What will be effect on diffraction pattern of grating if i) the number of lines in grating is increased. ii) grating element is of the order of a wavelength or less iii) if the rulings are

Subject: Applied Physics 2

Topic: Interference And Diffraction Of Light

Difficulty: High

1 Answer

i) with the increase in number of lines the secondary maxima relative to the principal maxima decreases and becomes negligible. The principal maxima become more sharp and intense.

ii) we know (a+b) sinθ = nλ

For the first order minima ,Sinθ =λ/(a+b)

If a+b = λ, then

Sinθ = 1 or θ =90°

Thus, a theoretical limit is reached the first order maximum will not be visible.

If (a+b) <λ sinθ becomes greater than 1, which is not possible. Hence no spectrum is formed and the grating is rendered useless.

iii) If the rulings are made closer, the angular spacing between the maxima increases.

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