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?