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Why Light Beams Don't Go On Forever
StarTalk
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Apr 30, 2026
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10:13
Why Light Beams Don't Go On Forever
From
StarTalk
Neil deGrasse Tyson
(host)
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Executive Summary
The visibility of a light beam from the side is not due to the light itself, but from the light scattering off particles like dust and water vapor in the atmosphere.
In a perfect vacuum, a beam would be invisible from the side.
The intensity of light from a source follows the inverse square law (1/d²), meaning its brightness decreases by the square of the distance from the source.
For an observer viewing a vertical searchlight from the ground, the perceived brightness drops off with the fourth power of the distance (1/d⁴).
This is because the inverse square law applies twice: once for the light traveling up to the particles, and again for the scattered light traveling back down to the observer's eye.
This extremely rapid drop-off in perceived brightness is why searchlight beams appear to terminate abruptly in the sky; their intensity quickly falls below the detection threshold of the human eye.
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Processed May 4, 2026
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