Physics Question from JEE Main 2023 · 29 January, Shift 2

In an experiment measuring the refractive index of a glass slab using a travelling microscope, a student measures the real thickness of the slab as $5.25 \, \text{mm}$ and the apparent thickness as $5.00 \, \text{mm}$. The travelling microscope has $20$ divisions in one cm on the main scale, and $50$ divisions on the Vernier scale are equal to $49$ divisions on the main scale. The estimated uncertainty in the measurement of the refractive index of the slab is $\dfrac{x}{10} \times 10^{-3}$, where $x$ is: