Question

Light is incident normally on a grating 0.25 cm wide with 1250 lines. Find the angular separation of the two sodium lines in the first order spectrum. Can they be seen distinctively if the lines are 5895 Å & 5901 Å.

Answer 1

Data : Number of lines / cm (N)= 5000

m=1

λ1=5895 Å

λ2=5901 Å

Formula :

(a+b)sinθ=Nλ

mN=λ/dλ

Calculations : 

---1

----2

---3

Number of lines / cm required for grating is 983

Number of lines / cm available for grating is 5000

angular separation is 0.0180

This lines will be resolved well by grating