CHAPTER VIII.
LIGHT.

Light is emitted from a luminous body. A luminous body is one in which all of the particles are conceived to be in violent motion, which motion is transmitted to a supposed ether. The existence of such ether cannot be demonstrated, but it is supposed to exist because it is impossible to think of anything being transmitted except through some medium. This ether is a rather imponderable substance; it is supposed to pervade all space; and it exists in all matter and in every vacuum. It is supposed to be elastic without weight and capable of transmitting motion without loss of energy or friction. It is, however, assumed to be modified to some extent by the matter in connection with which it exists. Thus the rate of transmission of light waves is different for air, glass, water, and other substances through which they may pass and such bodies as are entirely opaque are supposed to suppress the ether vibrations, resulting in light entirely.

Light is then a mode of motion of this universal ether which pervades all space even to the farthest star. The motion of this ether is conceived to be about as illustrated in Figure 53. If we take a heavy string or a small rope and, stretching it reasonably taut, jerk it forward and back quickly a few times, it will be seen to move and assume the appearance shown in the figure; and that portion shown between the two vertical lines represents one complete wave.

The assumption of this ether and the vibratory motions of it form the only explanation that is capable of accounting for all of the phenomena of light. All other theories advanced have failed to stand the test; sooner or later some phenomena have appeared which could not be explained by them.

Light waves are known to travel through space in straight lines until they meet with some medium which is capable either of reflecting, refracting, or absorbing them. The rectilinear propagation of light, consisting of vibratory motion, is one of the most difficult parts of the theory to explain. It involves rather deep study and more mathematics than the scope of this work will warrant. Suffice it to say that the rectilinear propagation is brought about through the interference of light waves. An analogy of this can be found in a stream of water. It is well known that a stream of water issuing from the nozzle of a garden hose moves in a straight line until gradually forced into a spray by the resistance of the air. Yet the water passing through the hose is interfered with on all sides and on all sides there is a tendency to deflect it. If the water were to move along slowly so that we could observe the action on one side independently of the balance of the stream, we should see a series of waves being formed by every particle of the hose which offers resistance to the flow. The waves formed in the interior of the hose on all sides are interfered with by waves from all other directions and the result is motion in a straight line. In a similar way it may be conceived that the millions of light waves emitted from a luminous body interfere with each other and thus cause light rays to move in a straight line.

Light is a form of energy and can be converted into other forms of energy. It can be converted into heat, for instance, or can be used to produce chemical effects. Light differs from heat only in the rate of vibration and the length of the ether waves. Heat can be reflected in the same manner as light, as the following experiment will show: Arrange two ordinary reflectors as shown in Figure 54. In the focal point of one, place a heated iron ball or something of the kind; if this be hot enough, it will ignite paper suspended in the focal point of the other reflector, although a thermometer placed anywhere between the two reflectors will give only a small indication of a rise in temperature.

All light rays as well as heat rays are in themselves invisible; we can see only the object which emits or reflects them. If a beam of light be allowed to enter a darkened room, as shown in Figure 55, we shall probably be able to see the whole path of the rays illuminated, as well as the spot on the floor. But this will be because of particles of dust in the air which reflect the rays to our eyes. If we introduce some smoke into the room, we shall see the light much more plainly because there are now more particles of matter to reflect it. On the other hand, if special precautions are taken to have the air absolutely clear of dust, we shall be able to see nothing but the spot on the floor.

Light travels through space at the rate of about 186,000 miles per second. White light is a combination of light of many colors, but the speed of transmission is the same for all colors. The length of waves and the rate of vibrations, however, vary. The red rays have the longest waves and the slowest rate of vibration; they vibrate about three hundred ninety-five billion times per second and the wave length is about 0.0008 millimeters. The violet rays possess a wave length of about seven hundred sixty-three billion per second. There are light waves which are longer than the red rays and these are known as infra red. They are not visible to the eye but their existence can be proved in many ways. Light waves shorter than the violet are also invisible and are known as ultra violet. These have much importance in photography and to this class belong the X ray.

