An object P is focused by a microscope M. A glass slab of thickness 2.1 cm is introduced between P and M. If the refractive index of the slab is 1.5, by what distance should the microscope be shifted to focus the object again?

0.70 cm

The thickness of the glass slab, d = 2 cm

and refractive index of the slab μ = 1.5

shift due to the glass slab is given by

Δ T = (1−1/μ) d = (1−1/1.5)2.1 = 0.7 cm

So, the microscope should be shifted 0.70cm to focus the object again.