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

If you shone a flashlight beam from Earth, would you be able to see it from the moon? It depends. Find out why -- and get a lesson in how light works -- in this episode of BrainStuff.

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Episode Transcript

Speaker 1 (00:00):
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Welcome to Brainstuff from how Stuff works dot com where
smart happens. H I am Marshall Brain with today's question,

(00:29):
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

(00:52):
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

(01:14):
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

(01:37):
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

(01:58):
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

(02:22):
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

(02:43):
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.

(03:06):
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

(03:27):
the how stuff works dot com home page. This episode
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