In: Physics
A diffraction grating is made up of slits of width a with separation d. The grating is illuminated by monochromatic plane waves of wavelength λ at normal incidence. What is the angular width of a spectral line observed in the first order if the grating has N slits? State your answer in terms of the given variables. Δθw =
We know from diffraction equations
dsin
 = m
-------------- (1)
for first order, m = 1
The angular width is given as
w =
 /
Ndcos
w =
dsin
 / Ndcos
w =
sin
 / Ncos
from trigonometry, we know that
sin
 / cos
 =
tan
so
w =
tan
 / N
from equation (1)
dsin
 = 
sin
 = 
 /
d
 = arcsin
(
 / d)
so,
w =
tan (arcsin (
 / d)) / N