Real and apparent depth
A thick slab of glass appears to be about two-thirds of its real thickness when viewed from vertically above. Similarly, water in a pond appears to be only three-quarters of its true depth. Fig. 23.8 shows how this comes about. Rays from a point 0 at the bottom of the slab are refracted away from the normal where they leave the glass and then enter the pupil of the eye as though coming from a virtual image I above O. For clarity, the horizontal scale of this diagram has been exaggerated. The diameter of the eye
pupil is, of course, so small that only rays very close to the normal will enter it.