Question

Two waves of same frequency but of amplitudes $a$ and $2a$ respectively superimpose over each other. The intensity at a point where the phase difference is $3\pi /2$, will be

• A. $a$
• B. $3a^2$
• C. $5a^2$
• D. $9a^2$

Answer 1

The correct option is C $5a^2$

The intensity at any point is directly proportional to the square of amplitude: $I\propto a^2$

when phase difference is an odd multiple of 90, the dot product goes to 0.

Hence, $I=a^2+(2a{)}^2$ Therefore, $I=5a^2$