Question

Assertion :In YDSE, if separation between the slits is less than wavelength of light, then no interference pattern can be observed. Reason: For interference pattern to be observed, light sources have to be coherent.

• 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 B Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion

Reason is true. Light sources have to be coherent for interference pattern to be observed. But this is not the explanation for the assertion.

Explanation is that for a YDSE interference pattern, fringe width is given by $\beta =\frac{D\lambda }{d}$. Now if $\lambda >d$, fringe width would be too large. Thus even the first minima would be observed too far from the central maxima, which would not be possible experimentally.