Question

Two light beams of intensities $4 I$ and 9I interfere on a screen. The phase difference between these beams on the screen at point $A$ is zero and at point $B$ is $\pi$. The difference of resultant intensities, at the point A and B, will be___ $I$

Answer 1

$ I _{ net }= I _1+ I _2+2 \sqrt{ I _1} \sqrt{ I _2} \cos \phi $
$ I _{\max } \text { for } \phi=0 \& I _{\min } \text { for } \phi=\pi $
$ I _{\max }=\left(\sqrt{ I _1}+\sqrt{ I _2}\right)^2=(\sqrt{9 I }+\sqrt{4 I })^2=25 I $
$ I _{\min }=\left(\sqrt{ I _1}-\sqrt{ I _2}\right)^2=(\sqrt{9 I }-\sqrt{4 I })^2= I $
$ I _{\max }- I _{\min }=25 I - I =24 I $