Question 21N.1.HL.TZ0.28

Monochromatic light of wavelength $\lambda$ in air is incident normally on a thin liquid film of refractive index $n$ that is suspended in air. The rays are shown at an angle to the normal for clarity.

What is the minimum thickness of the film so that the reflected light undergoes constructive interference?

A.  $\frac{\lambda }{4n}$

B.  $\frac{\lambda }{3n}$

C.  $\frac{\lambda }{2n}$

D.  $\frac{\lambda }{n}$