How does the apparent brightness of a light source depend on its distance from you?

Question

BlackTom AI · Accepted Answer

To understand how the apparent brightness of a light source changes with distance, consider how light spreads: as you move away, the same amount of light covers a larger area, so the flux you receive diminishes.
Option 1: 'It varies as the inverse square of the distance.' This is the correct description for how apparent brightness (or intensity) falls off with distance in three-dimensional space; doubling the distance makes the brightness drop by a factor of four, because the light spreads over a sphere whose area grows as r^2.
Option 2: 'It does not vary with distance at all.' This would imply brightness is constant with distance, which contradicts the geometric spreading of light; in reality, brightness decreases as you move away.
Option 3: 'It varies inversely with the distance.' An inverse relationship 1/r would mean brightness halves when distance doubles, which contradicts the actual 1/r^2 dependence for radiant flux from a point source in free space.
Option 4: 'It varies directly as the distance.' This would mean brightness increases as you move farther away, which is physically incorrect for light spreading in space.
Option 5: 'It varies directly as the distance squared.' That would imply brightness grows with the square of distance, the opposite of what happens; again, this misrepresents the geometric dilution of light.
In summary, only the inverse-square relationship correctly captures the way apparent brightness diminishes with increasing distance from a light source, while the other options describe incorrect dependencies.

ASTRON 2140 AU2025 (23428) Quiz 2 - 2025 Oct 10

How does the apparent brightness of a light source depend on its distance from you?

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