Question

Which one of the following plane transmission gratings of width W , and number of lines per centimeter N , will have the maximum resolving power in the first order?

(a) W = 1 cm N = 5000

(b) W = 1.5 cm, N = 4000

(c) W = 2 cm, N = 2400

(d) W = 3 cm, N = 2400

Answer 1

Correct option (b) W = 1.5 cm, N = 4000

Explanation:

The resolving power of a grating is given as resolving power = nN

Where n is order and N is total number of line of grating. Thus, resolving power of a grating is proportional to the total number of liens on it.

(a) total number of lines = W x N = 1 x 5000 = 5000

(b) total number of lines = 1.5 x 4000 = 6000

(c) total number of lines = 2 x 2400 = 4800

(d) total number of lines = 3 x 1500 = 4500

Hence, total numbers of lines are maximum for option (b).