Question

White light, with a uniform intensity, is perpendicularly incident on a water film of refractive index $ 1.33 $ and thickness $ 320 \,nm $ , that is suspended in air. At what wavelength is the light reflected by the film brightest to an observer?

• A. 459 nm
• B. 567 nm
• C. 623 nm
• D. 690 nm

Answer 1

Answer: (B)

The film appears bright due to interference of reflected rays
The film will appear bright, when
$2\mu t \,cos\,r=\left(2n+1\right)\frac{\lambda}{2}$
$\Rightarrow 2\times1.33\times320 \,nm\,\cos\,0^{\circ}=\left(2\times1+1\right)\frac{\lambda}{2}$
$\Rightarrow \lambda=567\,nm$