The principle of reversibility of light
The principle of reversibility of light, which states that the paths of light rays are reversible, has already been mentioned in connection with reflection on page 237. The same holds true in the case of refraction and, indeed, for rays passing through
any optical system. The refractive indices for a ray passing fron air to glass and from glass to air may be distinguished by using the symbols ang and gna respectively. Thus, using Fig. 23.1,
The equation will be used later on in the chapter in connection with critical internal reflection. If ang = 1.5 it follows that gl1a = 1 1.5 = 0.67.