Two mirror `M_(1)` and `M_(2)` make an angle `phi` with line `AB`. A point source `S` is kept at a distance `r` from the point of intersection of mirror. A small hemispherical annular mask is kept close to `S` so that no ray emanting from `S` directly reaches the screen kept at a distance `b` from the source. Interference pattern of rays reflected from `M_(1)` and `M_(2)` is oberved on the screen. Find the fringe width of this interference pattern `lambda=` wavelength of light being emitted by source.

Options

A. `(lambda(b+2rsin^(2)phi))/(2rsin2phi)`
B. `(lambda(b+2rcos2phi))/(2rsin2phi)`
C. `(lambda(b+rcos2phi))/(2rsin2phi)`
D. `(lambda(b+2rcos2phi))/(rsin2phi)`

Explanation

All light rays hitting the screen will be appearing to be coming from `S'_(1)` and `S'_(2)`
`rArr D=b+2r sin^(2) phi`
`d=4r sinphi cosphi`
fringe width `=(lambdaD)/(d)`