<p>Visible light from a torch has a 50% (blue light) to 90% (red light) chance of making it through the Earth's atmosphere, depending on the amount of dust and aerosols in the atmosphere and how close the Moon is to the horizon.</p>

<p>Light travels in straight lines. The Moon has an angular diameter of only 0.5 degrees as seen from the Earth. Most ordinary torch beams diverge by much more than this, so only a small fraction of the photons from a torch would pass through a circle of angular diameter 0.5 degrees. Hence only a small fraction of the emitted photons would hit the Moon.</p>

<p>Finally, you said that the torch was pointed "towards the Moon". Actually, you pointed it to where the Moon was 1.5 seconds ago, and the torch light reaches the distance of the Moon a further 1.5 seconds later. Thus the Moon has travelled 3 seconds in its orbit. However, since the orbital period (360 degrees) is a month, then it only covers an angle of 1.5 arcseconds in 3 seconds, so this shift is not relevant for your experiment (though it is for those bouncing lasers off reflectors on the Moon, which proves that light does in fact reach the Moon).</p>

CC BY-SA 3.0
2017-07-20T19:23:28.807Z
2017-07-20T19:23:28.807Z
ProfRob
<p>Yes. If there is nothing opaque in the way to obstruct it, it will reach the moon. However, the intensity of light (from a point source like a torch) diminishes with the square of distance. So, by the time it reaches the moon, it will be extremely faint.</p>

<p>Further information on the inverse square law as applied to light from a point source: <a href="https://en.wikipedia.org/wiki/Inverse-square_law#Light_and_other_electromagnetic_radiation" rel="nofollow noreferrer">https://en.wikipedia.org/wiki/Inverse-square_law#Light_and_other_electromagnetic_radiation</a></p>

CC BY-SA 3.0
2017-07-17T12:39:06.390Z
A torch, aka flashlight,is not a point source, nor is it uncollimated.
Carl Witthoft
CC BY-SA 3.0
2017-07-17T18:57:48.193Z
@barrycarter you misread my comment.  A torch **is** partially collimated by way of the parabolic reflector behind the bulb.
Carl Witthoft
CC BY-SA 3.0
2017-07-18T03:30:34.383Z
For the distances in question, light from a torch approximates a point source quite reasonably. That is, however, a side point to the answer. The question is whether photons from a torch reach the moon, and the answer is that they do unless something gets in the way. The intensity only really becomes a factor in the follow-up question "Could we see it?".
hartacus
CC BY-SA 3.0
2017-07-18T11:29:09.613Z
@barrycarter  That is 100% incorrect.  I'm not sure what you mean by "perfect laser," since collimation is unrelated to lasing action, but if you are thinking of a perfectly collimated beam, that spreads due to self-interference.  Take a look at the wikipedia pages on wave propagation
Carl Witthoft
CC BY-SA 3.0
2017-07-15T02:12:50.370Z
2017-07-15T02:12:50.370Z
hartacus
<p>If a torch is switched on and pointed towards the moon, would the light from the torch reach to the moon?</p>

CC BY-SA 3.0
2017-07-15T03:25:26.163Z
Related XKCD:    https://what-if.xkcd.com/13/
userLTK
CC BY-SA 3.0
2017-07-17T12:40:14.673Z
@barrycarter you're misusing "disperse" there.  Better to say "beam spread" to indicate that most of the photons from a flashlight will miss the moon entirely.
Carl Witthoft
CC BY-SA 3.0
2017-07-17T18:56:54.123Z
@barrycarter but that is not the definition within the field of optics, and of astronomy.
Carl Witthoft
<the-moon><light>
Does the light from a torch reach to the moon?
Ashish Gawande
# Does the light from a torch reach to the moon?

(Asked by: username_0 on 2017-07-14)

If a torch is switched on and pointed towards the moon, would the light from the torch reach to the moon?

**Tags:** the-moon, light
> **Comments:**
> **username_1**: Related XKCD:    https://what-if.xkcd.com/13/  
> **username_2**: @barrycarter you're misusing "disperse" there.  Better to say "beam spread" to indicate that most of the photons from a flashlight will miss the moon entirely.  
> **username_2**: @barrycarter but that is not the definition within the field of optics, and of astronomy.  

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## 2 answers

### Answer by username_3 on 2017-07-20. Score: 2

Visible light from a torch has a 50% (blue light) to 90% (red light) chance of making it through the Earth's atmosphere, depending on the amount of dust and aerosols in the atmosphere and how close the Moon is to the horizon.

Light travels in straight lines. The Moon has an angular diameter of only 0.5 degrees as seen from the Earth. Most ordinary torch beams diverge by much more than this, so only a small fraction of the photons from a torch would pass through a circle of angular diameter 0.5 degrees. Hence only a small fraction of the emitted photons would hit the Moon.

Finally, you said that the torch was pointed "towards the Moon". Actually, you pointed it to where the Moon was 1.5 seconds ago, and the torch light reaches the distance of the Moon a further 1.5 seconds later. Thus the Moon has travelled 3 seconds in its orbit. However, since the orbital period (360 degrees) is a month, then it only covers an angle of 1.5 arcseconds in 3 seconds, so this shift is not relevant for your experiment (though it is for those bouncing lasers off reflectors on the Moon, which proves that light does in fact reach the Moon).


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### Answer by username_4 on 2017-07-15. Score: 1

Yes. If there is nothing opaque in the way to obstruct it, it will reach the moon. However, the intensity of light (from a point source like a torch) diminishes with the square of distance. So, by the time it reaches the moon, it will be extremely faint.

Further information on the inverse square law as applied to light from a point source: <https://en.wikipedia.org/wiki/Inverse-square_law#Light_and_other_electromagnetic_radiation>

> **Comments:**
> **username_2**: A torch, aka flashlight,is not a point source, nor is it uncollimated.  
> **username_2**: @barrycarter you misread my comment.  A torch **is** partially collimated by way of the parabolic reflector behind the bulb.  
> **username_4**: For the distances in question, light from a torch approximates a point source quite reasonably. That is, however, a side point to the answer. The question is whether photons from a torch reach the moon, and the answer is that they do unless something gets in the way. The intensity only really becomes a factor in the follow-up question "Could we see it?".  
> **username_2**: @barrycarter  That is 100% incorrect.  I'm not sure what you mean by "perfect laser," since collimation is unrelated to lasing action, but if you are thinking of a perfectly collimated beam, that spreads due to self-interference.  Take a look at the wikipedia pages on wave propagation  

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