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Speaker 1: Welcome to Brainstuff from how Stuff works dot com where
Speaker 1: smart happens. H I am Marshall Brain with today's question,
Speaker 1: Could I see a flashlight beam from Earth on the Moon?
Speaker 1: This is a great dot question because it makes you
Speaker 1: think about how light works. When you turn on a flashlight,
Speaker 1: you're creating a source of photons. The photons leave the
Speaker 1: flashlight and they immediately start to spread out into a
Speaker 1: cone shaped beam, provided that they don't hit anything. Each
Speaker 1: individual photon travels through space forever. So it's not that
Speaker 1: the photons run out of gas on the way to
Speaker 1: the Moon stop. What happens instead is that by the
Speaker 1: time they reach the Moon, the photons have spread out tremendously,
Speaker 1: so few photons hit your eye at any one time
Speaker 1: when you're standing on the Moon that you cannot detect
Speaker 1: the flashlight. So the answer to your question is it
Speaker 1: depends on both the flashlight and the size of your eye.
Speaker 1: If the flashlight in question is a little pen light
Speaker 1: flashlight powered by a couple of double A batteries, and
Speaker 1: if the eye in question is your naked eye, then
Speaker 1: the answer is no. You cannot see the flashlight from
Speaker 1: the moon. The cone of a typical flashlight is gigantic
Speaker 1: by the time it reaches the moon, and the photons
Speaker 1: are spread out too thinly for your eyes to detect.
Speaker 1: If you were to use a much bigger flashlight, for example,
Speaker 1: an aircraft searchlight or something like that, or if you
Speaker 1: were to increase the size of your eye by using
Speaker 1: a telescope, then it's possible for you to detect the
Speaker 1: flashlight from the moon. The other alternative would be to
Speaker 1: replace the flash light with a small laser. The cone
Speaker 1: of divergence of a laser is extremely small compared to
Speaker 1: a flashlight. For example, there are lasers whose beams are
Speaker 1: so tightly focused that by the time the light reaches
Speaker 1: the moon it's only diverged into a circle about half
Speaker 1: a mile in diameter. You could probably see tightly focused
Speaker 1: laser light like that with your naked eye. The other
Speaker 1: alternative is to increase the size of your eye with
Speaker 1: a telescope. A telescope collects light over a large area
Speaker 1: with its lens or mirror. This is why people use
Speaker 1: large telescope to detect light from distant stars. Even though
Speaker 1: the stars are very bright compared to a flashlight, they're
Speaker 1: also incredibly far away. Most stars are many light years away,
Speaker 1: and one light year equals ten trillion kilometers or six
Speaker 1: trillion miles by the time the star's light reaches Earth. Therefore,
Speaker 1: the light is incredibly dim. The whole space telescope helps
Speaker 1: astronomers to see light from distant objects even more clearly.
Speaker 1: In fact, it's been said that the hubble can detect
Speaker 1: the light from a single match on Pluto. Do you
Speaker 1: have any ideas or suggestions for this podcast? If so,
Speaker 1: please send me an email at podcast at how stuff
Speaker 1: works dot com. For more on this and thousands of
Speaker 1: other topics, go to how stuff works dot com and
Speaker 1: be sure to check out the brain Stuff blog on
Speaker 1: the how stuff works dot com home page. This episode
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