General Science, Bertha May Clark [miss read books TXT] 📗
- Author: Bertha May Clark
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105. Why the Image seems to be behind the Mirror. If a candle is placed in front of a mirror, as in Figure 62, one of the rays of light which leaves the candle will fall upon the mirror as AB and will be reflected as BC (in such a way that the angle of reflection equals the angle of incidence). If an observer stands at C, he will think that the point A of the candle is somewhere along the line CB extended. Such a supposition would be justified from Section 102. But the candle sends out light in all directions; one ray therefore will strike the mirror as AD and will be reflected as DE, and an observer at E will think that the point A of the candle is somewhere along the line ED. In order that both observers may be correct, that is, in order that the light may seem to be in both these directions, the image of the point A must seem to be at the intersection of the two lines. In a similar manner it can be shown that every point of the image of the candle seems to be behind the mirror.
FIG. 62.—The image is a duplicate of the object, but appears to be behind the mirror.
It can be shown by experiment that the distance of the image behind the mirror is equal to the distance of the object in front of the mirror.
FIG. 63.—The surface of the paper, although smooth in appearance, is in reality rough, and scatters the light in every direction.
106. Why Objects are Visible. If the beam of light falls upon a sheet of paper, or upon a photograph, instead of upon a smooth polished surface, no definite reflected ray will be seen, but a glare will be produced by the scattering of the beam of light. The surface of the paper or photograph is rough, and as a result, it scatters the beam in every direction. It is hard for us to realize that a smooth sheet of paper is by no means so smooth as it looks. It is rough compared with a polished mirror. The law of reflection always holds, however, no matter what the reflecting surface is,—the angle of reflection always equals the angle of incidence. In a smooth body the reflected beams are all parallel; in a rough body, the reflected beams are inclined to each other in all sorts of ways, and no two beams leave the paper in exactly the same direction.
Hot coals, red-hot stoves, gas flames, and candles shine by their own light, and are self-luminous. Objects like chairs, tables, carpets, have no light within themselves and are visible only when they receive light from a luminous source and reflect that light. We know that these objects are not self-luminous, because they are not visible at night unless a lamp or gas is burning. When light from any luminous object falls upon books, desks, or dishes, it meets rough surfaces, and hence undergoes diffuse reflection, and is scattered irregularly in all directions. No matter where the eye is, some reflected rays enter it, and the various objects are clearly seen.
CHAPTER XI REFRACTIONFIG. 64.—A straw or stick in water seems broken.
107. Bent Rays of Light. A straw in a glass of lemonade seems to be broken at the surface of the liquid, the handle of a teaspoon in a cup of water appears broken, and objects seen through a glass of water may seem distorted and changed in size. When light passes from air into water, or from any transparent substance into another of different density, its direction is changed, and it emerges along an entirely new path (Fig. 64). We know that light rays pass through glass, because we can see through the window panes and through our spectacles; we know that light rays pass through water, because we can see through a glass of clear water; on the other hand, light rays cannot pass through wood, leather, metal, etc.
Whenever light meets a transparent substance obliquely, some of it is reflected, undergoing a change in its direction; and some of it passes onward through the medium, but the latter portion passes onward along a new path. The ray RO (Fig. 65) passes obliquely through the air to the surface of the water, but, on entering the water, it is bent or refracted and takes the new path OS. The angle AOR is called the angle of incidence. The angle POS is called the angle of refraction.
FIG. 65.—When the ray RO enters the water, its path changes to OS.
The angle of refraction is the angle formed by the refracted ray and the perpendicular to the surface at the point where the light strikes it.
When light passes from air into water or glass, the refracted ray is bent toward the perpendicular, so that the angle of refraction is smaller than the angle of incidence. When a ray of light passes from water or glass into air, the refracted ray is bent away from the perpendicular so that the angle of refraction is greater than the angle of incidence.
The bending or deviation of light in its passage from one substance to another is called refraction.
FIG. 66.—A fish at A seems to be at B.
108. How Refraction Deceives us. Refraction is the source of many illusions; bent rays of light make objects appear where they really are not. A fish at A (Fig. 66) seems to be at B. The end of the stick in Figure 64 seems to be nearer the surface of the water than it really is.
