The dotted line is drawn perpendicularly to the surface, and it is seen that the ray of light on entering is bent towards this line. On emerging, on the other hand, it is bent to an equal extent away from the perpendicular, so that it proceeds in a course parallel to, but not coincident with, its original direction. If we suppose the new medium, in place of being more dense than the old, to be less dense, then the conditions are exactly reversed,—the ray is bent away from the perpendicular on entering, and towards it on leaving.
It must be observed that the laws of refraction apply only to rays of light which fall upon the medium at an angle: if they enter perpendicularly—in the direction of the dotted lines in the last figure—they pass straight through without suffering refraction.
Notice also, that it is at the surfaces of bodies that the deflecting power acts. The ray is bent on entering, and bent again on leaving; but whilst within the medium it continues in a straight line. Hence it is evident that by variously modifying the surfaces of refractive media the rays of light may be diverted almost at pleasure. This will be rendered clear by a few simple diagrams.
In the figures given below, and in the following page, the dotted lines represent perpendiculars to the surface at the point where the ray falls, and it is seen that the usual law of bending towards the perpendicular on entering, and away from it on leaving the dense medium, is in each case correctly observed.
| Fig. 1. | Fig. 2. |
Fig. 2, termed a prism, bends the ray permanently to one side; fig. 3, consisting of two prisms placed base to base, causes rays before parallel to meet in a point; and conversely, fig. 4, having prisms placed edge to edge, diverts them further asunder.
| Fig. 3. | Fig. 4. |
The various forms of Lenses.—The phenomena of the refraction of light are seen in the case of curved surfaces in the same manner as with those which are plane.
Glasses ground of a curvilinear form are termed Lenses. The following are examples.