Eli5 If you shine a laser up in the sky for ~3 seconds, does the beam eventually fizzle out or does it keep going until it hits an object?

Question

Eli5 If you shine a laser up in the sky for ~3 seconds, does the beam eventually fizzle out or does it keep going until it hits an object?

In: 5

Answer 1

Light continues until it is absorbed. Some of it will be absorbed hitting molecules in the air. Some will get absorbed hitting molecules in space (it’s not a complete vacuum). If you’re pointing it at the moon then a good portion or all of the photons will be absorbed by it. If you’re aiming at clear section of sky then a good portion will continue traveling through space essentially for ever.

Even though lasers tend to have small divergence (how much they spread out), they do spread out over time. A typical laser pointer might be 5 mW with a 2mm aperture (width of laser beam departing the pointer), and a divergence of 1 mrad (it deviates from a cylindrical beam by 1/1000 of a radian). That 2mm beam will become a 385m wide beam by the time it reaches the Moon (discounting dispersion due to the atmosphere).