A parallel coherent beam of light falls on fresnel biprism of refractive index `mu` and angle `alpha` .The fringe width on a screen at a distance `D` from biprism will be (wavelength =`lambda`)

Options

A. `(lamda)/(2(mu-1)alpha)`
B. `(lamdaD)/(2(mu-1)alpha)`
C. `(D)/(2(mu-1)alpha)`
D. none of these

Answer

A

Explanation

`beta=((a+b)lamda)/(2a(mu-1)alpha)=((1+(b)/(a))lamda)/(2(mu-1)alpha)`
For parallel beam, `a=infty,` so `beta=(lamda)/(2(mu-1)alpha)`