Popular Science Monthly/Volume 11/October 1877/Simple Experiments in Optics
By ELIZA A. YOUMANS.
THE little work of Mayer and Barnard, designed to introduce beginners to the experimental study of optics, is so much needed, so skillfully done, and may be so helpful to teachers and students of all ages, that it is desirable to offer a few illustrations of the method of experiment adopted, and to point out some of the cheap and simple ways which Prof. Mayer has hit upon for exemplifying and proving optical phenomena. We shall make free use of his text as well as his cuts in the present article. Fig. 1, for example, represents the arrangement adopted to prove that light moves in straight lines. He first gets three little blocks, two or three inches square; then three slips of pine, three inches by four and one-eighth of an inch thick; and then three postal-cards, through which a small aperture is to be made. The authors say: "Just here we need a tool for making small holes and doing other work in these experiments; and we push, with a pair of pliers, a cambric needle into the end of a wooden pen-holder or other slender stick, putting the eye-end into the wood, and thus making a needle-pointed awl." This is an excellent little contrivance, and we suggest to the pupil to make a set of them with different-sized needles, which he will find very useful. Now, lay the postal-cards flat on a board, one over the other; measure off a half-inch from one end of the top postal-card, and with the awl punch a hole through them all just half-way from each side. Trim the holes with a pen-knife, and then take one of the cards and one of the wooden slips and put the card squarely on one of the wooden blocks, and, placing the slip over it, tack them both down to the block. Place one of the blocks near a lighted lamp, as shown in the figure, and another at the
Fig. 1.—Experiment proving that Light moves in Straight Lines.
opposite side of the table, where the observer can sit to look through the aperture. When the light is seen through both openings, draw the third card into line between the others, when the ray will be seen to pass through all three cards. Next, take a piece of thread and stretch it against the sides of the three cards as they stand, and it will be seen that they are exactly in line, and, as the holes in the cards are at the same distance from their edges, it is proved that the beam of light that passes through all the holes must also be straight. If the position of the blocks is changed, so that the directions of the holes in the cards are different, the same effect will be observed, so that it is demonstrated that light moves in exactly straight lines in all directions from the source of illumination. Of course, a pupil can learn from a book that light moves in straight lines, but this will be a matter of hearsay or authority, and not of direct knowledge, while if he makes this experiment he will have begun to prove things for himself, and the preparation for it, and trial in different ways, will be a good exercise in manipulation.
Now, if the student wishes to prove the variation in the quantity or intensity of light at varying distances, he can do it in the simple way shown in Fig. 2. A small slit is cut in the card near the lamp, through which the light passes. A sheet of white paper, resting against some books at the opposite side of the table, forms a screen, upon which the light falls. A bit of paper, an inch square, is held by the point of the awl, the handle of which is stuck in some wax on the table. Set the needle-awl, with the bit of paper, about twelve inches from the lamp, and then darken the room. Upon the screen, which is placed two feet from the lamp, will then be seen the shadow of the square bit of paper. With a lead-pencil trace an outline of this shadow on the screen, and then move it a foot farther back; and note how much the shadow is increased in size. With the pencil trace this shadow on the screen, and then laying the paper
Fig. 2.—Experiment with Shadows.
on the table and measuring the two shadows, you will see how they compare in size, and get a clew to the principle of inverse squares, as it is called.
Fig. 3 represents the means used in showing that the angle of the ray as it strikes the mirror is the same as that at which it is reflected. A and B are two of the postal-cards and their blocks used in the first experiment, turned with their inside faces toward each other, and separated by three more blocks of the same dimensions as those supporting
Fig. 3.—Angle of Incidence and Reflection.
the cards. The flame is placed even with the hole. On the middle block rests a piece of glass, coated on the bottom side with black varnish. The eye looks through the hole H upon the glass, where it sees a small spot of light that is the reflection of the ray from the lamp through the hole A. The point of the needle is placed directly over this spot, and held in position by the wooden handle with a piece of wax. A strip of paper, filling the distance from A to B, and four inches wide, is held upright between the cards, with the bottom resting on the mirror. The edge of this is marked with a pencil at the hole A, and again at the needle-point. A straight line joining these marks will form an angle at the bottom of the paper that is identical with the angle of incidence. By reversing the ends of the paper, and comparing this line with one from B to the needle, both will be found alike. The angles of incidence and reflection agree. In regard to the reflection of light. Prof. Mayer remarks:
"The clouds, the water, the grass, rocks, the ground, buildings, the walls inside, clothing and furniture, and everything we can see, reflect light in every direction again and again, and thus it is that all spaces, without and within, are filled with light so long as the sun shines. At night the sun sinks out of sight, and still it is light for some time after, for the sunlight is reflected from the sunset-clouds and the sky. Sometimes, upon a summer's day, when broken clouds partly hide the sun, you will see long bars of dusky light streaming from openings in the clouds. These long bars are beams of sunlight shining upon dust and fine mist floating in the air, and we see them because each speck and particle reflects light in every direction."
