Given below are two statements: one is labelled as Assertion (A)...

The angular fringe width in Young's double slit experiment is given by $\theta = \dfrac{\lambda}{d}$, where $\lambda$ is the wavelength of light and $d$ is the distance between the slits.

When the apparatus is immersed in water (a denser medium), the wavelength of light decreases to $\lambda' = \dfrac{\lambda}{\mu}$, where $\mu$ is the refractive index of water.

Since the angular fringe width is directly proportional to the wavelength ($\theta \propto \lambda$), a decrease in wavelength results in a decrease in the angular fringe width. Therefore, the Assertion is correct.

The Reason correctly states that wavelength decreases in a denser medium and that angular fringe width is directly proportional to wavelength. Thus, the Reason is correct and provides the exact explanation for the Assertion.

Answer: Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.