Refraction, Glass Slab & Apparent Depth question from MHT CET 2024 (11 May, Shift 1)

Answer: (c)

1. Number of waves in thickness $t$: $N = \dfrac{t}{\lambda_0/\mu} = \dfrac{\mu t}{\lambda_0}$.
2. Equal $N$: $\mu_g t_g = \mu_w t_w \Rightarrow 1.56 \times 5 = \mu_w \times 6$.
3. $\mu_w = \dfrac{7.8}{6} = 1.30$.
4. So the answer is (c) 1.30.