Question

In a biprism experiment, the slit is illuminated by red light of wavelength 6400 A and the crosswire of eyepiece is adjusted to the centre of 3^rd bright band. By using blue light it is found that 4^th bright band is at the centre of the cross wire. Calculate the wavelength of blue light.

Answer 1

For bright band `x_n =(nDλ)/d` 

For red light, distance of 3^rd bright band is

`x_3=(3Dλ_R)/d`

For blue light, distance of 4^th bright band is

`x_4=(4Dλ_B)/d`

By condition,

(Distance of 3^rd bright band of red light) = (Distance of 4th bright band of red light)

`(3Dλ_R)/d = (4Dλ_B)/d`

`3/4λ_R =λ_B`

`λ_B = 3/4xx6400`

λ_B = 4800 A