Question

For light diverging from of finite point source 

• A. The wave front is parabolic
• B. The wave front is cylindrical
• C. The intensity at the wave front does not depend on the distance
• D. The intensity decreases in proportion to the distance squared

Answer 1

The Correct Option is D

The inverse square law of light propagation states that the intensity of light decreases in proportion to the square of the distance from the source. This means that as light spreads out from a point source, its intensity (or brightness) diminishes with the square of the distance from the source. Therefore, the correct statement is:

(D) The intensity decreases in proportion to the distance squared.

Answer 2

For light diverging from a finite point source:

The wave front is spherical, not parabolic. When light spreads out from a point source, it forms concentric spherical wave fronts around the source.

The wave front appears cylindrical only in the far-field (Fraunhofer) region, where the distance from the source is significantly greater than the source's dimensions.

The claim that intensity at the wave front remains unchanged with distance is incorrect. As light propagates outward, its intensity decreases due to the spreading of light over a larger area.

The intensity decreases in proportion to the square of the distance. This follows the inverse-square law, meaning the intensity III is inversely proportional to the square of the distance rrr from the source:
\(I \propto \frac{1}{r^2}\)

Thus, the correct statement for light diverging from a finite point source is:
"The intensity decreases in proportion to the distance squared."