Final answer:
The inverse-square law states that the intensity of light or radiation decreases with the square of the distance from the source, which is answer (a). This applies to both light propagation and gravitational forces.
Step-by-step explanation:
Astronomers use the term inverse-square law to describe how the intensity of light or radiation decreases as the square of the distance from the source. The correct answer to the provided question is (a) The intensity of light or radiation decreases with the square of the distance from the source. This phenomenon is comparable to the way gravity works, where the force of gravity is also inversely proportional to the square of the distance between two masses.
As light radiates away from its source, it spreads out, resulting in less energy per unit area as distance increases. For example, a star that is twice as far away as another of the same luminosity will appear four times dimmer, and if it is three times farther away, it will appear nine times dimmer, because the intensity is inversely proportional to the square of the distance (r²). This principle applies to any point light source, such as when you move away from a streetlight at night.