The Science Notebook
Gilbert Light Experiments - Part 3

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NOTE:  This book was published in 1920, and while many of the experiments and activities here may be safely done as written, a few of them may not be considered particularly safe today.  If you try anything here, please understand that you do so at your own risk.  See our Terms of Use.  

Pages 51-75


commands him to rise. Two concealed boys at T pull the rug and the boy appears to rise. He can then be turned upside down and back again. You can repeat with the boy lying down.


Experiment No. 41.
Magic money.

Stand the two mirrors vertically on the table sidewise to a good light and place a coin between them. Look over each mirror in turn into the other (Fig. 74). Have you multiplied your money wonderfully?

Experiment No.42.
Magic lights.

Repeat the above in the dark with a lighted candle between the mirrors


(Fig. 75). Do you find many, many lights?

Experiment No. 43.
Magic army.

Put a number of lead soldiers on a narrow strip of paper and draw them between the vertical mirrors (Fig. 76). Do you see an immense army marching in perfect order?

Experiment No. 44.
Magic dancers.

Cut out of paper or cardboard a small figure of a man dancing. Attach him to a string and make him dance between the mirrors in a good light (Fig. 77). Do you find a multitude of dancers who keep time perfectly ?

Experiment No. 45.
Magic silver or copper mine.

Separate the mirrors by two blocks, place them one above the other and face to face (Fig. 78); place a silver or copper coin on the lower mirror. Do you find yourself looking down into a very deep hole with many silver or copper coins in it?

Why you see Many Images in Parallel Mirrors. You


see many images between two parallel mirrors because the image formed in one mirror is an object in the other, and so on.

In Fig. 79, two mirrors, A and B, 4 inches apart are facing each other and a candle between them is 1 inch from B and 3 inches from A.

In B the image Bl is formed 1 inch behind B and in A the image Al is formed 3 inches behind A.

Now image Al is 7 inches in front of B and it forms an image B2 7 inches behind B; similarly image Bl is 5 inches in front of A and forms an image A2 6 inches behind A. Again, A2 is 9 inches in front of B and forms an image B3 9 inches behind B, and so on.

You see many images because the light which enters your eyes has been reflected one or more times. If you are looking at Bl, the light which enters your eye appears to come from Bl, but it comes from the candle and is reflected from B.


If you are looking at B2, the light appears to come from B2, but B2 is an image of A1, and the light goes from the candle and is reflected twice before it enters your eye.

Image B3 is an image of A2, which in turn is an image of Bl, and you see B3 by means of light which has been three times reflected. Similarly you would see B4, B10, and B50 by means of light reflected 4, 10, and 50 times.

It is good practice to locate the images in parallel mirrors and to trace the paths of the light.

Why the Images become dim. The images become dimmer the farther they are


away: first, because some light is absorbed by the mirrors at each reflection; and, second, because the light has traveled a long distance in being reflected back and forth between the mirrors.

Experiment No.46
The trench periscope.

To illustrate how the periscope works, look over the top of a tall book as shown in Fig. 80. Place one mirror against the book at an angle of 45 and hold the second mirror above the book at the same angle. Can you see over the top easily without being seen yourself? Turn the upper mirror until it looks backward (Fig. 81), Can you see back over your head, but is everything upside down ? Turn the upper mirror until it looks sidewise (Fig. 88). Can you see things, but are they turned on their sides?


Now let us see why the image is right side up in some cases and not in others,

The mirrors in the regular periscope are parallel to each other, and you can locate the image in each mirror in turn as you


did in the case of parallel mirrors.

Let the arrow, Fig. 83, represent the object; its image in A is Al and the top and bottom of Al are as far behind the mirror extended as the top and bottom of the arrow are in front.

Now let us suppose mirror B to be extended as shown by the dotted line, then Bl is the image of Al in this extended mirror and the top and bottom of Bl are as far behind B as the top and bottom of Al are in front of B, and therefore Bl is right side up.

