Two coherent monochromatic point sources `S_1` and `S_2` are placed in front of an infinite screen as shown in figure. Wavelength of the light emitted by both the sources is zero. Initial phase difference between the sources is zero.

Initially `S_(1)S_(2)=2.5 lambda` and the number of bright circular rings on the screen in `n_(1)`. if the distance `S_(1)S_(2)` is increased and made `5.7 lambda`, the number of bright circular rings becomes `n_(2)`. the difference `n_(2)-n_(1)` is

Explanation

For `S_(1)S_(2)=2.5 lambda`, max path difference `=2.5 lambda`
min path difference =0
Between `2.5 lambda` and 0 lie `2 lambda` and `lambdarArr ` two circular bright fringes
`n_(1)=2`
For `S_(1)S_(2)=5.7 lambda`, max path difference =`5.7 lambda`
min path difference =0
Between `5. 7lambda` and 0 lie `5lambda,4lambda,2lambda,lambdarArr ` five circular bright fringes.
`rArr n_(2)=5`
`:. n_(2)-n_(1)=5-2=3`