The Science Notebook
Gilbert Light Experiments - Part 5

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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.

 Page 101- End


seat the party around the table in the dark, light the alcohol, and look at your neighbors' faces and at your own in a mirror. Do you all look like ghosts? You do, because the salt in the flame gives only yellow light, and since your rosy cheeks and rosy lips absorb this color they appear black.


Our boys at the front painted their faces black (Fig. 153) before they started out on night raids, because the black paint absorbed the light and prevented their faces from being seen.


When substances are vaporized in a flame and the flame is viewed through a spectroscope (Fig. 154) the spectrum seen is crossed by bright lines. Each substance has its own particular lines, and when we know these lines we can tell what substances are in the flame. This is the basis of spectrum analysis. In the spectroscope shown here the light passes through a narrow slit, through tube A, through four prisms, and into the telescope B in which the enlarged spectrum is seen.



When the light from the stars is viewed in the spectroscope, the spectrum is crossed by dark lines exactly corresponding to the bright lines mentioned above. These are called the Fraunhofer lines, after their discoverer. If, in the spectrum of light from the sun, for example, we see dark lines exactly corresponding to the bright lines produced by iron in the spectrum on the earth, we know that there is iron in the sun, and so on.


Lighthouse lenses have at the center a comparatively thin lens and around this prismatic sections with greater and greater angle toward the edge, (1) Fig. 155. Panels (2) made up in this way are placed completely around the light F (3). This gives a large, short focus lens which does not absorb as much light as a solid thick lens would absorb.


Lenses are of two kinds, converging and diverging. Converging lenses are thicker at the middle than at the edges, and we may think of them as made up of sections of prisms,


Fig. 156 (1), the angles of the prisms being greater the nearer they approach the edges. These lenses converge parallel rays to a point F, called the focus.

Diverging lenses are thinner at the middle than at the edges, and we may think of them as made up of sections of prisms, Fig. 156 (3), with their thin edges toward the center. These lenses diverge parallel rays and make them appear to come from a point P, called an unreal or virtual focus.


Experiment No. 98.
Converging lenses.

Allow sunlight to pass through the slit in your darkened room, hold a converging lens in the beam (Fig. 157) and make a dust. Do you see that the light comes to a point and diverges afterward?

Repeat with the other converging lens. Is the light again brought to a point but at a different distance from the lens?


Experiment No. 99.
Diverging lens.

Repeat this experiment with your diverging lens. Is the light diverged or spread?

Experiment No. 100.
Focal lengths.

Remove your shutter, focus the light with a converging lens, hold a piece of paper at the point where you get the smallest and brightest image of the sun (Fig. 158) and measure the distance from the lens to the paper. The point is the focus and the distance is the focal length of the lens.

Repeat with the other converging lens. (Do you find the focal lengths of the lenses to be 4 inches and 8 inches respectively?

Experiment No. 101.
Focal length of diverging lens.

Punch two nail holes exactly 1 inch apart in a piece of paper, put this in front of the diverging lens, and measure the distance at which the spots of sunlight appear 2 inches apart on a paper behind the lens. This is the virtual focal length.  Is it 4 inches?

Experiment No. 102.
Is it hot?

Put your hand at the focus of each converging lens in turn (Fig. 159). Is the sunlight hot? It is, because all the light and heat


which falls on the lens is concentrated at the focus.

Repeat with the diverging lens. Is there no heat?

Experiment No. 103.
To light a match with sunlight.

When the sun is hot at mid-day put a match on a piece of paper and focus sunlight on it with the short focus lens (Fig. 160). Does it light? Why?

Experiment No. 104.
Magic cannon.

Repeat Experiment No. 59, but light the match by means of the short focus lens (Fig. 161).


When the parallel waves from the sun fall on a converging lens, which is thicker at the middle than at the edges (Fig. 163), the portions of the waves that go through the thick part are slowed up more than the portions which go through the thinner parts, and as a. result the waves are so curved in that they converge at the focus and diverge afterward. The waves are shown


in 1 and the rays in 2. This explains why these lenses converge the light.

When parallel waves fall on a diverging lens, which is thinner at the center than at the edges, the portions which go through the center are less delayed than the portions which go through the edges and the waves are so curved out that they diverge after passing through the lens. The waves are shown in 1, Fig, 163, and the rays in 2. This explains why these lenses diverge the light.

