We know that the farther off a bright thing is, the less bright it seems. The Moon, or a planet like Venus, is brighter than the stars to our eyes, though not a ten thousandth part so bright in the reality, because it is near. Light will travel on forever unless it is stopped. Still, somehow, light does get fainter as it travels. The reason is that the light spreads in all directions as it travels, and so gets less intense at any particular place.
If you have ever played with a magic lantern, you know what a bright spot of light it throws when it is placed near the screen, but when it is moved farther away the light gets wider. The law governing this is known precisely. If the distance is doubled, the light is one-fourth as bright; if the distance triple, one-ninths bright; if quadrupled, one-sixth as bright. To get the intensity, you must take the square of the distance – i.e. multiply it by itself, and then the intensity is so much less. We say that it varies inversely as the square of the distance. If it varied directly, the light would be sixteen times as bright when the distance was four times as great, instead of being one-sixteenth as bright. This ‘law of inverse squares’ is true for the intensity of light, sound, magnetism, heat and gravitation.