In a biprism experiment , fringe width is measured as 0.4 mm . When the eyepiece is moved away from the biprism through 30 cm, the fringe width increase by 50% if the two virtual soruces are 0.6 mm apart, find the wavelength of light used.

Explanation

Here `beta_(1)=0.4 mm`
`=0.4xx10^(-3)m`
`beta_(2)=(150)/(100)beta_(1)=1.5beta_(1)`
`D_(2)=D_(1)+30 cm`
`= D_(1)+0.3 m, d`
`=0.6 mm = 0.6xx10^(-3)m`
As `beta_(1)=(D_(1)lambda)/(d)`
and `beta_(2)=(D_(2)lambda)/(d)`
`therefore (beta_(1))/(beta_(2))=(D_(1))/(D_(2))`
or `(beta_(1))/(1.5beta_(1))=(D_(1))/(D+0.3)`
or `D_(1)=0.6 m`
`therefore` Wavelenth of light used,
`lambda = (beta_(1)d)/(D_(1))=(0.4xx10^(-3)xx0.6xx10^(-3))/(0.6)`
`=4xx10^(-7)m=4000 Å`