If the light comes from an object near a converging lens the waves are curved when they reach it, and one of three things may happen.

If the object is at a distance from the lens greater than the focal length (1, Fig. 164), the curvature of the waves is reversed and the light is brought to a point on the other side of the lens


at a distance greater than the focal length.

If the light is at the focus(2, Fig. 164), the curvature of the waves is so altered that they are parallel after they pass through the lens.

If the light is nearer to the lens than the focus (3, Fig. 164), the curvature of the waves is altered by the lens, but they still diverge and will never converge.


Experiment No. 105.

Arrange a candle, 4-inch converging lens, and screen as in Fig. 165. Place the lighted candle 3 feet from the lens and move the screen until you get an image. Is it inverted and small? Repeat with candle at 3 feet and 1 foot. Is the image larger each time?

Place candle at twice the focal length, that is, 8 inches. Are the candle image and candle the


same size? Place candle at 6 inches. Is the image larger? Place candle at 5 inches. Is the image larger still?  Place candle at the focus. Is the image very large? Place candle at 3 inches and 2 inches, that is, closer than focus.  Are no images formed?

Repeat with the converging lens of 8-inch focus. Place candle at distance of 4 feet, 3 feet, 2 feet, 16 inches or twice the focal length, 15 inches, 12 inches, 8 inches, and 6 inches. Are the results similar?

Is the image smaller than the candle when the candle is at a greater distance from the lens than twice the focal length? Is it larger when the candle is at a distance less than twice the focal length and greater than the focal length?

Experiment No. l06.
Picture shows.

With the candle, converging lens, and screen, as in Fig. 166, get the image of the candle on the screen, then hold your hand behind the candle and close to it. Do you get an inverted picture of your hand in natural colors?

Hold a black and white drawing upside down and close to the candle. Do you get a picture right side up?

Repeat with colored drawings, colored flowers, and so on.  Do you get colored pictures?

Repeat with all kinds of things and use four or five candles to get more light.

Experiment No. 107.
A picture of out-of-doors.

In the daytime, go to the side of the room away from the window and get a picture of distant objects on the screen (Fig. 167). Do you


find a beautiful inverted picture in natural colors of everything out-of-doors?

Measure the distance from lens to screen. This is again the focal length of the lens. At night get a picture of a distant light and measure the focal length.

Experiment No. 108.
The lenses and your eyes.

Hold the converging lenses in turn at arm's length and look at distant objects. Is the image small and inverted?

Hold them about one foot from your eye and look at your finger held closer to the lens than its focal length. Is the image large and right side up?

Repeat with the diverging lens. Is the image always right side up and small?


In Fig. 168 (1) the object OB is at a greater distance than the focal length. All the rays which fall on the lens from any point B meet at the point M and, therefore, the image of B is at M. We cannot trace all the rays, but it is necessary to trace only two. The two most easily traced are the parallel ray BR and the ray BP which goes through the center of the lens. Ray BE goes through the focus F after it goes through the lens; ray BP goes straight ahead, or nearly so, because the two sides of the lens are nearly parallel at the center.

The rays from all other points between B and O meet at points between M and I and, therefore, MI is the inverted image of BO.


In (2), BO is inside the foous; therefore BR and BP diverge after they pass through the lens and do not form an image. Your eye, however, makes an image because it sees the rays as though they came from MI. This explains why you see anything inside the focal length as enlarged and right side up.

In (3), BO is outside the virtual focus of the diverging lens. BR and BP diverge after they pass through the lens and your eye sees the image MI. This explains why diverging lenses always give images small and right side up.


Spectacles are lenses, and opticians measure the power of the spectacle lenses as follows: If the lens has a focal length of 1 meter it is said to have a power of 1 diopter; if it has a focal length of 1-2, 1-3, or 1-10 meter it is said to have a power of 2, 3, or 10 diopters; and so on. That is, the shorter the focal length the greater the power.


A meter is 100 centimeters long. You will find on most ordinary rulers 30 divisions on the side opposite the inch divisions; each of these divisions is 1 centimeter, and 100 of these make a meter.

Experiment No. 109.
Power of your lenses.

Measure in centimeters the focal length of the 8-inch lens. Do you find it to be 20 cms.? Is the power of the lens then

                      -----   = 5 diopters?

