Question

Calculate the smallest thickness of the soap bubble which produces a constructive interference for a red light of the wavelength $650\text{nm}$.

Take refractive index to be $1.33$.

• A. $112.2\text{nm}$
• B. $102.2\text{nm}$
• C. $132.2\text{nm}$
• D. $122.2\text{nm}$

Answer 1

The correct option is D $122.2\text{nm}$

For the constructive interference in soap film,

$2\mu t=(n-1/2)\lambda$

For minimum thickness, $n=1$

$\therefore 2\mu t=\lambda /2$

$\Rightarrow t=\frac{\lambda }{4\mu }=\frac{650}{4\times 1.33}=122.2\text{nm}$

Hence, option $\left(A\right)$ is the correct answer.