Question

Assertion [A]: For light diverging from a point source, the intensity at the wavefront does not depend on the distance.
Reason [R]: In a diverging beam of light from a point source, spherical wavefront is observed.

• A. and (R) are true and (R) is the correct explanation of (A).
• B. Both (A) and (R) are true, but (R) is not the correct explanation of (A).
• C. is true but (R) is false.
• D. is false but (R) is true.

Hint:

For a point source, intensity decreases with distance (\(I \propto 1/r^2\)) even though wavefronts are spherical.

Answer 1

The Correct Option is D

This is an assertion-reason question, so we judge each line on its own first and only then ask whether one explains the other.

The assertion claims that the intensity at a wavefront spreading out from a point source stays the same no matter how far you travel from the source. That is not right. As a spherical wavefront expands, the same total energy spreads over a bigger sphere of area $4\pi r^2$, so the intensity actually falls as $I \propto 1/r^2$. So the assertion is false.

The reason says that a point source sends out spherical wavefronts. That part is true, every point on the wavefront is the same distance from the source, so the wavefront is a sphere.

So we are left with a false assertion sitting next to a true reason. Since the assertion itself is wrong, there is no question of the reason "explaining" it.

So the correct choice is option (4): (A) is false but (R) is true.