WEBVTT - Could I see a flashlight beam from Earth on the moon?

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<v Speaker 1>Welcome to Brainstuff from how Stuff works dot com where

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<v Speaker 1>smart happens. H I am Marshall Brain with today's question,

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<v Speaker 1>Could I see a flashlight beam from Earth on the Moon?

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<v Speaker 1>This is a great dot question because it makes you

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<v Speaker 1>think about how light works. When you turn on a flashlight,

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<v Speaker 1>you're creating a source of photons. The photons leave the

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<v Speaker 1>flashlight and they immediately start to spread out into a

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<v Speaker 1>cone shaped beam, provided that they don't hit anything. Each

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<v Speaker 1>individual photon travels through space forever. So it's not that

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<v Speaker 1>the photons run out of gas on the way to

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<v Speaker 1>the Moon stop. What happens instead is that by the

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<v Speaker 1>time they reach the Moon, the photons have spread out tremendously,

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<v Speaker 1>so few photons hit your eye at any one time

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<v Speaker 1>when you're standing on the Moon that you cannot detect

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<v Speaker 1>the flashlight. So the answer to your question is it

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<v Speaker 1>depends on both the flashlight and the size of your eye.

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<v Speaker 1>If the flashlight in question is a little pen light

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<v Speaker 1>flashlight powered by a couple of double A batteries, and

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<v Speaker 1>if the eye in question is your naked eye, then

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<v Speaker 1>the answer is no. You cannot see the flashlight from

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<v Speaker 1>the moon. The cone of a typical flashlight is gigantic

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<v Speaker 1>by the time it reaches the moon, and the photons

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<v Speaker 1>are spread out too thinly for your eyes to detect.

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<v Speaker 1>If you were to use a much bigger flashlight, for example,

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<v Speaker 1>an aircraft searchlight or something like that, or if you

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<v Speaker 1>were to increase the size of your eye by using

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<v Speaker 1>a telescope, then it's possible for you to detect the

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<v Speaker 1>flashlight from the moon. The other alternative would be to

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<v Speaker 1>replace the flash light with a small laser. The cone

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<v Speaker 1>of divergence of a laser is extremely small compared to

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<v Speaker 1>a flashlight. For example, there are lasers whose beams are

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<v Speaker 1>so tightly focused that by the time the light reaches

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<v Speaker 1>the moon it's only diverged into a circle about half

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<v Speaker 1>a mile in diameter. You could probably see tightly focused

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<v Speaker 1>laser light like that with your naked eye. The other

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<v Speaker 1>alternative is to increase the size of your eye with

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<v Speaker 1>a telescope. A telescope collects light over a large area

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<v Speaker 1>with its lens or mirror. This is why people use

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<v Speaker 1>large telescope to detect light from distant stars. Even though

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<v Speaker 1>the stars are very bright compared to a flashlight, they're

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<v Speaker 1>also incredibly far away. Most stars are many light years away,

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<v Speaker 1>and one light year equals ten trillion kilometers or six

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<v Speaker 1>trillion miles by the time the star's light reaches Earth. Therefore,

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<v Speaker 1>the light is incredibly dim. The whole space telescope helps

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<v Speaker 1>astronomers to see light from distant objects even more clearly.

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<v Speaker 1>In fact, it's been said that the hubble can detect

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<v Speaker 1>the light from a single match on Pluto. Do you

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<v Speaker 1>have any ideas or suggestions for this podcast? If so,

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<v Speaker 1>please send me an email at podcast at how stuff

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