A parallel beam of monochromatic light ofwavelength 5000Å is inc...
Å is incident on a diffraction grating with 500 lines per mm. The diffraction pattern is observed on a screen placed at a distance of 1 m from the grating.
(a) What is the distance between two consecutive bright fringes on the screen?
(b) What is the angular separation between the first-order and second-order maxima?
(c) How many bright fringes will be observed on the screen if the width of the grating is 4 cm?
(d) What is the width of the central maximum on the screen?
Solution:
(a) The distance between two consecutive bright fringes can be calculated using the formula:
d sinθ = mλ
where d is the distance between the adjacent slits in the grating, θ is the angle between the incident beam and the direction of the mth maximum, m is the order of maximum, and λ is the wavelength of the incident light.
In this case, d = 1/500 mm = 2 × 10^-3 cm, m = 1, and λ = 5000 Å = 500 nm. Therefore, we have:
sinθ = mλ/d = 500 × 10^-9 / (2 × 10^-3) = 0.25
θ = sin^-1(0.25) = 14.48°
The distance between two consecutive bright fringes can be calculated using the formula:
y = D tanθ
where y is the distance between two consecutive bright fringes on the screen, and D is the distance between the grating and the screen.
In this case, D = 1 m. Therefore, we have:
y = D tanθ = 1 × tan(14.48°) = 0.25 m
Therefore, the distance between two consecutive bright fringes is 0.25 m.
(b) The angular separation between the first-order and second-order maxima can be calculated using the same formula as above:
θ2 - θ1 = sin^-1(mλ/d) - sin^-1((m-1)λ/d)
where m = 1 for the first-order maximum and m = 2 for the second-order maximum.
In this case, d = 1/500 mm = 2 × 10^-3 cm and λ = 5000 Å = 500 nm. Therefore, we have:
θ2 - θ1 = sin^-1(500 × 10^-9 / (2 × 10^-3)) - sin^-1(2 × 500 × 10^-9 / (2 × 10^-3))
θ2 - θ1 = 14.48° - 28.96° = -14.48°
Therefore, the angular separation between the first-order and second-order maxima is 14.48°.
(c) The number of bright fringes that will be observed on the screen can be calculated using the formula:
N = w/d
where N is the number of bright fringes, w is the width of the grating, and d is the distance between adjacent slits in the grating.
In this case, w = 4 cm and d = 1/500 mm = 2 × 10^-3 cm. Therefore, we have:
N = w/d = 4 × 10^-2 / (2 × 10^-3) = 20