Repeat with the 4-inch lens. Is its focal length 10 cms. and its power

                        -----   = 10 diopters?

Experiment No. 110.
Power of spectacles.

Measure in centimeters the focal length of your father's or mother's spectacles and calculate their power in diopters.

Experiment No. 111.
Conjugate foci.

Get the image of a candle as in Fig. 169, mark the position of the screen and the candle, and then exchange them. Do you again find an image, but of different size?

Repeat at different distances.

Two points so situated with respect to a converging lens that an object at either forms an image at the other are called conjugate foci. There are an infinite number of pairs of such points for each converging lens.



If Do is the distance of an object from a lens and Di is the distance of its image from the lens, then

1         1         1
  ---   +     ---      =     ---
Do            Di                  F

where F is the focal length of the lens. This is one relation between the object and its image.

The magnification of an image is the number of times it is larger or smaller than the object, and you can always find it by dividing Di by Do; that is, the magnification  =  Di   /   Do.

Experiment No. 112.
Where is the image?

Arrange the 4-inch lens with the candle 6 inches from it. Calculate where the image will be as follows:

Therefore Di is 12. The image will be 12 inches from the lens. Try it.

Now calculate and try where the image will be if the object is 5 inches, 7 inches, 8 inches, 12 inches, 20 inches from the lens, and so on.

Repeat with the 8-inch lens, using Do greater than 8 inches.

Experiment No. 113.
How big will the image be?

Arrange the candle 6 inches from the 4-inch lens and the image will be at 12 inches, as you found above.

Now, since magnification =  Di   /   Do,  it is 12 / 6 = 2, and the image will be 2 times as large as the object. Measure the height of the flame and of its image. Is the image 2 times as high as the flame? Try other distances and then the other lens.


Experiment No. 114.
Cylindrical lens.

Look at your finger through a tumbler of water. Does the tumbler of water act as a cylindrical lens and is your finger broad?


Experiment No. 116.
Treble your money.

Put a quarter in a tumbler half full of water, put a saucer over the tumbler, and invert both. Do you see a half dollar on the saucer and a quarter higher up? Why?

Experiment No. 118.
Heat through ice.

Place the concave mirror upside down on a sheet of clear ice 1/2 inch thick and let it melt into the ice. Do you get an ice lens? At noon, when the sun is hot, hold your hand at the focus of this lens. Is it hot?

Experiment No. 117.
A spectrum from ice.

Take a clear piece of ice, shave it to the shape of a prism, and hold it in sunlight. Do you get a beautiful spectrum?


A Magnifying Glass is simply a converging lens (Fig. 170) with the object PQ closer than the focus. The eye receives rays which are still diverging and sees the image pq enlarged. You have illustrated this above.

The Astronomical Telescope (Fig. 171) consists of two converging lenses, or systems of lenses, connected by a long tube. The lens nearest the object is called the objective, and the lens nearest the eye, the eyepiece.

The objective (Fig, 172) forms a real inverted image of the object BO inside the focus of the eyepiece. The eyepiece magnifies this, just as a magnifying glass does, and the eye sees the enlarged image IM.

When the telescope is focused on a distant object: the dis-



tance between the lenses is equal to the sum of their focal lengths; and the magnification is equal to the focal length of the objective divided by the focal length of the eyepiece.

Terrestrial telescopes have, between the objective and eyepiece, other lenses which turn the image right side up.

Experiment No. 118,
An astronomical telescope.

Arrange the converging lenses on a piece of board (Fig. 173) and focus on a distant object.

Measure the distance between the lenses. Is it equal to the sum of their focal lengths, that is, 8 + 4 = 12 inches?

Look at a distant object through the telescope with one eye and outside the telescope with the other eye. Is the magnification equal to focal length of objective / focal length of eyepiece, that is, 8 / 4 = 2 times?

Hold a piece of paper at the focus of the objective. Do you get an image?

Experiment No. 119.
To make a telescope.

Place 8-inch lens in ring hold-


er and wind dark wrapping paper around the holder to make a tube 10 inches long. Place 4-inch lens in the other ring holder and wind wrapping paper around the holder to make a tube 6 inches long. Slip the second tube into the first and your telescope is made (Fig, 174). Focus it on a distant object.

