Two coherent sources of light $S_1 $ and $S_2$ are placed in a line as shown. Two screens are placed perpendicular to $S_1S_2$ at A and B . It is observed that at point P a bright fringe is seen , then at point Q.

Question

(a)dark fringe occurs

(b)bright fringe occurs

(c)no fringe is seen

(d)screen B will be completely bright 

Answer 1

The path difference at point A due to light from source at $S_1 $ and $S_2$ is

$ S_2 A -S_2 A =S_2 -S_1 $

Since A is a bright point $|S_2 -S_1| $ must be an integral multiple of $\lambda$

At B also the path difference is $S_1 B -S_2 B =|S_1 -S_2| $

Which is an integral multiple of X

$\therefore$ B is also a bright point .

Hence d is the correct answer.