Streaming TV shows and movies directly to your home is a breeze with Netflix. As a Netflix member, you can instantly watch TV and movies on your PC, Mac, mobile device, or television. Get a free thirty day trial membership. Go to Netflix dot com slash stuff and sign up today. Welcome to Brainstuff from how Stuff works dot com where smart happens. H I am Marshall Brain with today's question, Could I see a flashlight beam from Earth on the Moon? This is a great dot question because it makes you
think about how light works. When you turn on a flashlight, you're creating a source of photons. The photons leave the flashlight and they immediately start to spread out into a cone shaped beam, provided that they don't hit anything. Each individual photon travels through space forever. So it's not that the photons run out of gas on the way to
the Moon 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 when you're standing on the Moon that you cannot detect the flashlight. So the answer to your question is it depends on both the flashlight and the size of your eye.
If the flashlight in question is a little pen light flashlight powered by a couple of double A 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 flashlight is gigantic by the time it reaches the moon, and the photons are spread out too thinly for your eyes to detect.
If you were to use a much bigger flashlight, for example, an aircraft searchlight or something like that, or if you were to increase the size of your eye by using a telescope, then it's possible for you to detect the flashlight from the moon. The other alternative would be to replace the flash light with a small laser. The cone of divergence of a laser is extremely small compared to
a flashlight. For example, there are lasers whose beams are so tightly focused that by the time the light reaches the moon it's only diverged into a circle about half a mile in diameter. You could probably see tightly focused laser light like that with your naked eye. The other alternative is to increase the size of your eye with a telescope. A telescope collects light over a large area with its lens or mirror. This is why people use
large telescope to detect light from distant stars. Even though the stars are very bright compared to a flashlight, they're also incredibly far away. Most stars are many light years away, and one light year equals ten trillion kilometers or six trillion miles by the time the star's light reaches Earth. Therefore, the light is incredibly dim. The whole space telescope helps astronomers to see light from distant objects even more clearly. In fact, it's been said that the hubble can detect
the light from a single match on Pluto. Do you have any ideas or suggestions for this podcast? If so, please send me an email at podcast at how stuff works dot com. For more on this and thousands of other topics, go to how stuff works dot com and be sure to check out the brain Stuff blog on the how stuff works dot com home page. This episode of brain Stuff is brought to you by Linda dot Com.
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