Question

A mixture of yellow light of wavelength 580nm and blue light of wavelength 450nm is incident normally on an air film of thickness 2.9x10^2 nm. The colour of the first order of reflected light is

A. Red

B. Blue

C. Violet

D. Yellow

Please also explain what is meant by first order reflected light and how the answer is B.

Answer 1

Bragg law
2 d sinθ = nλ means that d is for particular Miller indices (hkl) as d h,k,l:
2 d h,k,l sinθ = nλ
it could be written as: 2 (d h,k,l /n) sinθ = λ
which is equivalent to 2 d nh,nk,nl sinθ = λ and written as (in short):
2 d sinθ = λ
means order of reflection is included in d, which is for particular Bragg indices (nh,nk,nl) here.