In the second case, the mirrors are at right angles (Fig. 84). Al is the image of the arrow in A extended and Bl is the image of Al in B extended; Al is on its side and Bl is inverted for the reasons given above.

In the third case, the image is on its side in the upper mirror, and since the lower mirror is parallel to this image, the image in the lower mirror is still on its side. 

Experiment No. 47.
To make a


trench periscope.

Get a block of wood 4" X 4" X 6", measure down 1 inch from each end and draw a line across diagonally. This line will be at 45 to the length of the block. Cut the block through on this diagonal line, see right side Fig. 85.

Now attach a mirror to each diagonal face by means of tacks. Cut a piece of stiff cardboard 17 inches wide and as long as you wish to make the periscope. Tack this to the block, overlapping 1 inch on one side. Paste the overlapping parts together. Cut a hole 3" opposite the upper mirror and a hole 2" X 2" opposite the lower mirror, and your periscope is finished.

You can use this periscope in your trench battles; also you can use it on a train to see forward without putting your head out of the window. In this case, however, you should fasten the window glass over one of the holes to keep cinders out of your eyes.


Experiment No. 48.
Mirrors at different angles.

Stand the mirrors vertically and at right angles on the table (Fig. 86) and place a lighted candle


between them. Do you see four candles, the real candle and three images?

Make the angle 60. Do you see six candles, the original candle and five images?

Make the angle 45. Do you see eight candles?

Make the angle 30. Do you see twelve candles?

There are 360 in a complete circle, and the number of candles you see in each case is 360 divided by the angle between the mirrors. For example, when the angle is 90, you see


or 4 candles;

and when the angle is 60, you see


or 6 candles; and so on.

Experiment No. 49.
A one-boy crowd.

Stand the mirrors at 90 and put your face close to the mirrors. Are there four of you, yourself and three images?

Repeat with the mirrors at the angles mentioned above. Do you find yourself a crowd all in a circle?

Experiment No. 50.

Stand the mirrors at 90 on a piece of white paper and draw an arrow  pointing at one of the mirrors. Do some of the arrows point in one direction and some in the opposite direction? Keep one mirror in such a position that the arrow points directly at it and move the other mirror until the angle is 60. Do the six arrows point toward
each other in pairs?

Repeat with the mirror at the other angles mentioned above.

Experiment No. 51.
An infinite number of candles.

Light a candle and stand the mirrors close to it and gradually make them parallel. Do you see very, very many candles?

When the mirrors are parallel the angle between them is 0 and


is infinity, so you should see an infinite number of


images. You cannot, because some light is lost at each reflection and finally all is lost.

EDITOR'S NOTE:  (Not in the original text!)  While the experts say it is possible to define anything divided by zero as "infinity" as the author did here, today most math whizzes choose to say that anything divided by zero is "undefined."  In this case, there still are theoretically an infinite number of images, but the math would probably not be considered "right" today.  It may be "splitting hairs" to even bring this up, but the Notebook staff would not want anyone unduly upsetting a math teacher!

Experiment No. 52.
To locate the images in mirrors at an angle.

Draw two lines 4 inches long at right angles to represent two mirrors at right angles (Fig. 87) and extend them backward by dotted lines to represent the extended mirrors. Place a dot 1 inch from A and 2 inches from B, then image Al will be 1 inch behind A and image Bl 2 inches behind B. The third image A2B2 is an image of both Al and Bl; it is 1 inch behind A extended and 2 inches behind B extended.

It is harder to locate the images when the angle is 60 or smaller, but It will help you to know that the images are always all on the circumference of a circle of which the angle of the mirrors is the center.

Practice locating the images in mirrors at 60.

Experiment No. 68.
The kaleidoscope.

The kaleidoscope (Fig. 88) consists of two mirrors at an angle of 30 in a tube which has an eye opening at one end and at the other a chamber containing pieces of col-


ored glass. When you look through the tube and revolve it, the colored pieces of glass make beautiful twelve-sided figures by multiple reflection.

