The things that are the most common are often the ones that people know the least about. This is true of light. Few boys have even taken the trouble to get the essential facts about this subject. You can realize how important it is when you are told that without the sun - our main source of light - there would be no life at all. There would be no growth of plants, that's sure. I know as a boy my curiosity was always prompting me to ask questions. I wanted to know the facts and reasons for everything. I believe most boys are the same in that respect.

This book has been written so that you can get information first hand on a mighty interesting subject. It is in plain language, which you can readily understand, and a study of it will soon make you familiar with great scientists, who have made laws of great importance. Their discoveries make possible the use of a number of instruments that you know so well. The telescope - the opera glass - the moving-picture machine - are just a few which can be mentioned, and you know how necessary they are in the world to-day.

In learning facts about light and some of the inventions made by great men you will get a knowledge that very few boys have. You will be able to talk very interestingly about light and it will be easy for you to explain a great many questions that come up in connection with it Best of all, you will have a whole pile of fun and at the same time get a good understanding of the fundamentals of the science of light.

Sincerely yours.
 

Go outside with your watch in your hand and stand in the sunlight (Fig, 1) for just six seconds. In that short six seconds you will have received more than 1,000,000 miles of sunlight. Light travels at the enormous velocity of 186,000 miles per second, and therefore in six seconds you receive on your body 186,000 X 6 = 1,116,000 miles of sunlight.

Hold your hand in a beam of sunlight (Fig. 2) for six seconds and then withdraw it.

You will show soon that white light is a mixture of lights of all colors. Now the red light waves in the sunlight fall on your hand at the rate of 390 million million per second and in six seconds 390 X 6 = 2340 million million fell on your hand. Your hand received many more than this because waves of all colors fell on it, and of violet light alone it
received twice the above number.

Repeat this experiment with a candle flame, or oil lamp, or electric light. In each case your hand receives more than two thousand million million light waves in the six seconds.

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6  GILBERT BOY ENGINEERING

Boys, you are going to make many experiments with a beam of sunlight let into a darkened room, so prepare for them now thoroughly, as follows:

Select in your home a room with only one window, facing the south or east or west. Now cover this window so that no light can enter the room except through a small slit The best way to do this is to make a solid wooden shutter, with tar paper on one side, to cover the whole window and make a slit in it three Inchcs wide and two inches high.  If the window is too large for thin, make a wooden  shutter (Fig. 3) for the lower part and cover the upper part with black cloth, black paper, a quilt, a heavy blanket (Fig. 4), or anything that will shut out light.  Make the slit in the wooden part because you will want to change the

shape of the slit. To do this you will make slits of the right kind in black tar paper or cardboard and then tack these over the slit in the wooden shutter. Make the shutter so that you can take it down and put it up quickly, because you will want to experiment many times and your mother will, probably, not want to leave the window permanently darkened.

If you cannot make a dark room, you can make a dark box (Fig. 5), as follows: Take a packing box 1 1/2' X l 1/2'' X 2' or larger, bore a hole 2 1/2 inches in diameter in the center of one end, cover the open top with a piece of dark light-proof cloth, 4' X 3', tacked to the ends and one side. Plait this along the side to leave room for your head and shoulders. Make the box light-proof, turn it on its side with sunlight entering the slit, and you are ready to make your experiments.

It improves the box to paint it black on the inside. When you need a slit, cut it in cardboard and tack the cardboard over the 2 1/2-inch hole. This hole will just take your large ring lens holder when you experiment with lenses.

A dark room is more fun than a dark box, and the directions in this book assume that you have made one. You will find, however, that you can make most of the dark-room experiments in the dark box. It is an excellent idea to have both.

Darken the room on a day when the sun is shining in the window. Leave the room for five minutes to let the dust settle and then return. Do you find that you cannot see the beam between

the slit and a screen or the wall or floor?

Make a dust near the beam by shaking your coat or a carpet. Do you now appear to see the beam (Fig, 6)? You do, because the dust particles reflect light to your eyes.

On a clear night you cannot see the beam from the headlight of a locomotive ; but when there is mist, rain, or snow, you appear to see it because particles of these reflect light to your eyes. In either case you can see any object the beam falls on because it re-

fleets light to your eyes. You can also see the light if it falls directly on your eyes.

Make a dust near the beam. Does the light travel in a straight line from the slit to the wall or floor or paper screen?

Punch a nail hole in a piece of black paper or cardboard, tack the paper ot cardboard over the slit in your darkened room, and hold a sheet of tissue paper about one foot from the hole. Do you find on the paper a picture (Figs. 7 and 8) of the whole view out-of-doors opposite the hole? Is the picture inverted and in natural colors? Can you see men, horses, and automobiles moving?

