Refraction at plane surfaces
A pond or a swimming bath both appear much shallower than they actually are; a straight stick appears bent when partly immersed in water; and the landscape “shimmers” on a hot summer’s day. These, and many similar effects are caused by refraction, or the change in direction of light when it passes from one medium to another.
The terms used in connection with refraction are illustrated in Fig. 23.1, which represents the passage of a ray of light from air to glass. The angle of incidence, i, is the angle between the incident ray and the normal at the point of incidence. The angle of refraction, r, is the angle between the refracted ray and the normal. It is important to remember that. when a ray passes from one medium to a more optically dense medium, the ray bends towards the normal. Conversely, a ray passing from glass or water into air is bent away from the normal. Before going on to the next section it should be mentioned that refraction can be explained in terms of a change in the speed of light when it passes from one medium to another. This important aspect of refraction is dealt with in chapter 26 as part of our study of wave motion.