at the concave mirror and try to measure its focal point.
4. Bisect a cylinder and paste the outside of one half with silver paper to get a convex mirror.
Also get a parallel sided beam and observe the reflection and divergence of the parallel sided beam at the convex mirror.
Convex mirrors produce virtual, upright, diminished images, and inside F.
28.3.5 Reflection from a convex mirror (diverging mirror) convex surface
See diagram 28.3.5: Convex mirror.
Experiments
Observe the reflection and divergence of a parallel sided beam, by using the side of a water bottle with plating or stainless steel kettle as a convex mirror.
28.3.6 Light a match with reflectors
See diagram 28.3.6: Radiation.
Two reflectors are set at opposite ends of the lecture bench.
One contains a heater controlled by a variac.
The other has a match at the focal point of the reflector.
Turn the variac all the way up and wait.
The match will light in about 1 minute.
If it takes longer, something is wrong.
Alignment is critical!
28.3.7 Variable curved mirrors
Aluminized Mylar stretched over an open coffee tin makes a variable positive or negative mirror when the coffee tin is pressurized or evacuated.
28.3.8 Convex mirror and plane mirror
See diagram 28.3.8: Convex mirror and plain mirror.
Experiments
Set up a pin 30 cm in front of a convex mirror.
A plane mirror placed between the between the pin and the convex mirror 20 cm from the pin covers only the left half of the convex mirror.
The images of the pin in both the convex mirror and plane mirror coincide.
Convex mirrors produce upright, diminished, virtual images.
Magnification = hi / ho = v / u = 10 / 30 = - 1 / 3
u = 30 cm, v = -10 cm, so f = -ve
1 / f =1 / u + 1 / v
1 / f = 1 / 30 -1 / 10 = -2 / 30, so f = -15 cm.
28.3.9 Parabolic mirrors
2.0.5: Conic sections, parabola
Experiments
1. Cold candle in a parabolic mirror.
Hold your finger in the inverted image of a candle burning at the centre of curvature of a parabolic mirror.
2. Focus infrared light with parabolic mirrors and observe the rise in temperature.
Place a 650 W light globe at the focus of one of a first parabolic mirror to produce a parallel beam of light.
Place the second parabolic mirror about 2 meters in line to refocus the beam.
Place a flammable pellet at the focus and observe its ignition.
3. A hidden flower at the centre of curvature of a parabolic mirror appears in an empty vase.
4. Shine parallel rays at spherical and parabolic mirrors noting the difference in aberration.
28.3.10 Amusement park mirrors
Cylindrical mirrors are made with 25 cm of curvature.
Experiments
View the image of your nose in a 1 cm diameter steel ball through a short focal length lens.
28.3.11 Focus energy at a focal point
Experiments
1. Use a magnifier to focus sunlight on your arm.
Is the hottest spot on your arm the brightest spot?
Be careful!
2. Remove the projection head of an overhead projector and hold a piece of paper at the focal point until it bursts into flame.
28.4.04 Critical angle and total internal reflection
Critical angle is the angle of incidence in a more dense medium, which produces an angle of refraction of 90o in a less dense medium.
Total internal reflection occurs when the critical angle of incidence is exceeded.
Triangular prisms can change direction of light by total internal reflection if the angle of incidence > critical angle.
When a parallel beam of light from the lamp is aimed vertically upwards from beneath the cylindrical clear plastic trough containing water the beam is not deviated.
It passes straight up.
If the beam is aimed up at a small angle of incidence, i, then both an internally reflected beam, R, and a refracted beam, T, can be seen.
The beam, T, is passing from water to air, from a medium of higher optical density to lower optical density.
So the refraction is away from the normal, angle r > angle i.
As the angle of incidence, i, is increased, r increases until the direction of the transmitted beam, T, approaches the direction of the surface of the water.
When i reaches the critical angle the refracted ray just grazes the surface of the water, so angle of refraction becomes 90o.
When i > critical angle there is no refracted beam, because all the light is reflected as total internal reflection.
There is a sudden increase in the intensity of the reflected beam as the angle of incidence increases beyond the critical angle.
The critical angle is an angle of incidence in an optically more dense medium, which produces an angle of refraction of 90o in a less dense medium.
When the critical angle of incidence is exceeded, there is no refracted light at all, because all the light is totally internally reflected.
28.4.05 Prism as a mirror
See diagram 28.127.3: Right angle prism.
Use prisms to change direction of light by total internal reflection.
If glass has a critical angle of 42o, light entering a triangular right angle prism such that the angle of incidence to the glass / air interface is 45o will be totally internally reflected at an angle of 45o.
So the prism acts like a mirror.
28.6.01 Total internal reflection
See diagram 28.127.1: Candle behind fish tank.