We have mentioned above that white light is a combination of light rays of many colors. This can be proved by the following experiments: If we arrange to have a beam of sunlight pass through a small hole into a darkened room, it will pass to the wall on the opposite side in a straight line and give us white illumination upon a small spot. If we now arrange a prism in the path of this ray or beam of light, we shall find that the light no longer passes straight to the wall but that instead, it is bent in a certain direction and furthermore shows us a brilliant array of colors. This is illustrated in Figure 56. The rays are thus shown to be separated into their constituent colors; red is shown at the top and the following colors merge imperceptibly into one another—orange, yellow, green, blue, indigo, and finally violet at the bottom.

The reason for this change is that the rays of light on entering the glass are slowed down—those of the higher rate of vibration more than the others. The violet rays are thus said to be more refrangible than the red, for instance. The colors thus produced are simple colors. This is proved by the fact that if the light is passed on through another prism, it will be again reflected but will not be resolved into other colors; although whichever color is carried to the next prism will spread out and show finer gradations in its color.

The colors given above are those obtainable from the decomposition of sunlight and make up what is known as the solar spectrum. If instead of sunlight some other illuminant be used, the arrangement of colors will be different; and it has been found possible to tell from the colors of the spectrum what substances are burning, or heated to a luminous degree, in the source from which the light comes. This method is known as spectrum analysis.

There are several ways in which light, which has thus been separated into its fundamental colors, can be re-composed so as to give us white light again. One of these methods consists in arranging an inverted prism to receive the light, as shown in Figure 57. The rays leave the second prism parallel and produce the effect of white light. Another method consists in gathering the rays from the prism by a lens, as shown in Figure 58. Furthermore, if we take a disc and paint the colors of the solar spectrum upon it in the proper proportions, as indicated in Figure 59, and cause this disc to be rapidly revolved, we shall see it as almost white.

Another fact which goes to prove the undulatory, or motion, theory of light is that two sources of light arranged to oppose each other can actually be made to produce darkness. To do this, the waves of one source of light must be made so that they exactly oppose those of the other; thus they destroy each other and destroy what light there is in either. There are other methods, but this can be partially accomplished in the following manner: Two small mirrors of black glass or of metal are placed, as shown in Figure 60, very close together and so that they form an angle of nearly 180 degrees. A beam of light arranged to fall upon both of the mirrors will be reflected in such a manner that the two halves interfere with each other and cause bands of light and darkness to appear. The dark lines are due to the opposition and nullification of certain of the light waves.

The intensity of light diminishes directly as the square of the distance through which it is transmitted. This is illustrated in Figure 61. The light, starting from a point, is limited by the size of the first square at the left; it spreads out more and more, and illuminates larger and larger spaces. Exact measurement will show that the spaces illuminated by a ray of light are always exactly proportional to the square of the distance from the point of light. This law, however, applies strictly only if the distances considered are long compared to the source of light, so that the light may be considered as being a mathematical point, that is, having no physical dimensions. If the source of light, for instance, were of the same size as the first opening and of uniform intensity there would result the same intensity of illumination of a similar space at all distances. There would, however, be an outer fringe of light which would be proportional to the law of inverse squares. Many reflectors are arranged to throw very nearly parallel rays; and with these the intensity remains the same except for absorption, which is ordinarily not very great.

We see things only through the rays of light they reflect. All colored bodies have this peculiarity, viz., that they are capable of reflecting only such rays as make up the color the body is said to possess. A red body, for instance, absorbs all colors except red and reflects red only. A black body absorbs all rays and a perfectly dull black body is visible only by contrast; that is, we do not see it but we are aware that there is something invisible before our eyes. When we are in a perfectly dark room, we see nothing but we have blackness before our eyes. A perfectly white body is one which reflects all of the rays of light and absorbs none.

When we view things through colored glasses, we see them only in the colors which the glass will transmit. If we view a red body through a green glass or under a green light, it will appear black because it is capable of reflecting only red rays and in the green light there are no red rays; hence there is nothing to be reflected and the red appears black.

CHAPTER IX.
PRINCIPLES OF VISION.

