Question

What is the minimum thickness of a soap film needed for constructive interference in reflected light, if the light incident on film is of wavelength $900\text{nm}$ ?
Refractive index of film is $1.5$.

• A. $100\text{nm}$
• B. $150\text{nm}$
• C. $200\text{nm}$
• D. $250\text{nm}$

Answer 1

The correct option is B $150\text{nm}$

Condition for constructive interference,
$2\mu t=(2n+1)\frac{\lambda }{2}$
$\Rightarrow t=\frac{(2n+1)\times 900}{4\times 1.5}$
(for minimum thickness, $n=0$)
$\therefore t=150\text{nm}$