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Light moves along straight lines... Got it.

So if the light from that faraway star is traveling in a straight line, and that beam is, considering the distaces involved, at best one or two photons "wide", why, when I move six feet or so to my left, can I still see the star? IOW, why aren't there "dark" areas between the beams of starlight, where the stars "disappear"?

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    $\begingroup$ Lots and lots of photons $\endgroup$
    – anon01
    Oct 28, 2015 at 4:49
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    $\begingroup$ The star is not just emitting photons in one direction. It emits millions of photons in all directions. Therefore, we can see it in along varying lines of sight. $\endgroup$
    – Prahar
    Oct 28, 2015 at 4:57
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    $\begingroup$ Also, BTW, light moves in a straight line as long as the medium that is travelling in does not change. If it does, it reflects or refracts. The atmosphere of the earth causes lots of refraction so that light goes out in all directions. Thus, even if for some crazy reason, the photons were emitted along a single direction, they would be diffused by the atmosphere and give the same effect. However, note that in this case, the star would appear brighter or dimmer based on the angle that we view it in. $\endgroup$
    – Prahar
    Oct 28, 2015 at 4:57
  • $\begingroup$ But the photons are all being emitted from a single source,ie, the star, even if the photons are being scattered in all directions from the source, only those few that are traveling directly toward us are visible. they are not being generated from somewhere next to my eye $\endgroup$
    – Hep
    Oct 28, 2015 at 5:09
  • $\begingroup$ Albeit that the atmosphere does distort the rays, when we point Hubble at a star the light doesn't disappear as the telescope moves. $\endgroup$
    – Hep
    Oct 28, 2015 at 5:11

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I will try to address 2 points here:

1) Light does not necessarily move by rectilinear (straight line) propagation. is one example. If you also consider , then due to the curvature of space-time between 2 massive bodies, light will have to follow a curved path, known as a in transit.

2) The light from a star is not 1 beam wide, in fact, it has complete angular distribution from a celestial horizon to another. If you can't see a star at night, it may be due to its excessive dimness or due to it being beyond the horizon (notwithstanding atmospheric conditions). This very large angular distribution of its radiations allows you to see a star (don't forget that a star is in fact a sphere, not a point source of light).

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    $\begingroup$ Got it, a star is a spheroid. but let's start from the star... beams of photons radiate outward along rays... this particular ray points directly at us... the ray next to it heads off at a slightly different angle... the farther we get from the star, the wider the gap (vector space?) between the two rays becomes... by the time it gets here, the gap is huge... so why can't I step "out of path" of that ray? $\endgroup$
    – Hep
    Oct 28, 2015 at 6:45
  • $\begingroup$ I get what you mean, but since a photon is almost dimensionless, there can be an arbitrary number of them in any sub-angle you may bother to think of, although they might not all be visible (intensity considerations). $\endgroup$ Oct 28, 2015 at 6:49
  • $\begingroup$ So photons are so tiny I'd have to be something akin to an atom to "step out" of their shine, is that the gist? $\endgroup$
    – Hep
    Oct 28, 2015 at 7:44
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    $\begingroup$ @Hep: its a mistake to think of a photon purely as if it were a particle with an exact size and location. $\endgroup$ Oct 28, 2015 at 10:25
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    $\begingroup$ It don't think this has anything to do with a double slit experiment. You can't see a photon and step out of the the way even if you are an atom. Once you have seen the photon you have already absorbed it, and having seen the first photon you still don't know where the second photon is so you can't step out of (or into) its way either. $\endgroup$
    – gmatht
    Apr 17, 2019 at 15:10
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There, in fact can be, and thus are, dark areas which causes the light to disappear.

Feynman explains this in the link; The punch line is at 4:20. But I doubt you can understand it without looking the whole series of these lectures. This must be one reason why the stars are twinkling (or scintillating)

Of course one problem is that your eyes' diameter is too big to be able to "see" these dark areas completely. The other is that these are all present in various colours.

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  • $\begingroup$ Are you referring to a star's absorption spectrum? $\endgroup$ Oct 28, 2015 at 7:50
  • $\begingroup$ at 4 min 20 feynman is talking about reflection and interference between two surfaces, not a light source itself $\endgroup$
    – anna v
    Oct 28, 2015 at 9:51
  • $\begingroup$ @annav Yes, and As long as i know, the space is not a vacuum, and the direction of reflection is not very limited. Of course this aspect is very theoretical. In the praxis the different wavelengths make's this effect visible. But I think that some other kind of answer would only limit our thoughts $\endgroup$
    – Jokela
    Oct 28, 2015 at 10:35
  • $\begingroup$ What you addressed is a physical occurence and a valid point, but I don't think it was what the OP intended his question to be understood as. $\endgroup$ Oct 29, 2015 at 16:37
  • $\begingroup$ Yes. Light is constructed from single photons. And there is a distance between them. And their path is a straight line only in vacuum. As we are in Physics-platform, I think it's not proper to make same un-true simplifications, which would only disturb further learning; youtube.com/watch?v=iMDTcMD6pOw $\endgroup$
    – Jokela
    Nov 3, 2015 at 10:39

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