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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.
1861380_4cd0f547df1e481f9d2407a16254edc4.png


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.

Physics
NCERT
Standard XII

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