Question

Assertion :Two coherent point sources of light having non-zero phase difference are separated by a small distance. Then, on the perpendicular bisector of line segment joining both the point sources, constructive interference cannot be obtained. Reason: For two waves from coherent point sources to interfere constructively at a point, the magnitude of their phase difference at that point must be $2m\pi$ (where $m$ is a non-negative integer).

• A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
• B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
• C. Assertion is correct but Reason is incorrect
• D. Assertion is incorrect but Reason is correct

Answer 1

The correct option is A Both Assertion and Reason are correct and Reason is the correct explanation for Assertion

Consider light from two sources A and B, travelling to interference on the screen in between them at equal distances from them.

The rays travel equal path$=\sqrt{d^2+x^2}$ to meet at any point D on the screen.

So they do not have any path difference between them. But they have a constant non zero phase difference.

So the waves would not have a phase difference of $2m\pi$, so the constructive interference cannot take place.