If you switch on a lightbulb in a room its light has a visible range. So how about Sunlight? Does it light more on Mercury and less on Pluto? And what about planets that are not in the Solar System?


closed as off-topic by ForgeMonkey, Nathan Tuggy, gerrit, Hohmannfan Jun 10 '17 at 20:01

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  • $\begingroup$ If you switch on a lightbulb without a screen, it has no visible range. The light from the Sun goes in all directions with equal brightness. The light intensity is decreasing with increasing distance, but there is no range limit with no light at all beyond that limit. Otherwise you would see very few stars at night ( at a clear night without clouds and no artificial light in your area, after your eyes adapted to the dark for about half an hour). The sunlight at Pluto is indeed very weak, that is why Mercury is known for many centuries and Pluto only for 87 years. $\endgroup$ – Uwe Jun 10 '17 at 13:50

Light is electromagnetic energy conveyed by photons. Photons travel in straight lines at approximately 300,000,000 m/s. They do not lose any energy as they travel, so can travel indefinitely, only giving up their energy upon interacting with something. If such an interaction occurs, the photon is absorbed, and a new photon may be emitted by that something, depending on the circumstances of the interaction. Key point here is: barring something to get in the way, light travels indefinitely and without loss until it hits something.

A point source (more of an abstraction than a reality) radiates a given amount of power (energy per unit time) equally in all directions. At some distance from the point source (we can think of a spherical shell around it), that power will be evenly distributed over the area of that shell - so we have power per unit area. If you double the distance (the radius of the shell), the area goes up 4 times (math/geometry) so the power per unit area is only one quarter (referred to as the "inverse square" law). Note that we are not talking about a loss of energy with distance, only a "thinning out" because it is distributed over an increasing surface. Note also, that the power per unit area never actually gets to zero, but vanishingly approaches it with extreme distance.

The Sun is a huge ball of white hot gas. If you are really close to it, it doesn't look like a point source; it looks like a big wall of light. But, it does have a finite size - about a million miles across. If you get far enough away, say a hundred million miles (about how far away we are from it), it starts to look like a point source, so the further away you get, the more thinly the sunlight is spread, so the dimmer it seems. Because the light can potentially travel an infinite distance, there is no "range". There will come a point where it is no longer of significance, but that point is subjective.

So, because of the inverse square law, there is more light per unit area falling on Mercury than on Pluto, and much less on objects outside our solar system. If you are far enough away, it will be too little to measure with any practical instrument, but it will never be zero.

The light bulb in your room actually has no "range"; its light can continue forever too. We only perceive a limited range because of the limits of our perception relative to the amount of light being given off.

  • $\begingroup$ Even if our limit of perception would allow the recognition of single photons, when the probability of a photon is very low, one photon per day, month or year, it looks like there is a "range", but in fact the photons of light can go forever. $\endgroup$ – Uwe Jun 10 '17 at 15:49
  • $\begingroup$ Woah! @Anthony, thanks for such a great answer, it is more intriguing than I thought. $\endgroup$ – Prolog Jun 10 '17 at 15:58
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    $\begingroup$ @Uwe you are right. It is fun to know however that there may be some very subtle, single-photon 'awareness' possible at the limits of perception, though not everyone agrees. But for sure we can't say 'aha! I saw one' as far as visible light photons are concerned. nature.com/news/people-can-sense-single-photons-1.20282 $\endgroup$ – uhoh Jun 10 '17 at 17:16
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    $\begingroup$ @Uwe My colleague used to work in a field of astronomy where he encountered units like "photons per km² per century". $\endgroup$ – gerrit Jun 10 '17 at 19:01
  • $\begingroup$ @gerrit "photons per km² per century" is an incredible low probability of receiving a photon, that is "photons per m² per 100 Megayears" or "photons per mm² per 100 Terayears". Am I right that the unit "photons per km² per century" is only used in theory, not in praxis? I can't imagine a practical sensor useful for this unit. $\endgroup$ – Uwe Jun 13 '17 at 20:43

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