Question

Iridescent peacock feathers are shown in Figure P38.64a. The surface of one microscopic barbule is composed of transparent keratin that supports rods of dark brown melanin in a regular lattice, represented in Figure P38.64b. (Your fingernails are made of keratin, and melanin is the dark pigment giving color to human skin.) In a portion of the feather that can appear turquoise (blue-green), assume the melanin rods are uniformly separated by $0.25\mu m,$ with air between them.
Explain how it can also appear violet if light falls on it in a different direction.

Answer 1

For light incident at grazing angle ${60}^{\circ },$

We know that

$2d\sin \theta =m\lambda$ gives

$2(0.25\times {10}^{-6}m)\sin {60}^{\circ }=\lambda =433nm.$

This is violet.