Question

A bowl of water (n = 1.33) has a thin layer of grease (n = 1.60) floating on top of the water. When you illuminate the grease from above with monochromatic light, you get constructive reflection if the wavelength in air is 565 nm. Find the smallest possible thickness of the grease layer.

Answer 1

The grease layer must be 1/4 wavelength thick for constructive interference to occur (includes λ/2 phase change at grease water interface

λ/ 4 = 565E-9 m / (4 * 1.6) = 88 nm since the wavelength inside of the grease is 1/1.6 of the wavelength in air