Experiments
1. Put a short lighted candle behind a glass or plastic rectangular fish tank.
Fill the fish tank with water to a level just above the wick.
Look at right angles to the fish tank so that you can see the lighted candle directly opposite.
Raise and lower the level of your eye above and below the level of the water.
The top of the candle flame and the bottom of the candle flame around the wick are in one line.
Move your head to the left parallel to the front glass of the fish tank.
When your eye is above the water level, the top of the flame appears to move to the left.
When your eye is below the water level, the bottom of the flame appear to move to the left.
The angle between a line from the candle at right angles to the fish tank, the normal, and your line of sight, the incident ray, is increasing.
For most glass, when this angle reaches about 43o, the critical angle, the incident ray cannot pass into the water, so the image disappears.
Return to the first position where you first looked at the candle directly opposite you.
Lower your eye to the level of the bottom of the fish tank and look up at the bottom of the water surface.
See the reflection of the lower part of the candle that you saw when your eye was just below the level of the water.
Light from the candle up to the surface of the water is at an angle greater than the critical angle is reflected at the water surface, total internal reflection.
2. Stand a spoon in a glass of water at the edge of the table.
See diagram 28.127.2: Spoon in water.
Look up from just below the table surface at the spoon pointing down towards you.
The surface of the water acts like a mirror and so you see the reflection of the lower part of the spoon that is under water.
However, you cannot see the upper part of the spoon above water.
3. Shine a light into one of the two sides of a right angle reflecting prism.
The light reflects off the hypotenuse and passes out through the other side.
The light reflects, because the angle of incidence at the hypotenuse is greater that the critical angle for crown glass, 43o.
Reflecting prisms are used in binoculars, prismatic compasses and periscopes.
Prisms allow you to see around corners!
4. Drill a small hole in the side near the bottom of a tall beverage can and plug the hole.
Fill the beverage can with water and place it on the edge of a table over a basin.
Darken the room.
Shine an electric torch down into the beverage can.
Remove the plug to let the water stream fall into the basin.
The light from the torch bends with the falling stream to form a light spot on the bottom of the basin, because of the total internal reflection of light on the interface between water and air.
Some rays are refracted to your eyes.
5. Use a light pipe, a curlicue-shaped methyl methacrylate (Lucite) rod to show critical angle and total internal reflection.
See diagram 28.6.0: Lucite light pipe.
Let light enter the rod at one end of the light pipe to strike its walls at an angle greater than the critical angle of Lucite.
The light does not escape through the walls, but is reflected between the walls of the rod until it reaches the other end.
6. Trap and guide light by an unbroken stream of water flowing from a glass reservoir.
See diagram 28.6.01: Light through a water stream.
Aim a torch light or laser beam at the back of a stopper, through the opposite side of the glass reservoir.
Remove the stopper so that the light is guided by the stream of water and causes a brilliant spot on a sloping white screen in a fish tank.
7. Make an attractive table lamp by putting your light source facing upwards in a light proof container like a flower vase.
Then place thin bending plastic tubes in the flower vase.
28.6.1 Total internal reflection, fibre optic cable
Fibre optics cable contains optical fibres that are fine strands of glass surrounded by "cladding" glass that bends any light rays that strike it back towards the centre of the optical fibre.
Optical fibres allow you to see around corners, so surgeons may use optical fibres to see inside the body.
An optical fibre cable containing thousands of separated strands of glass is used as a decorative lighting device.
Experiment
See diagram 28.6.1: Glass rods.
Use a Plexiglas rod, Plexiglas curly Q and a section of fibre optic cable.
Hold their ends in the beam near the laser.
Vary the number of reflections in the rod by changing the angle.
28.6.2 Critical angle in ripple tank, refraction tank, aquarium
25.3.1.0 Ripple tank, wave tank.
See diagram 28.6.2: Ripple tank beam.
Experiments
1. Rotate a beam in a tank of water until there is total internal reflection at the surface.
Adjust the path of a beam with mirrors in a tank of water with fluorescein to show total internal reflection.
Rotate the light source through 270o to show refraction and total internal reflection for both air/plastic and plastic/air interfaces.
2. To demonstrate critical angle and total internal reflection, shine a beam through the side of a tank containing fluorescein.
28.216: "Pouring light" (diagram), critical angle
Rotate a mirror in the tank so the beam passes through the critical angle.
Vary the angle of incidence of ripple tank waves to a boundary with water depths of 13 and 3 mm.
Use an underwater light to illuminates powder on the surface of water to form a central spot of light.
4. Observe total internal reflection from a water / benzol surface.
5. A black soot-covered ball appears silver under water due to reflected light from air trapped on the surface of the ball, forming an air-water interface.
28.6.3 "Pour" light from a beverage can
28.6.1 Fibre optic cable "light pipes"
See diagram 28.216: "Pouring light".
