A travelling microscope is used to determine the refractive index...

Given here, main scale reading, $MS = \dfrac{1}{40}cm$

And, $50 \times VS = 49 \times MS$

Least count of travelling microscope is $LC = MS - VS$

$= MS - \dfrac{49}{50}MS$

$= \dfrac{1}{50} \times MS$

$= \dfrac{1}{50} \times \dfrac{1}{40}cm = \dfrac{1}{2000} \times {10}^{- 2}m$

$= 5 \times {10}^{- 6}m$