Could I see a flashlight beam from Earth on the moon?
This is a good thought question because it makes you think about how light works. When you turn on a flashlight, you are creating a source of photons (see How Light Works for details on photons). The photons leave the flashlight and they immediately start to spread out in a cone-shaped beam. Provided that they don’t hit anything, each individual photon travels through space forever. So it is not that the photons “run out of gas” on the way to the moon and stop. What happens instead is that, by the time they reach the moon, the photons have spread out tremendously. So few photons hit your eye at any one time that you cannot detect the flashlight. So the answer to your question is, “It depends on both the flashlight and on the size of your ‘eye'”. If the flashlight in question is a little penlight flashlight powered by a couple of AA batteries, and if the eye in question is your naked eye, then the answer is, “no — you cannot see the flashlight from the moon”. The cone of a typical flas