In 1870, John Tyndall demonstrated to the Royal Society that light could be guided by a stream of falling water.
So this experiment is often called "Tyndall's experiment".
This principle is used for "light pipes", fibre optic cables and decorations using light shining up through a bunch of tubes.
Experiment
Remove the top of a beverage can.
Punch a hole in the side of the drink near the bottom and close the hole with a stopper.
Pour water into the beverage can until it is three quarters full.
Put the beverage can next to a sink in a dark room.
Hold an electric torch vertically down in the top of the beverage can, so all the light shines down into the water.
Remove the stopper and let the water pour into the sink.
The light from the electric torch appears to pour out with the water.
Most of the light cannot escape from the falling water, because the critical angle is exceeded and it reflects off the water surface by total internal reflection.
28.6.4 Diamonds
See diagram 28.6.8: Diamond, planned shape.
Diamonds have a small critical angle, because of the large refractive index.
Light tends to become trapped inside, because it may undergo total internal reflection more than once before refracting out of the diamond.
Most light rays approach the diamond at angles of incidence greater than the critical angle so the diamond tends to sparkle.
For total internal reflection, incoming light must strike the diamond face at an angle of more than 48o with the horizontal.
After the first reflection light striking the second inclined face is reflected if the face makes an angle of less than 43o 43' with the horizontal.
However, the angle to produce the best possible dispersion for reflective brilliance is 40o50'.
The sparkling effect can be enhanced by cutting a diamond gemstone with a planned shape.
The diagram shows the total internal reflection within a diamond gemstone with and without a planned shape.
Refractive index with air:
Water 1.333, lowest optical density, Crown Glass 1.52, Diamond 2.417, highest optical density
28.6.5 Right angle prism inverter
A right angle prism placed in a projected beam inverts the image.
A beam entering the hypotenuse of a right angle prism is inverted and reversed.
28.6.6 Optical disc with prism semicircle
A single beam of light on the optical disc shows total internal reflection when passed through a prism.
A beam of light entering a semicircular glass normal to the curved surface is reflected off the flat side.
28.11.1 Water concave lens, ice lens
See diagram 28.1.17: Water lens.
Experiments
1. Make a single turn of copper wire around a nail to form a loop.
Dip the loop into water, take it out and look through it.
Such a lens may magnify four or five times.
Tap the wire sharply against the edge of the glass so that a drop of water falls off.
, because of adhesion between the wire and the water, the liquid remaining will form a lens, which is very thin at the centre, i.e, a concave lens.
2. Put the water lens in a freezer to make an ice lens.
28.11.2 Water drop magnifier, water lens
Experiments
Place a drop of water carefully on a plate of glass.
Bring your eye close to the drop and look at something small through the water drop and glass.
The water drop serves as a simple magnifier.
28.11.3 Model refracting telescope
Experiments
Arrange a long focus lens on the end of an optical bench pointing at an object through the window.
Bring a piece of white cardboard up on the opposite side of the lens to the place where the sharpest image of the scene is formed.
Bring a short focus lens up behind the cardboard until the cardboard is a little nearer the lens than its focal length.
Remove the cardboard and look through the two lenses at the object.
28.11.4 Projector for filmstrips or slides
See diagram 28.1.20: Projector.
Experiments
The base of the instrument is a piece of plywood 40 × 10 × 3 cm.
A plywood board 10 cm wide and 25 cm long fits into a groove cut across the base.
It serves as filmstrip carrier.
A hole 35 by 23 mm cut in this wood serves as an aperture or gate to limit the light passing to one frame of the strip.
Fix the strip close to the gate in a vertical position with staples made from wire paper clips.
Bent them to the width of the film and cut the ends short and sharpen them using a file.
Then press them into position on the plywood board.
No reels are necessary.
The strip can be moved on from one frame to the next by pulling on the end of the film until there is sufficient 'curl' to hold it stationary.
The lamp is a motor car head lamp in a holder mounted on a block.
It is adjustable so that it can be slid between two strips of wood nailed to the base.
Use a carafe or flask of water as a condensing lens placed so that the whole of the gate is illuminated by the image of the lamp.
When so positioned, the lamp and condensing flask are fixed in place with glue.
The object lens is mounted on a piece of wooden dowel that is a tight fit in a hole drilled into a block of wood arranged, like the lamp support, to slide between two wooden guides.
Adjust the lens by sliding the rod in or out of the hole so that the centre of the lamp, condenser and objective are all the same height above the baseboard.
Use a plywood, metal or cardboard case to enclose the lamp and the condenser.
Use the apparatus in a darkened room.
28.11.6 Microscope as a micro projector
If a very bright light source is used, the image from the eyepiece of a compound microscope can be reflected on to a screen using a mirror.
A powerful slide projector is a good source of light.