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