صور الصفحة
PDF
النشر الإلكتروني

But this is not all. Bodies which are not themselves luminous, give forth again a part of any light which falls upon them, and thereby become visible. It is in this way that the light of the sun is so generally diffused in the daytime, even in the interior of houses, and in other places not exposed to his direct rays. Every illuminated object becomes a luminary in its turn, and thus, by an infinite series of successive reflections, that general illumination is produced to which we give the name of daylight. The atmosphere itself greatly contributes to this uniform diffusion of the solar rays. Every particle of air becomes in fact a luminous centre, which sends forth light in every direction. Were it not so, the atmosphere would be invisible, for we can see an object only when light passes from it to the eye. If, therefore, the eye ceased to receive light from this great ocean of air in which we live, the sky over our heads would lose its beautiful azure, and appear totally black.

REFLECTION OF LIGHT. WAEN light falls on an opaque body, part of it is always absorbed and lost, at least so far as our senses are concerned. From every point of the illuminated surface, another portion of the same light is again sent out in every direction; and we have already learned that it is by means of this irregular reflection, as it is often called, that bodies having no light in themselves become visible. But if the surface receiving the light is polished, another portion still suffers reflection of a totally different kind, according to a definite law of direction now to be explained. It is this regular reflection from polished surfaces which people generally mean, when they speak, without any qualifying term, of the “reflection of light.”

The most familiar example of a polished reflector is a common mirror or looking-glass. In general, metallic surfaces are the best reflectors, and, even in the looking-glass, it is not the surface of the glass, but the metallic coating on the back, which really produces the effect. This coating, which consists of an amalgam of tin and quicksilver, has an intense metallic lustre, and therefore a very high reflecting power. Glass itself reflects more feebly, as any one may see by blackening a plate of it on one side, so as to exclude light from behind, and then using the other side as a reflector. In many cases, reflectors are made by polishing metallic surfaces, without the intervention of glass. These are usually denominated specula, and are much employed in the construction of optical instruments. The great law of regular reflection is easily understood. Fig. 49.

Let A O B represent a plane reflecting surface of whatever kind, and let O P be a line perpendicular to it. If a ray of light fall upon the surface AO B in the direction PO,

it will be reflected in the same

Bline from 0 to P. But if a ray fall slantingly on the reflector, as for example, in the line RO, it will be reflected in the direction OS, on the opposite side of the perpendicular. The line RO, be it observed, makes with the perpendicular an angle ROP, which is called the angle of incidence; the reflected ray makes with the perpendicular an angle SOP, which is called the angle of reflection; now the great principle which governs all regular reflection is, that these two angles, the angle of incidence and the angle of reflection, are always equal to one another. In other words, the incident ray and the reflected ray are equally oblique; the latter inclines as much, if we may so speak, to the one side, as the former does to the other side. We need not go far to find illustrations of this important law. If you stand right before a mirror, you see your own image, because the light falling upon the mirror from your body is thrown back to you. If you move a little to one side, you will see the image no longer, but a person placed as far towards the other side will see it, and his image will, in like manner, be visible to you. The rays from his body, falling obliquely on the mirror, are reflected obliquely towards you; the rays from your body, for a similar reason, are reflected towards him.

When you look at an image of any object in a plane mirror, the image appears to be as far behind the mirror, as the object itself is before it. Suppose A B to represent a mirror with the reflect

FIG. 50. ing surface turned to the

А right, CD an object placed ---- -----------------. ) before it, and cd the image of that object as seen by an eye at E. The point C gives forth rays in all directions, of which a small pencil, falling upon the mirror at 0, is so reflected that the rays enter the eye in the same directions as if they had come from the point c. At this latter point, therefore, the eye perceives an image of the point C. The same is the case with the point D, whose image is formed at d; and since every other point of the object produces an image of itself in the same way, the general result is, that an image of the whole object is seen, equally distant from the reflector with the object itself, and similar to it in every respect, but in a reversed position.

For many purposes, plane reflectors are not suitable. In telescopes, for example, a concave speculum is often used, and the image which it forms of any heavenly body is viewed through a lens or eye-glass. Such an instrument is called a reflecting telescope. It is not possible to explain here the mode in which images are produced by concave and convex reflectors. A pretty good idea of some of the phenomena may be got by looking at the reflection of one's own face in a tea-kettle, or in a silver spoon. But it is worthy of special remark, that the image of a distant object, formed by a concave mirror, is not one which can be seen by looking into the mirror, but is a real picture suspended in the air a

little in front of the reflecting surface, and may be rendered visible by placing a screen in a proper position to receive it. Such is the image of the sun, for example, formed in a reflecting telescope. The spot occupied by this image is called the focus of the reflector. There all the rays falling upon the reflector from the sun are collected, not exactly to a point, but into a very small space. The heat in that space is intense; sufficient, in the case of a large mirror, to set wood on fire, and melt silver and copper coins.

It has already been stated, that only a part of the light falling on any polished surface is reflected. This may, perhaps, be owing to the want of perfect polish. But it has to be borne in mind that the actual quantity reflected depends not only on the nature of the reflector, but also on the direction of the incident rays. The more oblique their course, the larger is the proportion reflected. Take as an example the surface of still water, which reflects but feebly. Very seldom, and only in favourable circumstances, can one see his own image reflected with anything like distinctness by a sheet of water. He may bend over it, but the only rays from his own body which can reach his eye after reflection are so near the perpendicular, that very few of them are reflected at all. Yet it is no uncommon thing to see, by reflection from the calm surface of a lake, a distinct inverted image, apparently stretching far down into its azure depths, of the trees, cliffs, precipices, and mountains on the farther shore. One such observation has been recorded by a devout lover of nature, in language whose very simplicity gives it beauty:

“The swan in still Saint Mary's Lake
Floats double, swan and shadow!”.

WORDSWORTI.

REFRACTION OF LIGHT. As the light which falls upon a polished surface is never wholly reflected, so the light which enters a transparent substance, or medium, is never wholly transmitted. Some

[ocr errors]
[ocr errors]

portion of it is always absorbed in its course. Thus a mountain looks dim and shadowy when seen from a great distance, because of the vast mass of air between it and the eye; and even the light of the sun is partly absorbed in passing through the atmosphere.

When a pencil of light passes from one transparent medium into another of a different kind, or of different density, there is not only a part of it reflected, and another part absorbed, but the remainder is turned out of its course, and enters the new medium in a different direction from that which it has already pursued. This effect is termed refraction. Suppose for

Fig. 51. example, that AB is a plate of glass of equal thickness throughout, and that CO and DP are perpen- A S dicular to either of its two parallel surfaces. If a ray fall upon the upper surface of the glass at 0, in the direction of the perpendicular CO, it will not be refracted, but will continue its course right through the plate in the same straight line. But it is not so in the case of a ray which falls on the glass in any other direction, as R 0. Instead of pursuing a straight course towards T, such a ray will be bent (or rather broken) at 0, and will proceed towards P, in the straight line OP. It will be observed that O P is less oblique than R0; in other words, the ray has been refracted in passing from air into glass, towards the perpendicular. When the refracted ray arrives at P, and emerges again from the glass into the air, a somewhat similar change will take place; but this time it will be turned into a more oblique direction, or bent farther away from the perpendicular. And the result of the two refractions is, that the

T

« السابقةمتابعة »