Question

A soap film of thickness $t$ is surrounded by air and is illuminated at near normal incidence by monochromatic light with wavelength $\lambda$ in the film. With respect to the wavelength of the monochromatic light in the film, what film thickness will produce maximum constructive interference?

• A. $\frac{1}{4}\lambda$
• B. $\frac{1}{2}\lambda$
• C. $1\lambda$
• D. $2\lambda$

Answer 1

Answer: (A)

$\frac{1}{4}\lambda$

Since phase change by takes place at 'a' in the figure due to reflection from optically denser medium, the condition for constructive interference is given by-

$2t=(m+\frac{1}{2})\lambda$

For m=0,

$t=\frac{\lambda }{4}$