Illustrate the working of the kaleidoscope as follows: Draw two lines at an angle of 30 on a piece of white paper. Stand the mirrors on a block above these lines with the angle toward a good light (Fig. 89). Now put pieces of colored paper and other small objects on a strip of paper and draw the paper under the angle, while you look down between the mirrors with your eye near the angle. Do you see a series of twelve-sided figures?


The Sphinx. This illusion shows an Egyptian head without a body (Fig. 90). The hypnotist shows the audience an empty box with a glass front. He closes and locks the door over the front, places the box exactly on the center of the table, unlocks it, opens the door, and, behold, there is an


Egyptian head in the box. The hypnotist stands near the audience and addresses the head. "O ancient Sphinx, awake! Awake! Awake !" The sphinx slowly opens its eyes and stares straight ahead. The hypnotist then addresses questions to it and it answers in very deep and very dead tones, and so on. Finally the hypnotist locks the box, brings it forward to the audience, opens it, and there is nothing in it but a handful of ashes.

The mechanism of this illusion is illustrated in Fig. 91. The table is on three legs. A, B, C, with mirrors at 60 between A, C, and A, B. The curtains at the back and sides are exactly alike, and to the audience the images of the side curtains appear to be the back curtain, and the space under the table appears quite empty.

Cabinet of Proteus. The performer puts his assistant into the cabinet (Fig. 92), closes the doors a moment, makes passes, open doors (Fig. 93), and the assistant is gone. Closes doors again, makes passes, opens doors, and out comes an entirely different man. Closes doors again, makes passes, opens doors, and out comes a lady. Closes doors again, makes passes, opens doors,


and out comes assistant.  To the audience, the cabinet appears entirely empty except for a post, C, with a strong light at the top. There are, however, two hinged mirrors, ab and ab. Fig. 94, at an angle of 60 and the post covers the angle. The sides and back are exactly alike and the images of the sides in the mirrors appear to the audience to be the back. The man, lady, and assistant, of course, hide behind the mirrors. Members of the audience stand behind and beside the cabinet all through the performance. The assistant swings the mirrors against the sides before be comes out the last time, and then members of the audience are asked to examine the cabinet, when, of course, they find nothing.

Illusion Show. Pharaoh's thumb. Make a table out of cardboard (Fig. 95) and stand it on three legs, each of which is exactly 5 1/4 inches from the other two, and place your two mirrors between A and B and A and C. Surround it by screens on three sides, making the sides and


back exactly alike and exactly the same distance from the table.

Now have an assistant put his arm through a hole in the back curtain and put his blackened thumb up through a hole in the table top, and you are ready to begin the act.

Explain to the audience that you have succeeded in bringing to life the thumb of an ancient pharaoh by your hypnotic power. Explain that the thumb was lost in battle, fell on the sands of the desert and dried but did not decompose. This pharaoh was a great hypnotist, which makes it easier for you to bring his thumb back to life. Explain also that the thumb will answer any question about the future. If the thumb moves forward it is, yes; if it doesn't move at all it is, no.

Now open the curtains, address the thumb, "O Thumb of an ancient Pharaoh, awake! Awake! Awake!" (slowly and with passes). The thumb does not move. You now ask, "O ancient and sacred Thumb, will Charles get his wish?" (Thumb slowly nods, yes.) "O ancient and sacred Thumb, will Henry get through his examinations?" (Thumb does not move. No.) And so on.

Vaudeville Act. The acrobat. You can put on a short but very funny act with a mirror (Fig. 96) placed at an angle to the audience.



Experiment No. 54.
Converging sunlight.

Open the slit in your darkened room to its full size and allow the sunlight to fall on the concave (curved-in) side of your curved mirror. Make a dust. Is the sunlight converged to a point and does it diverge beyond this point (Fig, 97) ? This point is the focus of the mirror.

