Question

If Young's double slit experiment is performed in water,
(a) the fringe width will decrease
(b) the fringe width will increase
(c) the fringe width will remain unchanged
(d) there will be no fringe

Answer 1

(a) the fringe width will decrease

As fringe width is proportional to the wavelength and wavelength of light is inversely proportional to the refractive index of the medium,
Here, $$\begin{gathered} {\lambda }_{{\rm M}}=\lambda /\eta \\ {\lambda }_{{\rm M}}=\mathrm{wavelength}\mathrm{in}\mathrm{medium} \\ \lambda =\mathrm{wavelength}\mathrm{in}\mathrm{vacuum} \\ \eta =\mathrm{refractive}\mathrm{index}\mathrm{of}\mathrm{medium} \end{gathered}$$
Hence, fringe width decreases when Young's double slit experiment is performed under water.