The Compound Microscope (Fig. 175) is the same in principle as the astronomical telescope, but the objective has very great power, that is, it has a very short focal length. The objective forms a real image, im, Fig. 176, of BQ, and the eyepiece forms the enlarged image IM of im.

The Opera Glass (Fig. 177) has a converging lens C for objective and a diverging lens c for eyepiece. The objective would form an inverted image ab of AB, but the eyepiece diverges the light and the eye sees the erect image A'B'. The ordinary opera glass consists of two such instruments;


they are shorter than the ordinary telescope and, therefore, more convenient.

Experiment No. 120.
An opera glass.

Arrange the lenses on a piece of board as in Fig, 178, Focus on an object. Is the image erect and are the lenses closer together than in the telescope?

Experiment No. 121.
To make an opera glass.

Place 8-inch lens in ring holder and wind around it a tube of wrapping paper 3 inches long. Place 


the diverging lens in the other ring holder and wind a tube 3 inches long. Insert the second tube in the first and your opera glass is made. Focus it on a distant object.

The Prism Binoculars (Fig. 179) are made with lenses similar to those in an astronomical telescope, but the light is reflected four times by means of glass prisms. This reflection makes the image erect and shortens the length of the tube.

The Projecting Lantern (Fig. 180) consists of a light-proof box, a source of bright light, a condensing lens, a lantern slide, and a projecting lens. The bright light,produced by electricity, acetylene, or as


here, by a limelight, is converged on the lantern slide by the condensing lens and an image of the inverted slide is thrown on the screen by the projecting lens.

The Postcard Lantern consists of a light-proof box, two electric lights which throw light on the postcard but not directly on the lens, a postcard slide, and a converging lens which throws an image of the postcard on the screen.

Experiment No. 122.
Magic-lantern shows.

Place 4-inch lens in ring holder in a hole in a large piece of cardboard, place a black book 6 inches from lens and a white screen 13 inches from lens on the other side, light the candles, and hold small objects against the book. Are their images thrown on the screen in natural colors and magnified twice?

Experiment No. 123.
To make a postcard lantern.

You can have lots of fun with a lantern made as follows:

Get a cardboard or wooden box (Fig. 181) about 8" X 6' X 6", put the 8-inch lens in ring holder and in a wrapping paper tube


4 inches long; put the tube into a hole in one side of the box and paint the opposite side of the box black. Place an electric light or oil lamp on each side of the postcard and close to it, and arrange two shades to prevent the direct light from falling on the lens. Hold a postcard, or other object, against the black end, focus the lens on a white screen about 2' X 2', and your lantern is finished. The illustration shows the lantern with the top and one side removed. The top should have a trapdoor at the rear end through which you can insert and remove the postcards. The audience is seated on the side of the screen away from the lantern.

Experiment No. 124.
Fun at night.

You can put on a magic-lantern show with oil lamps or electric lights as shown in Fig. 188. The doorway between two rooms is covered by two heavy curtains and the 8-inch lens in a ring holder is inserted in a hole in a piece of cardboard and pinned between the two curtains. A black book stands 10 inches from the lens, and is illuminated by two strong lamps; two screens prevent the direct light of the lamps from striking the lens. A white tissue paper or cloth screen, 2' X 3', is on the opposite side of the door 40 inches from the lens, the audience is beyond the screen, and if now you


hold postcards, drawings, and other small objects upside down against the book, the lens will throw erect and enlarged images on the screen, and your show is on.

The Photographic Camera is simply a light-proof box with a converging lens in one side and a plate holder in the other. The lens L (Fig. 183) throws an inverted image ba of the object AB on the plate S. 

Experiment No. 125.
To illustrate the camera.

Put your converging lenses in turn in a ring holder, and put the holder in a hole in one end of a cardboard box (Fig, 184). Cover the box and your head with a dark cloth and move the screen back and forth until you get a picture.

The Camera Obscura (Fig.185) has a combined lens and reflecting prism at the top which throws a picture down on the table in front of the artist.

Experiment No. 126.
To make a camera ob-



Arrange the 8-inch lens, mirror, and box as in Fig, 186. Cover the front of the box and your head with a black cloth. Do you get a beautiful picture on the white paper at the bottom of the box?

Experiment No. 127.
A moving-picture show.

Use the camera obscura on a table outdoors or near a window and let two of you get under the black cloth and look at the picture, while two others go through funny antics outdoors about 30 feet from the camera. Do those under the cloth see a very funny moving-picture show? Change places and repeat.