This is a fascinating experiment, and it shows best when the sun is shining on the landscape and not on the window.

The picture is inverted because light travels in straight lines. The sunlight which falls on any part of a cloud, for example, is reflected in all directions in straight lines, and a very small part of this light passes through the hole to the bottom of the tissue paper. Also the sunlight which falls on any object on the ground is reflected in all directions in straight lines, and a very small part of this passes through the hole to the top of the tissue paper, and so on. That is, the picture is inverted because light travels in a straight line from each object through the hole to the paper.

The picture is in natural colors because each object reflects light of its own color and absorbs the remainder. That is, the blue sky, green grass, and red bricks reflect blue, green, and red light respectively, and so on.

Move the paper farther from the hole. Is the picture larger  but dimmer? It is larger because the rays from different parts of the view cross at the hole and diverge afterward. It is dimmer because only a certain amount of light passes through the hole, and it covers a larger area the farther the paper is from the hole. Punch a second nail hole two inches from the first Do you now get two pictures? Do they blur where theyoverlap? Punch many holes. Do you get as many pictures, but do they blur more and more? Open the slit to its full size. Do you find that there is no picture at all, but just white light? There is no picture because light from all parts of the view falls on all parts of the picture and the combination of all colors produces white light

Make a hole the size of a lead pencil in a new piece of black paper and tack the paper over the slit. Do you get a brighter picture, but is it more indistinct than with the nail hole? Remove the tissue paper. Is there a picture on the opposite wall ? This will show only if the sun is shining brightly on the view, and if your room is completely dark except for the light which passes through the hole.

Allow a beam of sunlight to pass through a nail hole into your darkened room, catch it on a piece of paper and move the paper back and forth. Is the image round (Fig. 9) and is it larger the farther the paper is from the hole?

The image is round because the sun is round. It increases in size because the light rays from the sun travel in straight lines and

cross at the hole. The upper side of the sun sends out light in all directions in straight lines; a very small part of this passes through the hole (H, Fig. 11) and makes an image of itself at the bottom of the picture; similarly light from the lower side of the sun makes an an image of itself at the top of the picture, and so on. Reflect the light to the farthest part of the room by means of a mirror. Is the image larger the farther it is from the hole? Punch two holes. Do you get two images (Fig. 10)? Punch many holes. Do you get many images, but do they overlap?

Open the slit entirely. Do you get only a bright spot in the shape of the slit? This spot is made up of many, many round images, and you will notice that the edges and comers are somewhat blurred and not sharp.

Take a new piece of black paper, make a triangular hole one-quarter inch on a side, tack it over the slit and get an image of the sun at one inch from the hole, then, at greater and greater distances. Is the image at first triangular and does it become more and more blurred at the sides and corners until finally it is round? The image is made up of many small, round images of the sun, and when these are large

compared to the size of the hole they overlap and produce a round image.

Repeat with a square hole one-quarter inch on each side. Are the results similar?

You have probably noticed that sunlight produces round images of the sun when it passes through any small opening; for example, in a shutter or blind, between the leaves of trees, and so on. The explanation is that given above.

Make a nail hole in the middle of the bottom of a cardboard box, cover the open top with a piece of tissue paper (Fig. 12), hold the hole toward a brightly lighted landscape, cover your head and the tissue paper with a black cloth or blanket to shut out all the light (Fig. 13), and look at the tissue paper. Do you see a beautiful image of the landscape inverted and in natural colors?

This is a beautiful experiment and it is explained as above.

You can actually make pictures through a pinhole, as follows: Remove the lens from a camera, cover the opening with heavy tin foil and pierce the foil with a pin. Now to take the pic-

ture, cover the pinhole, arrange the plate or film in position, uncover the pinhole for a short time, cover it, and develop your negative as usual.

Now, boys, before we go any further let us get some clear ideas about light.

Light is that which produces on the eyes the sensation of sight.

Medium. A medium is anything through which light travels; for example, air, water, glass, and the ether.

Ether. The ether is supposed to be a very thin and elastic medium which fills all space, not only the space between the planets, but also the space between the smallest particles (molecules) of solids, liquids, and gases.

How Light is Produced. Light is produced by the vibration of very hot particles of matter.

For many reasons, scientists believe that the smallest particles of all substances are vibrating, that is, moving back and forth in all directions, all the time, and that the hotter they are the faster they vibrate. Now in the flame of a candle, oil lamp, or gas jet there are particles of unbumed carbon which are very hot and are, therefore, vibrating rapidly. These vibrating particles set the ether in the flame in vibration, and these vibrations spread out

in all directions in the form of spherical waves in the ether. These ether waves are light waves or heat waves.

