Ray Optics

In determining the refractive index of a glass slab using a travelling microscope, the following readings are tabulated

(a) Reading of travelling microscope for ink mark $=5.123 \mathrm{~cm}$

(b) Reading of travelling microscope for ink mark through glass slab $=6.123 \mathrm{~cm}$

(c) Reading of travelling microscope for chalk dust on glass slab $=8.123 \mathrm{~cm}$

From the data, the refractive index of a glass slab is