Question

How many orders will be observed by grating having 4000 lines/cm, if it is illuminated by light of wavelength in the range 5000 to 7500 A˚?

Answer 1

Grating equation is (a+b) sinθ =nλ , N= 4000 lines/cm

For maximum orders sinθ = 1

Therefore , \(n_{max} = \frac{(a+b)}{λ} = \frac{1}{Nλ} \)

For 5000 Å, \(n_{max} = \frac{1}{4000 \times 5000 \times 10^{-8}} = 5\)

For 7500 Å, \( n_{max} = \frac{1}{4000 \times 7500 \times 10^{-8}} = 3 \)

So, in the wavelength range, 5000 to 7500 Å ,the observed number of orders range between 3 to 5