A given amount of light emitted by the Sun is spread over a sphere of radius r. As the light propagates the radius gets bigger and bigger. The light per area is

A given amount of light emitted by the Sun is spread over a sphere of radius r. As the light propagates the radius gets bigger and bigger. The light per area is proportional to 1 / 4πr². If the radius grows 5 times then the area of the sphere grows 25 times.