Part 70

Mrs. B. That is a disputed point, upon which I cannot pretend to decide. In some respects, light is obedient to the laws which govern bodies; in others, it appears to be independent of them: thus, though its course is guided by the laws of motion, it does not seem to be influenced by those of gravity. It has never been discovered to have weight, though a variety of interesting experiments have been made with a view of ascertaining that point; but we are so ignorant of the intimate nature of light, that an attempt to investigate it, would lead us into a labyrinth of perplexity, if not of error; we shall, therefore, confine our attention to those properties of light, which are well ascertained.

Let us return to the examination of the effects of the radiation of light, from a luminous body. Since the rays of light are projected in straight lines, when they meet with an opaque body through which they are unable to pass, they are stopped short in their course; for they cannot move in a curve line round the body.

Caroline. No, certainly; for it would require some other force besides that of projection, to produce motion in a curve line.

Mrs. B. The interruption of the rays of light, by the opaque body, produces, therefore, darkness on the opposite side of it: and if this darkness fall upon a wall, a sheet of paper, or any object whatever, it forms a shadow.

Emily. A shadow, then, is nothing more than darkness produced by the intervention of an opaque body, which prevents the rays of light from reaching an object behind it.

Caroline. Why then are shadows of different degrees of darkness; for I should have supposed, from your definition of a shadow, that it would have been perfectly black?

Mrs. B. It frequently happens that a shadow is produced by an opaque body, interrupting the course of the rays from one luminous body, while light from another, reaches the space where the shadow is formed; in which case, the shadow is proportionally fainter. This happens when the opaque body is lighted by two candles: if you extinguish one of them, the shadow will be both deeper, and more distinct.

Caroline. But yet it will not be perfectly dark.

Mrs. B. Because it is still slightly illuminated by light reflected from the walls of the room, and other surrounding objects.

You must observe, also, that when a shadow is produced by the interruption of rays from a single luminous body, the darkness is proportioned to the intensity of the light.

Emily. I should have supposed the contrary; for as the light reflected from surrounding objects on the shadow, must be in proportion to the intensity of the light, the stronger the light, the more the shadow will be illumined.

Mrs. B. Your remark is perfectly just; but as we have no means of estimating the degrees of light, and of darkness, but by comparison, the strongest light will appear to produce the deepest shadow. Hence a total eclipse of the sun, occasions a more sensible darkness than midnight, as it is immediately contrasted with the strong light of noonday.

Caroline. The reappearance of the sun, after an eclipse, must, by the same contrast, appear remarkably brilliant.

Mrs. B. Certainly. There are several things to be observed, in regard to the form, and extent, of shadows. If the luminous body A (fig. 3.) is larger than the opaque body B, the shadow will gradually diminish in size, till it terminates in a point.

Caroline. This is the case with the shadows of the earth, and the moon; as the sun, which illumines them, is larger than either of those bodies. And why is it not the case with the shadows of terrestrial objects? Their shadows, far from diminishing, are always larger than the object, and increase with the distance from it.

Mrs. B. In estimating the effect of shadows, we must consider the dimensions of the luminous body; when the luminous body is less, than the opaque body, the shadow will increase with the distance. This will be best exemplified, by observing the shadow of an object lighted by a candle.

Emily. I have often noticed, that the shadow of my figure, against the wall, grows larger, as it is more distant from me, which is owing, no doubt, to the candle that shines on me, being much smaller than myself.

Mrs. B. Yes. The shadow of a figure as A, (fig. 4.) varies in size, according to the distance of the several surfaces B C D E, on which it is described.

Caroline. I have observed, that two candles, produce two shadows from the same object; whilst it would appear, from what you said, that they should rather produce only half a shadow, that is to say, a very faint one.

Mrs. B. The number of lights (in different directions) while it decreases the intensity of the shadows, increases their number, which always corresponds with that of the lights; for each light, makes the opaque body cast a different shadow, as illustrated by fig. 5. which represents a ball A, lighted by three candles, B, C, D; and you observe the light B, produces the shadow b, the light C, the shadow c, and the light D, the shadow d; but neither of these shadows will be very dark, because the light of one candle only, is intercepted by the ball; and the spot is still illuminated by the other two.

Emily. I think we now understand the nature of shadows very well; but pray, what becomes of the rays of light, which opaque bodies arrest in their course, and the interruption of which, is the occasion of shadows?

Mrs. B. Your question leads to a very important property of light, Reflection. When rays of light encounter an opaque body, they cannot pass through it, and part of them are absorbed by it, and part are reflected, and rebound; just as an elastic ball rebounds, when struck against a wall.

By reflection, we mean that the light is turned back again, through the same medium which it had traversed in its first course.

Emily. And is light, in its reflection, governed by the same laws, as solid, elastic bodies?

Mrs. B. Exactly. If a ray of light fall perpendicularly on an opaque body, it is reflected back in the same line, towards the point whence it proceeded. If it fall obliquely, it is reflected obliquely, but in the opposite direction; the ray which falls upon the reflecting surface, is called the incident ray, and that which leaves it, the reflected ray; the angle of incidence, is always equal to the angle of reflection. You recollect that law in mechanics?

Emily. Oh yes, perfectly.

Mrs. B. If you will shut the shutters, we will admit a ray of the sun's light, through a very small aperture, and I can show you how it is reflected. I now hold this mirror, so that the ray shall fall perpendicularly upon it.

Caroline. I see the ray which falls upon the mirror, but not that which is reflected by it.