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Find the minimum number of lines required in a grating of width 2 cm so that the two Na lines of 5890 & 5896 $\mathring{A}$ are just resolved in the 1st order spectra.

Subject: Applied Physics 2

Topic: Interference And Diffraction

Difficulty: Medium

1 Answer
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RP = $\frac{\lambda}{d\lambda}$ = nN

$λ = \frac{5890+5896}{2} = 5893 A° $

dλ = 5896-5890 = 6 A°

$\frac{λ}{dλ}$ = 982

982 = 1 x N

Therefore N=982 lines /cm

For grating of width 2 cm N = 982 x2 = 1964

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