) When monochromatic light of wavelength 5.0 x 10"m is incident normally on a plane
diffraction grating, the second-order diffraction lines are formed at angles of 30° at the
normal to the grating. What is the number of the lines per millimetre of the grating?
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Answer:
Diffraction grating: d sinθn = nλ where n is an integer
λ=5.30×10
−7
m
When n = 1, θ1 = 15.4°
Slit separation, d =
sinθ
1
λ
=
sin15.4
o
5.30×10
−7
= 1.996×10
−6
m
When n =2
slit separation d =
sinθ
2
λ
⟹sinθ
2
=
d
λ
⟹θ
o
=sin
−1
(
1.996×10
−6
5.30×10
−7
)
⟹θ
2
=16.7
o
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