c
(c)Suppose slit width’s are equal, so they produces waves of equal intensity say \(I'.\) Resultant intensity at any point \({I_R} = 4\,I'{\cos ^2}\phi\) where \(\phi\) is the phase difference between the waves at the point of observation.
For maximum intensity \(\phi= {0^o} \Rightarrow {I_{\max }} = 4I' = I\)…\((i)\)
If one of slit is closed, Resultant intensity at the same point will be \(I'\)only i.e. \(I' = {I_O}\)…\((ii)\)
Comparing equation \((i)\) and \((ii)\) we get
\(I = 4{I_O}\)