Question

Among two interfering sources, let $S_1$ be ahead of the phase by ${90}^{\circ }$ relative to $S_2$. If an observation point P is such that $P{S}_1-P{S}_2=1.5\lambda$, the phase difference between the waves from $S_{1}and{S}_2$ reaching P is

• A. $3\pi$
• B. $\frac{5\pi }{2}$
• C. $\frac{7\pi }{2}$
• D. $4\pi$

Answer 1

Answer: (B)

$\frac{5\pi }{2}$

Given length difference between:

$P{S}_1-P{S}_2=\frac{3}{2}\lambda$

length difference of $\lambda$ implies $2\pi radians$

length difference of $\frac{3}{2}\lambda$ implies $3\pi radians$ (which

is the difference in phase which would exist due to the length difference.

Hence, the first wave also leads by $\frac{\pi }{2}$ This implies the

effective phase difference would be:

$3\pi -\frac{\pi }{2}=\frac{5\pi }{2}$

Hence option B.