Question

In biprism experiment, the 3^rd dark band is formed opposite to one of the slits. The wavelength of light used is ______.

(D = distance between source and screen, d = distance between the two narrow slits)

Options

• `d^2/(5 D)`

• `d^2/(7 D)`

• `d^2/(9 D)`

• `d^2/(3 D)`

Answer 1

In biprism experiment, the 3^rd dark band is formed opposite to one of the slits. The wavelength of light used is `bbunderline(d^2/(5 D))`.

Explanation:

Distance of n^th dark fringe from the centre of the interference pattern is,

y_n = `(n - 1/2) (D lambda)/d`

As the 3^rd dark fringe is formed opposite to one of the slits, its distance from the centre of the pattern will be `d/2`.

∴ `d/2 = (3 - 1/2) (D lambda)/d`

∴ `d/2 = (5/2) (D lambda)/d`

∴ λ = `d^2/(5 D)`