Similarly the light of an incandescent electric arc light or of the sun is produced by rapidly vibrating hot particles of matter.

Note. Heat waves are longer than light waves and do not produce the sensation of sight, but they are similar to light waves in all other respects.

Waves and Rays. If the dot in the center of Fig, 14 is a rapidly vibrating particle, the circles about it will give the position of its light waves after equal intcrvals of time, but the light waves are spherical instead of circular. The straight arrows drawn from the center represent light rays. They give the path along which the light is traveling in all directions from the center. The light waves are real and produce the scnaation of sight; the rays are not real, they are imaginary, straight lines which give the direction of the light and they are always at right alleles to the waves.

Parallel Waves and Rays. The waves (from the dot are larger the farther they are from the center, and when they are one hundred yards or a mile from the center they are very large indeed. If your eye receives light from any such distant point the small part of the waves which enter it are nearly parallel straight lines,

and since the rays are always at right angles to the waves they are also nearly parallel. This is particularly true if the distant point is the sun, at a distance of ninety million miles. Parallel waves and rays then are those from a distant source.

Beam. Pencil. A beam (Fig. 15) is a group of parallel waves and rays. A pencil (Fig. 16) is a group of waves and rays which converge at a pomt or diverge from it. The eyes (Fig. 17) are receiving diverging pencils of light from the candle which is sending out light in all directions.

Luminous and Non-luminous Bodies. Luminous bodies are those which give out light, such as the sun, electric light, gas jet, oil lamp, candle, and match. Non-luminous bodies are those which do not give out light, and which can be seen only by means of light from luminous bodies.

Transparent, Translucent, and Opaque Bodies. Bodies which you can see through are called transparent; such as air, water, and glass. Bodies which let light through, but which you cannot see through, are called translucent; as paper, ground glass, and cotton cloth. Bodies which do not transmit light are called opaque; for example, wood, brick, and metal.

No substance is entirely opaque; for example, even metals let through some light when they are in very thin sheets.

Straight Lines. Light always travels in straight lines from its source until it falls on some object or until it

passes into another medium. It then changes direction but again travels in straight lines from the object or in the new medium.

Hold a saucer in a candle flame (Fig. 18.)  Is it blackened?  The blackening is caused by the unbumed carbon particles.

Look along the surface of water in a pan and dip a pencil in and out. Do you observe circular waves (Fig. 19)? Throw a stone into a still pond or lake. Do you see circular waves? These illustrate light waves, but the light waves are spherical.

Boys, when you have attended movie shows where they have animated cartoons you have perhaps seen a dotted line move from the eye of the hero (or villain) to the object he is looking at. You might think from this that you see an object by means of light which goes from your eye to it. This is not the case, however, as you will now prove.

Take an unlighted candle into an absolutely dark room and 

look around. Can you see anything? You cannot because all the objects in the room are non-luminous, including yourself and your eyes. This proves that you do not see things by means of light which goes from your eye to the thing you are looking at. Now light the candle. The flame is a luminous body and you see it by means of light which goes from it to your eye.

Can you now see the non-luminous bodies? You can because light travels from the candle flame to these objects and from them to your eye (Fig, 20).

You have proved here that you see any object by means of light which travels from it to your eye and not the reverse.

Note. The thing you are "looking at directly or indirectly is called the object. Close one eye, look at the flame of a candle, and then move a book slowly between the flame and your eye (Fig. 21). Do you find that you cannot see the flame when the book has crossed the straight line between the flame and your eye? This proves that the

light from the flame travels in a straight line to your eye.

Close one eye and look at any part of some other object, then again move the book across the straight line from the part to your eye. Do you find again that you cannot again see the part when the book has crossed the straight line between the part and your eye?

Cut three pieces of cardboard about 5" X 3", punch a small hole in each at the same height, stand them upright on the table, place a candle flame in front of an end hole, and look at the flame through the three holes (Fig. 22). Shift the cardboards one at a time. Do you find that you can see the flame only when the holes are in a straight line?

Note. You will show later that light is bent out of the straight line when it is reflected from a mirror and when it is refracted in air to water, air to glass, and so on.

You have shown here that light which is not reflected or refracted travels in a straight line from the object to your eye,

Punch a nail hole in card C and arrange as shown in Fig. 23.   Do you sec an inverted image of the flame and is it larger the farther 

D is from C? Make the hole larger. Is the image brighter but more blurred? Can you explain these facts as in Experiments 5 and 6 and as illustrated in Figs. 11 and 23?

