In young's double slit experiment, a beam of parallel rays are incident on the slits making an angle of 60owith the plane of the slits as shown in the figure. Separation between the slits is d and D>>d. Then distance of the central bright fringe from the mid point o of the screen is

If P be the position of control bright fringe on the screen then path difference between the rays at position P must be zero. Initial path difference.NS1=d cos 60o=d/2 After emerging from the slits, path difference between the rays =S2M=d sin αNet path difference S2M-S1N=(d sinα-d/2)=0 ⇒sinα=1/2 ⇒α=30o ∴OPD=tan α ⇒OP=D tan 30o=D/3.