Experiment No. 55.
Diverging sunlight.

Turn the convex (curved-out) side of the mirror to the sunlight (Fig. 98). Is the sunlight reflected and diverged or spread?


Experiment No. 56.
Picture of the sun.

Remove the shutter, stand the mirror on the table in the sunlight, and focus the sunlight on a strip of paper 1/2 inch wide (Fig. 99). Is the picture of the sun round and very bright?

Experiment No.57.
The focus is very hot.

Focus the sunlight on your hand with the concave mirror (Fig. 100). Is it hot? It is, because all the heat of the sunlight is concentrated at the focus.

Experiment No. 58.
To light a match with sunlight.

Place a match in front of a narrow strip of paper (Fig. 101) and focus the sunlight on the head. Does the match light?

Experiment No. 59.
A magic cannon.

Stick a needle into the under side of a cork and stick a match on the other end of the needle (Fig. 108), with a small piece of paper at one side of the head. Insert the stopper in an empty bottle, focus the sunlight on the match head through the glass sides (Fig. 103). Does the match light and are the cork, needle, and match driven out with a satisfactory pop?


The lighted match heats the air and the expanding air drives out the cork.

Experiment No. 60.
Focal length of concave mirror.

Focus the sunlight on a narrow piece of paper and measure the distance between the back of the mirror and the paper. This is the focal length of the mirror. Do you find it to be about two inches ?

Experiment No. 61.
Focal length of convex mirror.

Make two pencil dots just 3 inches apart on a piece of cardboard and between these punch two holes just 1 inch apart. Hold the cardboard between the convex mirror and the sun and move it until the light which passes through the holes 1 inch apart is reflected to the dots 2 inches apart, and measure the distance from the back of the mirror to the card. This is the focal


length of the convex mirror. Do you find it to be 2 inches?

There is no real focus for a convex mirror because it  spreads the light, but the reflected rays appear to come from a point 2 inches behind the mirror. An unreal focus of this kind is called a virtual focus.

Experiment No. 62.

Go to the back of the room, turn the concave mirror toward the window, and hold a piece of paper three-quarters inch Wide near the focus (Fig. 104). Do you find a small inverted picture in natural colors of the window and of the things outside the window?

Have a friend move about near the window. Do you get his picture? 


Experiment  No. 63.
Your own image.

Look at yourself in the concave mirror. Are you upside down and small? Bring your eye closer to the mirror than the focus (3 inches) (Fig. 105). Is your eye large and right side up? Look at yourself in the convex side. Are you small and right side up in all cases?


Waves. When parallel waves (1), Fig. 106, strike the concave side of the mirror, they are reflected and so curved in that they converge at the focus and then diverge.

When parallel waves strike the convex side (2), they are reflected and so curved out that they diverge and never meet.

Rays. The curved mirror is part of a sphere and the center of the sphere is at C, Fig. 107(1).  The lines CA are radii of the sphere and they are perpendicular to the mirror. When parallel rays strike the concave mirror they make equal angles with these perpendic-


ular radii and cross at the focus F. The line through the center O of the mirror and through the center C of the sphere is called the principal axis of the mirror. You will notice that the parallel



rays which are above the principal axis before they strike the concave mirror are below it afterward and vice versa. This explains why the images you see in the concave mirror are reversed.

When your eye is nearer than the focus, it intercepts the rays before they can cross, and your image appears to be behind the mirror, right side up and large.

When parallel rays strike the convex side of the mirror. Fig. 107 (2), they make equal angles with the radii (CA extended); they diverge but appear to come from the focus F. This is the unreal or virtual focus.

The rays above the principal axis before reflection are above it afterward, and, therefore, the images in the convex mirror are right side up.

