Question

In a Young's double slit experiment setup with monochromatic light, fringes are obtained on a screen
placed at a certain distance from the slits. If the screen is moved by 5 cm towards the slits, the
change in fringe width is 20 um. Given the distance between two slits to be 1.2 mm, calculate the
wavelength of the light used.
​

Answer 1

Given info : In a Young's double slit experiment setup with monochromatic light, fringes are obtained on a screen placed at a certain distance from the slits. If the screen is moved by 5 cm towards the slits, the change in fringe width is 20 μm. and the distance between two slits to be 1.2 mm.

To find : The wavelength of the light used is..

solution : here, distance between slits, d = 1.2 mm = 1.2 × 10^-3 m

shifting screen towards slits, ∆D = 5 cm = 5 × 10^-2 m

change in fringe, ∆β = 20μm = 20 × 10^-6 m

using formula, β = λD/d

so, ∆β = λ∆D/d

now, 20 × 10^-6 = (λ × 5 × 10^-2)/(1.2 × 10^-3)

⇒24 × 10^-9 = λ × 5 × 10^-2

⇒ λ = 4.8 × 10^-7 m = 480 nm

Therefore the wavelength of light used in the Young's double slit experiment is 480nm.