Conversations on Natural Philosophy, in which the Elements of that Science are Familiarly Explained Part 39

_Mrs. B._ Do not proceed to the second, until we have agreed upon the definition of the first. All bodies that s.h.i.+ne, are not luminous; for a luminous body is one that s.h.i.+nes by its own light; as the sun, the fire, a candle, &c.

_Emily._ Polished metal then, when it s.h.i.+nes with so much brilliancy, is not a luminous body?

_Mrs. B._ No, for it would be dark, if it did not receive light from a luminous body; it belongs, therefore, to the cla.s.s of dark, as well as of opaque bodies, which comprehends all such as are neither luminous, nor will admit the light to pa.s.s through them.

_Emily._ And transparent bodies, are those which admit the light to pa.s.s through them, such as gla.s.s and water.

_Mrs. B._ You are right. Transparent, or pellucid bodies, are frequently called mediums, because they allow the rays of light to pa.s.s through them; and the rays which pa.s.s through, are said to be transmitted by them.

Light, when emanated from the sun, or any other luminous body, is projected forward in straight lines, in every possible direction; so that the luminous body, is not only the general centre, from whence all the rays proceed; but every point of it, may be considered as a centre, which radiates light in every direction. (Fig. 1, plate 15.)

_Emily._ But do not the rays which are projected in different directions, and cross each other, interfere, and impede each other's course?

_Mrs. B._ Not at all. The particles of light, are so extremely minute, that they are never known to interfere with each other. A ray of light, is a single line of light, projected from a luminous body; and a pencil of rays, is a collection of rays, proceeding from any one point of a luminous body, as fig. 2.

_Caroline._ Is light then a substance composed of particles, like other bodies?

_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 pa.s.s, 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 s.p.a.ce 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 diminis.h.i.+ng, 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 s.h.i.+nes 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 ill.u.s.trated 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 pa.s.s 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.

_Mrs. B._ Because it is turned directly back again; and the ray of incidence, and that of reflection, are confounded together, both being in the same line, though in opposite directions.

_Emily._ The ray then, which appears to us single, is really double, and is composed of the incident ray, proceeding to the mirror, and of the reflected ray, returning from the mirror.

_Mrs. B._ Exactly so. We will now separate them, by holding the mirror M, (fig. 6,) in such a manner, that the incident ray, A B, shall fall obliquely upon it--you see the reflected ray, B C, is marching off in another direction. If we draw a line from the point of incidence B, perpendicularly, to the mirror, it will divide the angle of incidence, from the angle of reflection, and you will see that they are equal.

_Emily._ Exactly; and now, that you hold the mirror, so that the ray falls more obliquely upon it, it is also reflected more obliquely, preserving the equality of the angles of incidence, and of reflection.

_Mrs. B._ It is by reflected rays only, that we see opaque objects.

Luminous bodies, send rays of light immediately to our eyes, but the rays which they send to other bodies, are invisible to us, and are seen, only when they are reflected by those bodies, to our eyes.

_Emily._ But have we not just seen the ray of light, in its pa.s.sage from the sun to the mirror, and its reflections? yet, in neither case, were those rays in a direction to enter our eyes.

_Mrs. B._ What you saw, was the light reflected to your eyes, by small particles of dust floating in the air, and on which the ray shone, in its pa.s.sage to, and from, the mirror.

_Caroline._ Yet I see the sun, s.h.i.+ning on that house yonder, as clearly as possible.

_Mrs. B._ Indeed you cannot see a single ray, which pa.s.ses from the sun to the house; you see, by the aid of those rays, which enter your eyes; therefore, it is the rays which are reflected by the house, to you, and not those which proceed directly from the sun, to the house, that render the building visible to you.

_Caroline._ Why then does one side of the house appear to be in suns.h.i.+ne, and the other in shade? for, if I cannot see the sun s.h.i.+ne upon it, the whole of the house should appear in the shade.

_Mrs. B._ That side of the house, which the sun s.h.i.+nes upon, receives, and reflects more light, and therefore, appears more luminous and vivid, than the side which is in shadow; for the latter is illumined only, by rays reflected upon it by other objects; these rays are, therefore, twice reflected before they reach your sight; and as light is more, or less, absorbed by the bodies it strikes upon, every time a ray is reflected, its intensity is diminished.

_Caroline._ Still I cannot reconcile to myself, the idea that we do not see the sun's rays s.h.i.+ning on objects, but only those which such objects reflect to us.