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 $$0.25 \mu \mathrm{m},$$ with air between them.Explain how it can also appear violet if light falls on it in a different direction.

Solution

## For light incident at grazing angle $$60^{\circ},$$We  know that$$2 d \sin \theta=m \lambda$$ gives $$2\left(0.25 \times 10^{-6} \mathrm{m}\right) \sin 60^{\circ}=\lambda=433 \mathrm{nm} .$$ This is violet.PhysicsNCERTStandard XII

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