Question

A monochromatic light beam of wavelength λ = 550 nm is directed at a film of water at 23° from the vertical as shown. What is the minimum thickness of the film such that I see the brightest reflection?

Answer 1

As per the given question,

Wavelength of the wave $(\lambda)=550nm$

$\theta=23^\circ$

$\sin \theta =n\frac{\lambda}{d}$

from bright fringe n = 1

Now, substituting the values

$\sin 23^\circ =1\times \frac{550\times 10^{-9}m}{d}$

$\Rightarrow d = \frac{5.5\times 10^{-9}}{0.39}$

$\Rightarrow d = 14.1\times 10^{-9}m$

$d=0.0141\mu m$