The distances of a point on the screen from two slits in biprism experiment is `1.8xx10^(-5)` m and `1.23xx10^(-5)` m if wavelength of light used is 6000 Å then fringe formed at that point is

Options

A. 10th bright
B. 10th dark
C. 9th bright
D. 9th dark

Answer

B

Explanation

Path difference between light rays coming from two slits,
`Deltax=1.8xx10^(-5)-1.23xx10^(-5)=10^(-5)(1.8-1.23)`
`=0.57xx10^(-5)=57xx10^(-7)=57000xx10^(-10)=57000` Å
It not an integral multiple of wavelength. Thus, only dark fringes are possible. For nth dark fringe, `Deltax=(2n-1)(lambda)/(2)`
`rArr 57000 Å=(2n-1)xx(6000"Å")/(2)`
`rArr 2n-1=2xx(57)/(6)=(57)/(3)=19`
`rArr 2n-1=19 or 2n=20 or n=10`
Thus, 10th dark fringe formed.