Experiment No. 128.
A submarine periscope.

Arrange the apparatus as in Fig, 187 with the mirror at 45 at the top of a long cardboard tube and observe
the paper under the black cloth. Do you get a fine picture on the paper?

This illustrates the construction of one type of submarine periscope.

The Stereoscope (Fig. 188) turns two pictures into one that stands out. The glasses are prismatic lenses placed edge to edge; they take light from the two pictures A1B1, A2B2, Fig. 189, and diverge it so that it appears to come from one pic-


ture AB. The pictures are taken in a stereoscopic camera, which is simply two cameras side by side and a short distance apart.

Your Eye (Fig. 190) has an outer horny membrane called the cornea and behind this a watery liquid called the aqueous humor, behind this a muscular lens called the crystalline lens and inside this another fluid called the vitreous humor. At the back is the nerve layer, the retina, which receives the sight impression, and behind the retina is a black coating which shuts out all light except that which comes through the lens. The colored part of the eye is the iris and the opening in the iris is the pupil. The iris contracts the size of the pupil in a strong light and enlarges it in a dim light.

The eye is very much like a camera, but there is one striking difference: the camera is focused by moving the lens back and forth; but the eye is focused by changing the shape of the lens and, therefore, its focal length. The muscles of the eye make the crystalline lens more convex when we view an object near at hand and less convex when we view one at a distance.


Spectacles. The eyes of short-sighted people focus the light in front of the retina F, Fig. 191 A, and this difficulty is overcome by spectacles with diverging lenses, L.

The eyes of long-sighted people focus behind the retina F, Fig. 191 B, and this difficulty is corrected by spectacles with converging lenses, L.

Experiment No. 129.
To look through your hand.

Your two eyes look along converging lines when you look at any object, and this leads to the following apparent magic. Roll a piece of paper into a tube, hold it beside your hand, look at your hand with one eye and through the tube with the other. Do you appear to see through your hand? Look through other things in this way.

Experiment No. 130.
To put the bird into the cage.

Draw a cage and a bird with centers about 2 inches apart on paper, stand a card on the line AB between them (Fig. 192), then look at


the cage with one eye and at the bird with the other. Does the bird enter the cage?

The Moving-Picture Machine (Fig. 193) throws 12 to 16 pictures on the screen each second and shuts off the light while one picture is changing to the next. The pictures are taken at the same intervals and differ very slightly one from the next (Fig. 194).

The "Why" of the Movies. The reason you see the pictures continuously and are not aware that the light has been shut off is that your eyes retain each picture for a short time after it has left the screen. You will now illustrate this.

Experiment No. 131.
Circles of fire.

Go into a dark room, light a match, blow it out but keep the live coal, and then wave


it in the air. Do you see circles of fire? You do, because your eye retains the impressions for some time.

Experiment No. 132.
To put the bird into the cage.

Draw a bird on one side of a piece of cardboard and a cage exactly opposite on the other side. Attach cords above and below and spin the cardboard. Does the bird appear to enter the cage? It does, because your eyes retain the pictures of the cage and bird for a short time.

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In the Dark!

A knock on the head with a hatchet or a stab with a knife doesn't sound pleasant, but you'll enjoy apparent treatment of this kind and so will your friends who watch your shadow show. Make your boy friend rise in the air - change him into a bird or a cat - create freakish images. It's easy! And laugh - your audience sure will enjoy it because it's new - nothing like it. An entertainment made for boys who want real fun. But that's only a few of the many things you can do with


One of these outfits will help you to understand a great many facts about light.  You can perform a number of experiments which explain the laws of light. Learn about the movie machine, the telescope and other optical instruments. There's a big book on Light with each set, it's a handy size, just right to put in your pocket

From this book and your set you'll get a knowledge of light that will be helpful to you always. It's great fun too, the kind you like. The outfit is complete with prisms, mirrors and all the apparatus you'll need to perform the experiments.

Ask your dealer to shew you this new Gilbert toy. If he hasn't it write

507 Blatchley Ave., New Haven, Conn.

In Canada - The A. C Gilbert-Menzies Co., Limited, Toronto, Ont.
In England - The A. C. Gilbert Co, 125 High Holborn, London, W. C. 1

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