YDSE: Film, Medium & Two Wavelengths question from NEET 2017

Answer: (c)

1. In a medium $\lambda' = \lambda/\mu$, so 8th bright: $y = \dfrac{8\lambda D}{\mu d}$.
2. 5th dark in air: $y = \dfrac{4.5\lambda D}{d}$.
3. $\dfrac{8}{\mu} = 4.5 \Rightarrow \mu = 1.78$
4. So the answer is (c) 1.78.