Question

Assertion :In Young's double-slit experiment, the two slits are at distance $d$ apart. Interference pattern is observed on a screen at distance $D$ from the slits. At a point on the screen which is directly opposite to one of the slits, a dark fringe is observed. Then, the wavelength of wave is proportional to the square of distance between the two slits. Reason: For a dark fringe, intensity is zero.

• 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

If at the point on the screen which is directly opposite to one of the slits, a dark fringe is observed, then $y=\frac{D\lambda }{2d}=\frac{d}{2}$

Thus $\lambda \propto d^2$

Also it is true that intensity of a dark fringe is zero.