Through the medium of our eyes we see objects by means of the light which is reflected from them. This light enters the eye and forms an inverted image of the object upon the retina, just as an inverted image is formed upon the ground glass of a camera. This impression made upon the eye is corrected automatically, so that, although we see everything upside down, we are not at all aware of so doing. The proof of this peculiarity of the eye is found in cases where persons born blind have later through operations acquired sight. In the eyes of slaughtered cattle also the image can be seen inverted. A further proof that we are able thus to adjust ourselves is found in the experience of persons using cameras with ground glass screens. The image on these screens is always inverted both horizontally and vertically. The user soon learns to see his object, although inverted, in the natural way, that is, vertically, because this is so plain that he must take it into account at every focusing. He does not, however, accommodate himself to the reversal from right to left because this is of no consequence ordinarily and is not noticed. Many photographers, who have been accustomed to the vertical inversion, still find themselves confused when trying to locate the right and left of a view seen through the lens.

Unless a special arrangement of lenses is provided, all images cast upon screens through small openings appear inverted. The reason for this can be seen from Figure 62. It is obvious that no other light can reach the bottom of the screen through the pin hole O, at the left of the figure, except that coming from the flame of the candle; also that no other light can reach the top of the screen except that reflected from the bottom of the candle at the right. Hence the image of the candle appears inverted.

A general understanding of the structure of the eye can be had from Figure 63. W indicates a watery substance in the front of the eye; I is the iris which has power to contract or expand and thus regulate the quantity of light admitted to the eye; P is the pupil; L is the lens; and R is the retina which connects with the optic nerve and the brain. The lens is made up of several parts having different indexes of refraction. The whole resembles an ordinary convex lens but has considerable power of adjustment. When looking at objects close by, for instance, the pupil can often be seen to bulge out which is its method of accommodating itself to objects close at hand.

To the iris falls the duty of regulating the quantity of light which is to reach the retina. If confronted by a bright light, it closes partially; in a dim light, it opens out wide. When subject to a flickering light, there is a tendency to follow the flickerings by rapid opening and closing, which causes pain. If subject to flickerings long enough, however, the pain becomes somewhat less, probably because the iris has come to rest on an intermediate point.

An image formed upon the retina remains for some time, the time varying with the intensity of the light. Very intense impressions are supposed to last about one twenty-fifth of a second; milder ones as long as one-tenth of a second. This tendency to retain images is known as the persistence of vision and can be noticed in many ways. A twenty-five cycle alternating current falls to zero fifty times in one second; and fifty times in each second there is a slight cooling off of the incandescent filament. Yet the variation in the intensity of the light is noticeable. Many of the sleight-of-hand tricks depend upon this persistence of vision and the projection of moving pictures would be impossible without it.

We are able to judge distance principally through the fact that we have two eyes. If our eyes were immovable, we should see two images for every object. But as they are movable and as both normally point directly at the object we are looking at, their axes form angles with each other and in this way we are enabled to judge the distance, as well as other qualities of objects.

When both eyes are centered upon an object, the impressions received by the brain from both sources are mixed, and the picture we become conscious of is a composite of the two images in the eyes.

This is verified by the fact that many persons with defective vision can see much more clearly with one eye than with both. They are not able to focus both eyes upon the same point and thus the perfectly clear picture which may exist in one eye is mixed with an uncertain picture in the other.

In youth normal eyes are able to adjust themselves to different intensities of light and different distances very rapidly. This power is largely curtailed as age advances. Where a young person can almost instantly, after gazing at some distant object, turn to a newspaper and read, the eyes of a person of advanced age generally require considerable time before they can adjust themselves in the same way. Quite frequently, however, very old people regain their powers of vision and become able to do without the glasses formerly used.

All of the above facts should be thoroughly understood by those having to do with illumination used by a mixed audience. Light that may seem perfectly satisfactory to one may be entirely unsuitable for another.

CHAPTER X.
REFLECTION.

Light may be reflected from opaque or transparent bodies such as glass. In the case of transparent bodies, the reflected rays are not noticeable unless the ground behind the reflecting body is dark. If there is much light behind a pane of glass, for instance, the pupil of the eye will be partially closed and not be able to see the faint light which is reflected. As we gradually darken the space behind the glass, the image begins to appear more and more distinct, partly from contrast with the dark background and partly on account of the increased opening of the pupil. This can be readily noticed if some evening out of a dimly-lighted room we look at some object just discernible. If we then turn on the light suddenly, the object will at once disappear but reflections will appear in the glass where there were none before.

