Question

The two light beams having intensities $I$ and $9I$ interfere to produce a fringe pattern on a screen. The phase difference between the beams is $\frac {\pi}{2}$ at point P and $\pi$ at point Q. Then the difference between the resultant intensities at P and Q will be:

• A. $2I$
• B. $6I$
• C. $5I$
• D. $7I$

Answer 1

Answer: (B)

$6I$

Answer 2

$IP = I+9I+2\sqrt {I\times9I} \ cos \frac {\pi}{2}= 10I$
$IP = I+9I+2\sqrt {I\times9I} \ cos \pi= 14I$
Then, the difference between the resultant intensities
$I_P - I_Q = 6I$

Hence, the correct option is (B): $6I$