The distances from a Fresnel prisum to a narrow slit and a screen are equal to `a = 25 cm` and `b = 100 cm` respectively. The refracting angles of the glass biprism is equal to `theta = 20`. Find the wavelength of light if the width of the fringe on the screen is `Delta x = 0.55 mm`.

Explanation

`Deltax = (l lambda)/(d)`
`l = a+ b`
`d = 2(n - 1) theta a`
`del = (n - 1) theta`
`d = 2del.a`
`n = R.I.` of glass
Thus `lambda = (2(n - 1) theta a Deltax)/(a + b) = 0.64 mu m`.