Refractive index related to real and apparent depth
The way refractive index is related to real and apparent depth is illustrated in Fig. 23.9. Once again the horizontal scale is exaggerated, but it is to be understood that OBC is a ray very close to the normal which enters the eye from a point 0 at the
bottom of the slab. The emergent ray BC appears to be coming from a virtual image I, so that Al is the apparent depth of the slab.
By using the principle of reversibility of light, the refractive index of glass is given by