Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
**Assertion (A):** If Young's double slit experiment is performed in an optically denser medium than air, then the consecutive fringes come closer.
**Reason (R):** The speed of light reduces in an optically denser medium than air while its frequency does not change.
In the light of the above statements, choose the most appropriate answer from the options given below.
Answer: (B) Both (A) and (R) are true and (R) is the correct explanation of (A)
Fringe width $\beta = \dfrac{\lambda D}{d}$.
In a medium of refractive index $\mu$, the frequency stays the same while the speed drops to $c/\mu$, so the wavelength becomes $\lambda/\mu$. Hence $\beta' = \beta/\mu$: fringes come closer.
Both statements are true, and the reduced speed at fixed frequency (smaller wavelength) is exactly why the fringes come closer. (R) correctly explains (A).
Solution by Sreeraj P, M.Sc Physics