The maximum intensity of fringes in Young's experiment is I . If one of the slit is closed, then the intensity at that place becomes I0. Which of the following relation is true ?

Suppose slit width’s are equal, so they produces waves of equal intensity say  I'. Resultant intensity at any point IR=4 I'cos2φ  where φ  is the phase difference between the waves at the point of observation. For maximum intensity  φ=0o⇒Imax=4I'=I …(i)If one of slit is closed, Resultant intensity at the same point will be I' only i.e.I'=IO   …(ii) Comparing equation (i) and (ii) we get I=4I0