Answer :
To refocus the microscope, it should be moved 2 cm upward after placing a glass slab of thickness 3 cm and refractive index 1.5 over the mark.The correct option is a) 2 cm upward
The student is asking about how to refocus a microscope after a glass slab with a known refractive index and thickness is placed over the observed mark. When the glass slab is introduced, it reduces the effective thickness that light has to travel through due to its refractive index. The formula to calculate the reduced thickness (optical path length) is t' = t/n, where t is the actual thickness and n is the refractive index. Here, t is 3 cm and n is 1.5. So, the reduced thickness is t' = 3 cm / 1.5 = 2 cm. The microscope needs to be moved by this reduced thickness in the direction opposite to the added slab to refocus. Hence, it should be moved 2 cm upward.
