Study the following four experimental set-ups I, II, III and IV for the experiment, "To trace the path of a ray of light through a rectangular glass slab." Which of the marked set-ups is likely to give best results (P1 and P2 are the positions of pins fixed on the incident ray)?

The pins must not be too close to each other, otherwise it would be hard to align the light passing through both of them when seen from the other side of the slab. Hence III is rejected. The incident ray of light must not be normal to the glass slab surface, otherwise it would directly go straight ahead giving no information about the refractive index. Hence I is rejected. Also the angle of incidence must be optimum for observing the shifted path of light after emergence from the other side of slab. A large angle of incidence can give poor results. Hence II is rejected. Correct answer is option IV.