Searchlight Reflectors. The reflectors on battleship searchlights (Fig. 108) are made in the shape of a parabola (Fig. 109). Parallel rays which strike parabolic reflectors converge exactly at the focus, and conversely if a light is placed exactly at the focus the reflected light consists of parallel rays which go straight forward. The reflectors on automobile and locomotive headlights are also parabolic, and the lamp is placed at the focus.

Spherical Aberration. Spherical mirrors do not converge all parallel rays at the focus because those which strike near the edge are reflected behind the focus (Fig. 110). This is called the spherical error or spherical aberration of the mirror. Conversely if a light is placed at its focus a spherical mirror does not reflect it in parallel rays. This explains why it is not used as a first-class reflector.



When light passes in a slanting direction from one medium to another, - for example, from air to water or the reverse, or from air to glass or the reverse, - part of it is reflected at the surface between the two media and part of it enters the second medium but is bent out of its path, from ABC to ABD, Fig. 111. This bending is called refraction. When light passes from air to any denser medium as water or glass, it is bent toward a line NN drawn perpendicularly through the surface at the point it enters. See Fig. 112 (1). When light passes from water or glass to air, it is bent away from the perpendicular NN. See Fig. 112 (2).



Experiment No. 64.
Air to water.

Allow a beam of sunlight to pass through the slit in your darkened room. Cut a slit 1 inch long and 1/4 inch wide in a piece of cardboard, put this over your mirror, and reflect sunlight into a glass pitcher full of water into which you have put 2 or 3 drops of milk (1), Fig. 113. Vary the slant of the beam of sunlight which strikes the water and view the beam in the water through the sides of the pitcher. Is some of the light reflected at the surface of the water? Does some of it enter the water and is it bent or refracted? Make the beam split on the side of the pitcher so that half is inside and half outside. Is the beam in the water bent toward an imaginary perpendicular at the point it enters? Repeat this with a glass of milky water (2). Repeat with a bottle of milky water (3). Use a bottle with flat sides.


Experiment No. 65.
Air to glass.

Make the beam split on the thick glass plate standing on its edge, on two matches, on paper (Fig. 114). You cannot see the light in the glass but you can see it on the paper below after it has passed through the glass. Is the light which passes through the glass bent toward an imaginary perpendicular NN drawn at the point it enters?

Let the sunlight enter through a slit 1 inch long and 1/4 inch wide. Split the beam of light on the edge of the glass plate and hold a piece of paper behind the plate. Tilt the plate to different angles. Is the light which passes through the glass plate always bent toward the perpendicular NN?

Experiment No. 66.
A glass of water.

Remove your shutter and stand a glass of water in sunlight near the window; fill the glass to the top and put paper around the sides to keep out the sunlight. Is the sunlight which strikes the water


surface bent down, as shown in  Fig. 116?

Explanation of Refraction. A beam of sunlight is bent or refracted when it passes from air to water because light travels more slowly in water than it does in air. Its velocity in water is only three-fourths of its velocity in air.

Now to see the connection between change in direction and change  in velocity, let us consider what would happen if a regiment of soldiers marched in a slanting direction BD from smooth ground to rough ground, as shown in Fig. 116. The men would march less rapidly on the rough ground and the direction of the marching lines would be changed. The line AB is still on smooth ground and is straight. Part of the line ab is on rough ground and this part is somewhat behind. The line cd has a larger part on rough ground and this part is behind. The line CD is wholly on rough ground and it is marching in a direction DE different from BD, and it would continue in this new direction. This is exactly what happens to parallel light waves. They are bent toward the perpendicular when they pass at a slant from air to water or glass because they travel more slowly in water or glass than they do in air. They are bent away from the perpendicular when they pass at a slant from water or glass to air because they travel faster in air than they do in water or glass.


Experiment No. 67.
A coin under water.

Put a coin in a glass of water and look down at it through the water (Fig. 117). Does it appear to be nearer than it really is?

You see the coin because light passes from it to your eyes. This light is in the form of spherical waves in the water, but

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