A thin film of oil of $\mu =1.2 $ floats on watch of $\mu_w =\large\frac{4}{3}$. A monochromatic light of wave length $\lambda= 9600 A^{\circ} $ falls normally and it appears dark. What must be minimum change in thickness of oil film which will appear bright if normally reflected by same light.

Question

$(a)\;10^{-7}\;m \\ (b)\;2 \times 10^{-7}\;m \\ (c)\;3 \times 10^{-7} \\ (d)\;5 \times 10^{-7}\;m $