The reflections from clear glass are much stronger at an angle than when the rays are thrown straight back. This can be seen by placing any object directly in front of a pane of glass with a dark background. If we place the eye so as to receive only those rays which are reflected directly back, we shall obtain but a weak reflection. If, however, we place the object a little to one side and stand close to the glass, we shall see the object almost as plainly as in a regular mirror.

A ray of light is always reflected at exactly the same angle at which it strikes, the reflecting body; that is, the angle of incidence is equal and opposite to the angle of reflection. This can be illustrated by Figure 64. If a mirror be attached to the pointer in the position shown at an angle of exactly ninety degrees and a beam of light be allowed to enter through the slit at the top, it will be reflected back exactly to the spot at which it entered. If we then turn the pointer slightly, we shall notice that the reflection of the beam of light moves twice as fast as the pointer and, when the pointer occupies the position indicated by broken lines, the light will be reflected at right angles to the line along which it enters. If the mirror is turned still more, the same law will hold; so that, if the mirror were turned through an angle of nearly ninety degrees, the reflected beam of light would in the same space of time make an angle of nearly one hundred eighty degrees.

Reflected light results in the formation of images in mirrors and other reflecting bodies and, by bearing in mind the law of reflection given above, we can readily explain how these images are formed and the manner in which they appear to us.

Let N, Figure 65 be an object in front of the mirror. The only rays that are reflected back to the eye are those that strike the mirror at the proper angle. All others are wasted with reference to the particular position of the eye. If the eye and the object reflected are equally distant from the mirror, we need but draw a line at right angles to the mirror and half way between the eye and the object and, from these two, draw lines to the point at which the perpendicular line strikes the mirror. The two lines thus drawn will give us the path of the incident and the reflected rays. The image will appear to lie in the direction from which the reflected ray comes and as far behind the mirror as the object is in front of it. If the eye and the object to be reflected are not equally distant from the mirror, it is more difficult to find the paths of the rays and it simplifies matters very much to use the following construction: Draw a line from the object before the mirror at right angles to the mirror and extend it behind the mirror as far as the object is in front of it. From this point behind the mirror, draw another line to the eye. By drawing a third line from the object to the point in the mirror where this line, from back of the mirror to the eye, crosses it, we shall obtain the paths of the rays and the position of the image in the mirror. The image will exist in the mirror at the point where the reflected and incident rays meet but will have the appearance of lying some distance behind the mirror. This is illustrated in Figure 65, N being the object reflected and M the apparent position of the image to the eyes as located in the cut.

In Figures 66 and 67 the same construction is used to show the appearance of arrows as they are reflected from a mirror to the eye. Objects standing erect over horizontal mirrors or arranged at right angles to mirrors and looked at, as in Figure 68, always appear inverted. This can be noticed in quiet ponds of clear water which give reflections of trees and other objects. Figure 68 shows two arrows, one horizontal, the other vertical; by the construction in the figure one appears inverted, the other not. If the arrow were placed in the position indicated by broken lines, the eye would see only the butt. If the arrow were placed a little nearer the horizontal, it would appear in its natural position; if a little more vertical, it would appear inverted in the mirror.

All objects seen in mirrors are reversed with reference to right and left. A pocket on the left side of a person facing a mirror will appear to be on the right side. Printed matter held before a mirror will appear just as it would if seen through the paper from the back side and will have to be read from right to left.

If an object be placed between two parallel mirrors as B, Figure 69, there will be a vast number of reflections visible at the point D. Several reflections of B will come to the eye in the manner indicated but there will be a large number of additional reflections. If the mirrors are exactly parallel and absolutely smooth, the number of reflections would theoretically be infinite. At each reflection, however, some light is absorbed and some diffused so that many of the reflections are not discernible. Two mirrors set opposite each other will also give many reflections of each other as indicated in Figure 70. The images seen in parallel mirrors all appear arranged in straight lines on both sides, as indicated in Figure 69. If now one of the mirrors be inclined so as to form an angle with the other, the long line of images will seem to become curved and finally lie in a circle. If the mirrors be placed at right angles to each other, as in Figure 71, there will be three reflections of the object C visible and these will reach the eye by the paths shown. If the mirrors be placed at an angle of sixty degrees to each other, five images will appear as shown in Figure 72, in which A is the object being reflected.

The following tabulation shows the number of images obtainable at different angles between the mirrors.

