If sunlight shines straight onto a peacock feather, the feather appears blue whe

Question

If sunlight shines straight onto a peacock feather, the feather appears blue when viewed from 24° on either side of the incident beam of sunlight. The blue color is due to reflection interference from the melanin bands in the feather barbules (i.e., they behave like a diffraction grating). Blue light with a wavelength of 465 nm appears brightest at 24° by these bands (this is the m=1 interference peak) while other wavelengths in the sunlight appear brightest at different angles. What is the spacing of the melanin bands in the feather? (answer in micro meters)

Explanation / Answer

The spacing of the melanin bands is found by this formula:

d = /sin()

We know that...

= 24 degree
= 465 nm

So...
d sin theta =m lemda

d=m lemda / sin theta

d = (1)(465 nm) (10^-9/1)/ sin(24°)(0.0174533rad/1 degree)

=1.14*10^-6 m

=1.14 micro meter

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