You can telegraph to your friends at night by means of flash-light signals (Fig. 24) and the Morse code. Use a short flash for a dot and a long flash for a dash. For the Morse code see page 19.

Arrange the apparatus as shown in Fig. 25 and have a friend make a similar arrangement in a window facing you. You can then telegraph by means of the Morse code. Uncover the light for a short time to produce a dot and for a longer time for a dash.

If you hold a book 1 foot from a lighted candle, it receives a certain amount of light; if you hold it 8 feet from the candle, it receives only one-fourth as much light ; if you hold it 3 feet from the candle, it receives only one-ninth as much light, and so on. That is, the intensity of the light on any object varies inversely as the square of the distance between the object and the source of light.

In Fig. 26 the light

which would cover 1 square at 1 foot would cover 4 and 9 equal squares at 2 and 3 feet and would therefore be one-fourth and one-ninth as intense.

Experiment No. 15.
To prove the law of intensity.

Screen A, Fig. 27, has a hole just 1 inch square and screen B a square 3 inches on each side, divided into 9 square inches. Place A 1 foot from the candle and B 2 feet. Does the light which passes through 1 square inch in A cover 4 square inches on B? Is it, therefore, only one-fourth as Intense on B as it is on A? Place B 3 feet from the candle. Does the light now cover 9 square inches? Is it, therefore, only one-ninth as intense? This proves that the intensity of light varies inversely as the square of the distance.

Experiment No. 16.
A greased spot.

Rub a small piece of butter on the center of a piece of paper and melt it.

Look at the spot by reflected Iight (Fig. 28). Is the spot dark? Look at it by transmitted light

(Fig.29). Is it bright? It is darker than the paper in the first case and brighter in the second, because more light goes through the greased spot than through the paper.

Put four candles 2 feet from the greased-spot screen and one candle 1 foot on the other side (Fig. 30). Trim the wicks until the flames are of equal size.

Is the greased spot as bright as the paper? It is, because the light which goes through from one side is exactly equal to that which goes through from the other, that is, the one candle throws as much light on the screen as do the four candles. This is explained by the law of intensity mentioned above. Each of the four candles is 2 feet from the screen and therefore throws just one-fourth as much light on the screen as one candle does at 1 foot.

Experiment No. 18.
Candle power of a lamp.

The candle power of a lamp is the

number of times greater or less its light is than that given by a standard candle.

Put the greased spot screen just 1 foot from a candle (Fig. 31) and move the lamp until the greased spot is as bright as the paper. Measure the distance from the lamp to the screen. If this is 2 feet, the lamp is 4-candle power; if 3 feet, 9-candle power ; 4 feet, 16-candle power; and so on. This follows from the law of intensity of light.

The instruments used to measure the candle power of lamps are called photometers, and the one you have here illustrated is named after its inventor, Bunsen's photometer (Fig. 32), A is the greased-spot screen, B the standard candle, and C the light tested.

Experiment No. 19.
Four against one.

Place a candle 1 foot from a screen and four equal candles just 2 feet from the screen, and place an object in front of the screen as in Fig. 33. Are the shadows of equal darkness?

If the candle flames are all of the same size, the shadows are equally dark because each light illuminates the shadow produced by the other and because the candle at 1 foot sends as much light to the screen as the four candles do at 2 feet from the screen. This is explained by the law of intensity.

Experiment No. 20.
Shadow photometer.

Put a

candle 1 foot from a screen (see Fig. 34, illustrating shadow photometer), stand a pencil in front of the screen, and move the lamp back and forth until the two shadows are equally intense. Measure the distance from the screen to the lamp. If this is 2, 3, 4, or 4 1/2 feet, the candle power of the lamp is 4, 9, 16, or 20 1/4, and so on.

Each light illuminates the shadow cast by the other and therefore when the shadows are equal the lights are equal, and the candle power is calculated as above from the law of intensity of light. This illustrates the shadow photometer.

Move a pencil from the screen toward the candle (Fig. 35). Does the shadow increase in size? It does, because light goes in all directions in straight lines from the flame, and the pencil intercepts more light the nearer it is to the flame.

Boys, yon can have the greatest kind of fun by giving shadow shows to your friends, and the preparation you need is very slight. Hang a sheet over a folding doorway as shown in Fig. 36.

Now opposite the door put a strong lamp on a stool, chair, or table, according to the show (Fig. 37). The audience is in darkness on the other side of the screen.

Dentist seated, bell rings, boy comes in with bandaged head, dentist seats him and examines tooth, boy howls, dentist takes very, very large pliers, and pulls out a very large cardboard tooth (Fig. 38). The tooth, of course, was stuck in boy's coat collar