Instead of being plain, mirrors may be either concave or convex. A concave mirror is hollowed out in conformity with a small section of the surface of a sphere. If a piece of glass be cut out of a hollow sphere, the inner side of it will show the surface of a concave, and the outer side, the surface of a convex mirror. A section of a concave mirror is shown in Figure 73. C is the center of curvature and any line drawn from the surface of the mirror to this center is at right angles, or normal, to the curvature of the mirror. A ray of light emanating from this center will be reflected straight back to it. If the source of light be moved a little nearer to the mirror, the light reflected will be spread out more and come to a focus farther back from the glass; if it be moved farther back from the glass, the rays will be focused nearer the mirror. Thus if a light be placed at A, its rays will be focused at D and a light placed at D will focus at A. This can be seen by the lines which represent the rays of light. The two points at which a source of light will thus focus are known as the conjugate foci of the mirror.

If such a mirror receives parallel rays of light, they will be reflected and come to a focus at a point midway between the mirror and the center of curvature. This point is known as the principal focus of the mirror, and the distance between it and the mirror is called the focal length of the mirror. A source of light placed at this point will throw out parallel rays from the mirror. If the light be moved closer to the mirror, the reflected rays will spread out; while if moved farther away, the light will come to a focus at some distant point, as shown above.

Figure 74 can be used to illustrate the manner in which a concave mirror reflects the light from an object placed before it. From the upper point of the large arrow, rays of light emanate in all directions. All that strike the face of the mirror are thrown to a certain point which can be found by tracing out the lines, using the small arrows as guides. At this point will appear the image of the top of the arrow. It will be noted that it is inverted. The rays from the lower part of the arrow are of course all reflected in the same manner.

With mirrors of this kind, the position of the object with reference to the focal length and center of curvature is of great importance. If the object be placed in the position shown as the image in Figure 74, the image will appear as though it were in place of the object; it will be much enlarged and also inverted. If the object is placed between the principal focus and the mirror, it will appear to lie behind the mirror as shown in Figure 75. In this case it will not be inverted.

When concave mirrors forming large sections of spheres are used, the rays reflected from the outer edges will not all meet exactly at the focal point. There will then be a somewhat fuzzy image formed. This is illustrated in Figure 76. In order to obtain a perfectly clear and distinct image, only the central part of concave mirrors should be used.

Convex mirrors are not much used. Sometimes glass spheres are set up to show miniature reflections of scenery; convex mirrors are also found in the lobbies of theaters and in places of amusement to amuse the patrons with the caricatures of themselves reflected in them.

All of the rays that strike a convex mirror are reflected back in such a manner that they seem to come from a common point behind the mirror. This is shown in Figure 77. The center of curvature here is behind the mirror but the paths of the various rays can be determined as before explained. Thus we shall find that every ray, striking the mirror from a certain point, is reflected back in a direction which gives it the appearance of coming from a certain point behind the mirror. Two such points are shown in Figure 77.

In Figure 78 we have drawn the arrow and the image it would produce in the mirror. If the mirror forms a section of a sphere, the object reflected will appear reduced in size in all directions. If the mirror forms merely a section of a cylinder, a person standing in front of it will appear very much shorter than natural but of full width thus presenting a ridiculous appearance. Convex and concave mirrors are often combined and if properly set, a person standing in front of one may see himself either very much elongated or shortened.

CHAPTER XI.
REFRACTION.

If a straight stick or pencil be plunged into a vessel containing water, it will appear to be bent. The reason for this appearance is given in detail in Figure 79. The only light which can reach the eye from the lower extremity of the stick must reach it by a path similar to that of the bended ray at the left. According to this, the rays of light leaving the bottom of the stick at E bend at the water’s edge and meet the eye as shown. To the eye the rays, by which it sees the end of the stick, appear to come from the direction F; hence the stick is seen crooked. When a ray of light passes from air into a denser medium such as water or glass, the ray appears to be bent somewhat, as illustrated at the left of Figure 79. This bending of the rays of light is called refraction.

The fact of refraction can be further verified by placing an object G into an empty vessel in the position shown. In this position the object is not visible to the eye. By slowly filling the vessel with water, the object will gradually appear and will seem to lie in the direction of the straight dotted line.