Fig. 4.—Light reflected by Floating Particles.
Fig. 4 shows the effect of particles in scattering the light. A clean glass jar stands upon a black cloth laid on a table in a dark room, and over its mouth rests a postal-card having a slit in it one inch long and one twenty-fifth of an inch wide. A beam of light enters the room from one side, and is thrown downward upon the postal-card by a hand-mirror. Now set fire to a small bit of paper, and drop it into the jar. When it is burned out, put the postal-card in place, and the vessel will be filled with smoke. The beam that is reflected downward from the mirror enters the slit, and you see a slender ribbon of light extending downward through the jar, while all around it is quite dark and black. Fig. 4 shows the light streaming through the opening in the card, and lighting up the particles of smoke in its path. Take off the card, and let the reflected beam fall freely into the jar: the smoke is now wholly illuminated, and the vessel appears to be full of light
To make a milk-and-water lamp:
"Take away the jar and put a clean glass tumbler in its place. Fill this with water and throw the beam of reflected light down upon it, and the water will be lighted up so that we can easily see the tumbler in the dark. Now add a teaspoonful of milk to the water and stir them together. Throw the beam of light down once more. This is indeed remarkable. The tumbler of milk-and-water shines like a lamp, and lights up the room so that we can easily see to read by its strange white light. Move the mirror and turn aside the beam of light, and instantly the room becomes dark. Turn the light back again, and once more the glass is full of light. Here the minute particles of milk floating in the water catch and reflect the light in every direction, so that the entire goblet seems filled with it, and the room is lighted up by the strange reflections that shine through the glass."
Fig. 5.—Experiment showing the Law of Refraction.
By the following simple contrivance, illustrated in Fig. 5, Dr. Mayer shows the pupil how he can demonstrate the law of the refraction of light: 1. We have a clear glass bottle, with a sheet of white paper, in which a perfectly round hole has been cut, pasted on its side; 2. Horizontal and perpendicular lines are drawn with ink upon the glass at right angles across each other, and within this circle, dividing it into four equal parts; 3. Water is poured into the bottle until its level is that of the horizontal line; 4. A postal-card containing a slit is placed as at D in the figure; 5. The mirror, B, reflects the beam into the bottle so that it may touch the water where the two lines cross. The light is seen to bend as soon as it enters the water. The index of refraction for all liquids may be determined by measuring the distance from where the beam enters to the perpendicular, then the distance from the perpendicular to where it vanishes, and dividing these into each other. The constancy of the quotient for each particular liquid can also be shown by having the beam strike the water at various angles, making these measurements and dividing. No matter how the distances vary from the perpendicular, when divided into each other they give the same result for the same liquid.
Fig. 6.—Total Reflection.
With a mirror on the table and our bottle arranged as in Fig. 6, total reflection will occur, that is, all the light of the beam will be thrown downward from the surface of the water.
"Sometimes, when walking along a road on a warm day, you may observe a curious quivering in the air just where the road seems to meet the sky, as it goes over a hill. The objects near this point appear to be distorted and to tremble, or they assume fantastic shapes. Here we have an instance of refraction caused by the heated air just above the surface of the road. The light passing through these layers of unequally-heated air is refracted unequally, and the objects that reflect the light appear distorted. In some instances the refraction may pass the critical angle, and we may see the objects apparently doubled by reflection. Warm, calm days by the sea show the same thing, when distant vessels appear repeated in the sky, or when distant land that is really below the horizon 'looms' up and glimmers upon the horizon in trembling headlands. This illusion is called the mirage, and takes place when refraction exceeds the critical angle and becomes reflection. "Fill a clear glass tumbler with water, and put a spoon in it, or dip one finger in the water, and hold it above your head, so that you can look into the water from below. You will find that you cannot see through the water up into the air above. The under-surface of the water will appear to shine like burnished silver, and the spoon or your finger will be reflected in it, as in a beautiful mirror. This illustrates total reflection, and shows that in this case all light thrown upward through the water is reflected from its surface. Look into the tumbler from above, and it appears full of clear water. Look into it from below, and it seems as if an opaque sheet of silver rested on the water, and shut out the view of everything above."