The light from the sun, moon, and stars can reach us only by passing through the atmosphere, but in Section 76, we learned that the atmosphere varies in density from level to level; hence all the light which travels through the atmosphere is constantly deviated from its original path, and before the light reaches the eye it has undergone many changes in direction. Now we learned in Section 102, that the direction of the rays of light as they enter the eye determines the direction in which an object is seen; hence the sun, moon, and stars seem to be along the lines which enter the eye, although in reality they are not.
109. Uses of Refraction. If it were not for refraction, or the deviation of light in its passage from medium to medium, the wonders and beauties of the magic lantern and the camera would be unknown to us; sun, moon, and stars could not be made to yield up their distant secrets to us in photographs; the comfort and help of spectacles would be lacking, spectacles which have helped unfold to many the rare beauties of nature, such as a clear view of clouds and sunset, of humming bee and flying bird. Books with their wealth of entertainment and information would be sealed to a large part of mankind, if glasses did not assist weak eyes.
By refraction the magnifying glass reveals objects hidden because of their minuteness, and enlarges for our careful contemplation objects otherwise barely visible. The watchmaker, unassisted by the magnifying glass, could not detect the tiny grains of dust or sand which clog the delicate wheels of our watches. The merchant, with his lens, examines the separate threads of woolen and silk fabrics to determine the strength and value of the material. The physician, with his invaluable microscope, counts the number of infinitesimal corpuscles in the blood and bases his prescription on that count; he examines the sputum of a patient to determine whether tuberculosis wastes the system. The bacteriologist with the same instrument scrutinizes the drinking water and learns whether the dangerous typhoid germs are present. The future of medicine will depend somewhat upon the additional secrets which man is able to force from nature through the use of powerful lenses, because as lenses have, in the past, been the means of revealing disease germs, so in the future more powerful lenses may serve to bring to light germs yet unknown. How refraction accomplishes these results will be explained in the following Sections.
110. The Window Pane. We have seen that light is bent when it passes from one medium to another of different density, and that objects viewed by refracted light do not appear in their proper positions.
FIG. 67.—Objects looked at through a window pane seem to be in their natural place.
When a ray of light passes through a piece of plane glass, such as a window pane (Fig. 67), it is refracted at the point B toward the perpendicular, and continues its course through the glass in the new direction BC. On emerging from the glass, the light is refracted away from the perpendicular and takes the direction CD, which is clearly parallel to its original direction. Hence, when we view objects through the window, we see them slightly displaced in position, but otherwise unchanged. The deviation or displacement caused by glass as thin as window panes is too slight to be noticed, and we are not conscious that objects are out of position.
111. Chandelier Crystals and Prisms. When a ray of light passes through plane glass, like a window pane, it is shifted somewhat, but its direction does not change; that is, the emergent ray is parallel to the incident ray. But when a beam of light passes through a triangular glass prism, such as a chandelier crystal, its direction is greatly changed, and an object viewed through a prism is seen quite out of its true position.
FIG. 68.—When looked at through the prism, A seems to be at S.
Whenever light passes through a prism, it is bent toward the base of the prism, or toward the thick portion of the prism, and emerges from the prism in quite a different direction from that in which it entered (Fig. 68). Hence, when an object is looked at through a prism, it is seen quite out of place. In Figure 68, the candle seems to be at S, while in reality it is at A.
FIG. 69.—Rays of light are converged and focused at F.
112. Lenses. If two prisms are arranged as in Figure 69, and two parallel rays of light fall upon the prisms, the beam A will be bent downward toward the thickened portion of the prism, and the beam B will be bent upward toward the thick portion of the prism, and after passing through the prism the two rays will intersect at some point F, called a focus.
FIG. 70.—Rays of light are diverged and do not come to any real focus.
If two prisms are arranged as in Figure 70, the ray A will be refracted upward toward the thick end, and the ray B will be refracted downward toward the thick end; the two rays, on emerging, will therefore be widely separated and will not intersect.
Lenses are very
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