In finding out refractive index of glass slab the following observations were made through travelling microscope 50 vernier scale division $$= 49$$ MSD; 20 divisions on main scale in each cm. For mark on paper MSR $$= 8.45 \text{ cm}$$, VC $$= 26$$. For mark on paper seen through slab MSR $$= 7.12 \text{ cm}$$, VC $$= 41$$. For powder particle on the top surface of the glass slab MSR $$= 4.05 \text{ cm}$$, VC $$= 1$$. (MSR = Main Scale Reading, VC = Vernier Coincidence) Refractive index of the glass slab is :