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Speaker 1: Welcome to Brainstuff from how Stuff works dot com where

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

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Speaker 1: Could I see a flashlight beam from Earth on the Moon?

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Speaker 1: This is a great dot question because it makes you

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Speaker 1: think about how light works. When you turn on a flashlight,

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Speaker 1: you're creating a source of photons. The photons leave the

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Speaker 1: flashlight and they immediately start to spread out into a

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

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Speaker 1: individual photon travels through space forever. So it's not that

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Speaker 1: the photons run out of gas on the way to

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Speaker 1: the Moon stop. What happens instead is that by the

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Speaker 1: time they reach the Moon, the photons have spread out tremendously,

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Speaker 1: so few photons hit your eye at any one time

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Speaker 1: when you're standing on the Moon that you cannot detect

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Speaker 1: the flashlight. So the answer to your question is it

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Speaker 1: depends on both the flashlight and the size of your eye.

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Speaker 1: If the flashlight in question is a little pen light

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Speaker 1: flashlight powered by a couple of double A batteries, and

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Speaker 1: if the eye in question is your naked eye, then

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Speaker 1: the answer is no. You cannot see the flashlight from

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Speaker 1: the moon. The cone of a typical flashlight is gigantic

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Speaker 1: by the time it reaches the moon, and the photons

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Speaker 1: are spread out too thinly for your eyes to detect.

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Speaker 1: If you were to use a much bigger flashlight, for example,

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Speaker 1: an aircraft searchlight or something like that, or if you

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Speaker 1: were to increase the size of your eye by using

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Speaker 1: a telescope, then it's possible for you to detect the

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Speaker 1: flashlight from the moon. The other alternative would be to

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Speaker 1: replace the flash light with a small laser. The cone

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Speaker 1: of divergence of a laser is extremely small compared to

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Speaker 1: a flashlight. For example, there are lasers whose beams are

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Speaker 1: so tightly focused that by the time the light reaches

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Speaker 1: the moon it's only diverged into a circle about half

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Speaker 1: a mile in diameter. You could probably see tightly focused

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Speaker 1: laser light like that with your naked eye. The other

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Speaker 1: alternative is to increase the size of your eye with

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Speaker 1: a telescope. A telescope collects light over a large area

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Speaker 1: with its lens or mirror. This is why people use

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Speaker 1: large telescope to detect light from distant stars. Even though

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Speaker 1: the stars are very bright compared to a flashlight, they're

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Speaker 1: also incredibly far away. Most stars are many light years away,

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Speaker 1: and one light year equals ten trillion kilometers or six

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

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Speaker 1: the light is incredibly dim. The whole space telescope helps

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Speaker 1: astronomers to see light from distant objects even more clearly.

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Speaker 1: In fact, it's been said that the hubble can detect

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Speaker 1: the light from a single match on Pluto. Do you

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Speaker 1: have any ideas or suggestions for this podcast? If so,

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Speaker 1: please send me an email at podcast at how stuff

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Speaker 1: works dot com. For more on this and thousands of

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Speaker 1: other topics, go to how stuff works dot com and

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Speaker 1: be sure to check out the brain Stuff blog on

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Speaker 1: the how stuff works dot com home page. This episode

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