Page images
PDF
EPUB

CONVERSATION III.

of the Refraction of Light.

CHARLES. If glass stop the rays of light, and reflect them, why cannot I see myself in the window ?

Tutor. It is the silvering on the glass which causes the reflection, otherwise the rays would pass through it without being stopped, and if they were not stopped, they could not be reflected. No glass, however, is so transparent, but it reflects some rays: put your hand to within three or four inches of the window, and you see clearly the im

age of it.

James. So I do, and the nearer the hand is to the glass, the more evident is the im

age, but it is formed on the other side of the glass, and beyond it too.

Tutor. It is ; this happens also in looking-glasses : you do not see yourself on the surface, but apparently as far behind the glass, as you stand from it in the front.

Whatever suffers the rays of light to pass through it, is called a medium. Glass, which is transparent, is a inedium ; so also is air, water, and indeed all fluids that are transparent are called media, and the more transparent the body, the more perfect is the medium.

Charles. Do the rays of light pass through these in a straight line?

Tutor. They do: but not in precisely the same direction in which they were moving before they entered it. They are bent out of their former course, and this is called refraction.

James. Can you explain this term more clearly?

Tutor. Suppose A B (Plate 1. Fig. 2.) to be a piece of glass, two or three inches thick ; and a ray of light s a, to fall upon it

at a, it will not pass through in the direction s s, but when it comes to a, it will be bent towards the perpendicular a b, and go through the glass in the course a x, and when it comes into the air, it will pass on in the direction x 2, which is parallel to s s.

Charles. Does this happen if the ray fall perpendicularly on the glass at p a?.

Tutor. In that case there is no refraction, but the ray proceeds in its passage through the glass, precisely in the same direction as it did before it entered it, namely, in the direction ph.

Fames. Refraction then takes place only when the rays fall obliquely or slantwise on the medium ?

Tutor. Just so: rays of light may pass out of a rarer into a denser medium, as from air into water or glass : or they may pass from a denser medium into a rarer, as from water into air.

Charles. Are the effects the same in both cases ?

Tutor. They are not: and I wish you to remember the difference. When light passes out of a rarer into a denser medium, it is drawn to the perpendicular; thus if s a pass

from air into glass, it moves, in its passage through it, in the line a x, which is nearer to the perpendicular a b than the line å s, which was its first direction.

Bụt when a' ray passes from a denser medium into a rarer, it moves

in a direction farther from the perpendicular ; thus if the ray x a pass through glass or water into air, it will not, when it comes to a, move in the direction a m, but in the line a s, which is further than a m from the perpendicular a P.

James. Can you show us any experiment in proof of this?

Tutor. Yes, I can: here is a common earthen pan, on the bottom of which I will lay a shilling, and will fasten it with a piece of soft wax, so that it shall not move from its place, while I pour in some water. Stand back till you just lose sight of the shilling.

Fames. The side of the pan now completely hides the sight of the money from me,

Tutor. I will pour in a pitcher of clear water.

James. I now see the shilling: how is this to be explained ?

Tutor. Look to the last figure, and conceive

your eye to be at s, a b the side of the pan, and the piece of money to be at x: now when the pan is empty, the rays of light flow from x, in the direction x am, but your eye is at s, of course you cannot see any thing by the ray proceeding along x a m. As soon as I put the water into the vessel, the rays of light proceed from * to a, but there they enter from a denser to a rarer medium; and therefore, instead of moving in a m, as they did when there was no water, they will be bent from the perpendicular, and will come to your eye at s, as if the shilling were situate at no

James. And it does appear to me to be at n.

Tutor. Remember what I am going to tell you, for it is a sort of axiom in optics : “ We see every thing in the direction of that line in which the rays approach us last.”

« PreviousContinue »