Chapter 45: Problem 32

Question: Light of wavelength $\lambda~=~585~{\rm nm}$ is incident normally on a thin soapy film (n = 1.33) suspended in air. If the film is $d~=~0.00121~{\rm mm}$ thick, determine whether it appears bright or dark when observed from a point near the light source.

Solution: Forming the quantity ${2 n d\over\lambda}~=~5.50$, and so we have the condition for a m = 5 maximim reflection as we have a $\pi$ phase change at the first interface but no phase change at the second interface.