§ 3. _Ordinary Refraction of Light explained by the Wave Theory_.

We have now to exhibit the bearings of this act of crystallization
upon optical phenomena. According to the undulatory theory, the
velocity of light in water and glass is less than in air. Consider,
then, a small portion of a wave issuing from a point of light so
distant that the minute area may be regarded as practically plane.
Moving vertically downwards, and impinging on a horizontal surface of
glass or water, the wave would go through the medium without change of
direction. As, however, the velocity in glass or water is less than
the velocity in air, the wave would be retarded on passing into the
denser medium.

[Illustration: Fig. 25.]

But suppose the wave, before reaching the glass, to be _oblique_ to
the surface; that end of the wave which first reaches the medium will
be the first retarded by it, the other portions as they enter the
glass being retarded in succession. It is easy to see that this
retardation of the one end of the wave must cause it to swing round
and change its front, so that when the wave has fully entered the
glass its course is oblique to its original direction. According to
the undulatory theory, light is thus _refracted_.

With these considerations to guide us, let us follow the course of a
beam of monochromatic light through our glass prism. The velocity in
air is to its velocity in glass as 3: 2. Let A B C (fig. 25) be the
section of our prism, and _a_ _b_ the section of a plane wave