Fig. 7 shows a neat and simple arrangement by which water can be used for a lens to illustrate refraction. It is merely a long pine box fourteen inches high, made of thin boards, with one side open, and
One of the most beautiful experiments in total reflection is that illustrated by Fig. 8. A Florence flask filled with water acts as a lens. The room is darkened, and the light coming from without is brought to a focus on the inside of the flask. A hole has been made through the glass, and as the water streams out the light is totally
Fig. 8.—The Fountain of Fire.
reflected so as to illuminate the stream as it falls into the pail below. Of this experiment the authors say:
"How magical! The curving stream of water is full of light, and appears like a stream of molten iron. The spot where it falls seems touched with fire. Put your finger in the stream of water, and it is brightly illuminated. Of course, the water soon runs down, and the display stops. To prevent this, bring water in a rubber tube from the water-pipes in the house, and then regulate the supply so that the receiver may be kepi full as fast as the water runs out.
"Place a piece of red glass behind the flask in the beam of sunlight, and the stream of water will look like blood. Touch it, and the hand will be crimson, and the scattered drops that fall in a shower into the tub will shine like drops of red fire. Place a green or blue glass behind the flask, and the stream of water will turn green or blue, and present a most singular appearance. Hold a goblet in the stream, and it will overflow with liquid light. Flashes and sparkles of fire will appear in it, and foam over the sides, shining with brilliant light.
"This beautiful experiment is as interesting as it is strange and magical, and it illustrates both refraction and total reflection. The flask makes a lens, and the falling stream of water is lighted up by the cone of light that enters it at the hole in the flask. Both the water and the light pass out of the hole together, the light inside of the water. That this is so, may be proved by permitting the water to escape, when the light will be seen shining out of the hole horizontally into the room. Why, then, does it not shine out into the room while the water is escaping? When the stream of water is flowing out, it falls in a curve into the tub on the floor. The beam of light, passing out with the water, meets its curved surface at such an angle that it is totally reflected. This beam of reflection again meets the surface of the water, and is again totally reflected. In this manner it is reflected from side to side, again and again, till it reaches the tub, and there we see it shining brightly. It is a prisoner in the water, and follows it down into the tub. When you put your hand in the falling water, you see that it is lighted brightly, and yet the stream by comparison is rather dark. If it were pure distilled water it would hardly be visible. As it is full of floating specks and motes, each of these reflects light, and these cause the water to appear full of light."This fountain of fire is a charming experiment for a school, and its double lesson makes it as interesting as it is beautiful."
Prof. Mayer uses Lis flask-and-water lens, as illustrated in Fig, 9, to get a solar microscope, and so well does it succeed that it is doubtful if it can ever be excelled for combined cheapness and efficiency. With some blocks of wood, a twenty-five cent microscopic glass lens,
Fig. 9.—The Solar Microscope.
and a slide carrying a microscopic object, he gets very striking effects. Animalcules in water, and all sorts of transparent microscopic objects, can be projected upon a curtain by its aid, so that a large number of people can be entertained by observing the effects produced.
Fig. 10 shows how a common iron top, such as may be found in any toy-store, may be transmuted into a color-top. The shape of the handle is of no importance. By fastening disks of various colors, made of drawing-paper, around it, all sorts of chromatic changes may be studied. With red, green, and violet, white will appear by spinning the top. With one-quarter green and three-quarters red a deep orange may be produced. Directions are given for using the top on the lantern, and casting the colors on the screen, so that a great many persons may see it at once.
Fig. 10.—The Color-Top.
Another method of producing these recompositions is illustrated in Fig. 11. We have here a square piece of board for a base, an upright block at the corner, two pieces of glass to which the dotted lines
Fig. 11.—Direct Recomposition of the Colors
run, and in which reflections are to be seen at the spots where the lines meet, and three pieces of paper at A, B, and C, painted respectively red, green, and violet. The image of A is supposed to go through both pieces of glass to the eye, while the reflected images of B and C are, by adjustment, to be made to overlap each other and the image of A. When this is accomplished a single white object is seen. This experiment is conducted most successfully near a window, and with the back toward the light. When the red and green images are super-imposed a yellow one is seen, and when the green and violet are super-imposed we have blue. The colors originally supposed to be primary were red, yellow, and blue. Prof. Mayer has here given a simple means of refuting this old theory.
Prof. Mayer describes the construction of a cheap and simple heliostat for directing the sunlight into the room, and keeping the beam in the same position for all these experiments. We refer the reader to the book for the details of its construction, and the full-page woodcuts by which it is illustrated. A great point has been gained for scientific education by thus putting it in the power of any student, with ordinary ingenuity, a few tools, and a few shillings, to make such a large number of interesting and instructive experiments.