A soap bubble has an index of refraction of 1.33. What minimum thickness of this bubble will ensure maximum reflectance of normally-incident light having a wavelength of 530 nm?
A) 199 nm
B) 76.9 nm
C) 398 nm
D) 24.9 nm
E) 99.6 nm

Question

Quizplus · Accepted Answer

For maximum reflectance in thin films under normal incidence, the path difference should be an odd multiple of half wavelengths in the medium. The minimum thickness (t) for maximum reflectance is thus given by $t = \frac{\lambda}{4n}$, where $\lambda$ is the wavelength of light in vacuum and $n$ is the refractive index of the film. Substituting $\lambda = 530$ nm and $n = 1.33$, we get $t = \frac{530}{4 \times 1.33} \approx 99.6$ nm.

A Soap Bubble Has an Index of Refraction of 1