Write down the formula for distance between two consecutive bright fringes or two consecutive dark fringes.

Explanation

For fringes of constructive interference (bright),
`x_(n)=(nlamdaD)/(d)` for `n^(th)` order of bright fringe.
Now for `(n+1)^(th)` order of bright fringe.
`x_(n+1)=((n+1)lamdaD)/(d)`
Distance between two consecutive bright fringes
`x._(n+1)-x._(n)=(2n+3)(lamdaD)/(2d)-(2n+1)(lamdaD)/(2d)`
`=(lamdaD)/(2d)[2n+3-2n-1]`
`=(lamdaD)/(2d)[2]`
`:.beta=(lamdaD)/(d)`
Hence, the distance between two consecutiv bright or the distance between two consecutiv dark fringe (s) are same. This is the equation width of fringe also.