a thin film of water (refractive index= 4/3) is 3100 angstom thick. if it is illuminated by a white light and normal incidence . the color of film in the reflected light will be

1) blue

2) black

3) yellow

4) red

Given,
$$\begin{gathered} \mathrm{Refractive} \mathrm{index} \mathrm{of} \mathrm{water}=4/3 \\ \mathrm{Thickness} \mathrm{of} \mathrm{film}=3100 \stackrel{0}{\mathrm{A}}=310\times {10}^{-9} \mathrm{m} \\ \mathrm{For} \mathrm{constructive} \mathrm{interference}, \\ \mathrm{nt}=(\mathrm{m}+1/2)\mathrm{\lambda }/2 \\ \mathrm{Thus}, \mathrm{the} \mathrm{wavelengths} \mathrm{that} \mathrm{are} \mathrm{strongly} \mathrm{reflected} \mathrm{satisfy}, \\ \mathrm{\lambda }=\frac{2\mathrm{nt}}{(\mathrm{m}+1/2)} \\ \mathrm{where}, \mathrm{m}=0,1,2,.... \\ {\mathrm{\lambda }}_1=\frac{2\times 1.33\times 310\times {10}^{-9}}{1/2}=1649\mathrm{nm} (\mathrm{Infrared} \mathrm{region}) \\ {\mathrm{\lambda }}_2=\frac{2\times 1.33\times 310\times {10}^{-9}}{3/2}=549 \mathrm{nm} \\ {\mathrm{\lambda }}_3=\frac{2\times 1.33\times 310\times {10}^{-9}}{5/2}=330 \mathrm{nm} (\mathrm{Ultraviolet} \mathrm{region}) \end{gathered}$$
 Clearly, 1649 nm and 330 nm wavelengths correspond to light which is invisible to the human eye. The only strong reflection occurs at 549 nm, which corresponds to the blue-violet region of the spectrum.
The reflected light is weak in the red region of the spectrum and strong in the blue-violet region. The film will, therefore, possess a pronounced blue colour.

The correct option is (1) Blue