Question

A water film $(\mu =1.33)$ is illuminated normally by a light $\lambda =6.65nm$. In order that the film appears bright, the minimum thickness of the film is:

• A. 3.325 nm
• B. 1.16 nm
• C. 2.5 nm
• D. 1.25 nm

Answer 1

The correct option is D 1.25 nm

$2\times 1.33tcosr=(2n+1)\frac{\lambda }{2}$
Form minimum thickness, n = 0, cosr = 1
$\therefore 2.66t=\frac{6.65\times {10}^{-9}}{2}nm=1.25nm$