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If I focus my torch into the night sky,what would be the farthest point the light can reach? Recently I looked at the night sky and noticed a light spark moving around the clouds for about 15 min.After observation I jumped into a conclusion that its probably because of the light house near by or any powerful light emitter.

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Why did you post this question in this SE? XD – Py-ser Jul 10 '14 at 7:13
@Py-ser It relates to "Astronomical observations, for all celestial objects across the entire spectral range". Though the light source is actually Earth based, it is still a perceived astronomical phenomenon. I think it is a good and relevant question. – called2voyage Jul 10 '14 at 13:36
In US English, a handheld battery-powered light is called a "flashlight"; the word "torch" usually refers to a more primitive burning light source. – Keith Thompson Jul 11 '14 at 20:44
up vote 7 down vote accepted

If you point a flashlight up, then the light will reach as far as the other ends of the universe, if one has a sensitive enough measuring device, so long as it is not absorbed. Of course, said device might require a lot of work, and a huge telescope. Lights can easily be seen by satellites, but it would take a huge telescope to see a single light from, say, the Moon, and even larger the further you go.

In the case of your moving light, I would suspect a laser of some kind. Lasers are focused enough where one could easily see a reflection from the clouds, and pretty focused as well. It could be a moving light source, such as a plane or balloon of some kind as well.

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Potentially photons emitted by your torch could travel to infinite distances, assuming the universe will keep on expanding. If the light gets further away, the photons of your torch will be spread out over an increasing area and the density (number of photons per square metre) will decrease rapidly. At some distance, only one photon will strike the area of your eye/detector. And you may not be able to observe a single photon. It is also likely (but not certain) that the photons will be absorbed by atoms.

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Some theories allow for photon decay. In that case, the average photon would decay after travelling for 1018 years, which would give us around 9.461×1030 kilometers. That's about a million times the current estimate for the diameter of the observable universe.

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Potentially forever assuming nothing blocks it's path fully, however past a relatively short distance a torch would not be observable without an immense effort (large optics.) It may be next to impossible to discern if there is competing light emission (for example, if you shine a torch to the sky in the middle of the day, then the Sun's reflection would overpower it to a potential observer.)

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