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So if our eye is considered a lens, a parallel beam (sort of like collimated light from a flash light) should converge to a single point, the focal point. So does that mean our eye will see a finite parallel beam as a single dot? That seems contradict real life experience, where I still see the parallel beam as a beam with finite width. What gives?

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... I still see the parallel beam as a beam with finite width.

Can you describe this scenario more? Normally you cannot see "beams" at all. You just perceive the transmission point.

But yes, for parallel beams, and assuming the lens is focused at infinity, and assuming the beams intersect the lens, then they would all focus to a single point. You would see a single point of light.

Basically, this is exactly the scenario when you observe a bright star in the night sky. Light rays from it are intersecting your lens at different points (near the center, near the top edge, halfway to the right edge, etc.) All these rays are brought to a focus at a single point on your retina and you perceive a single point.

Flashlights don't produce parallel rays. So when you look at a flashlight you're able to see different parts of the transmission lens.

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  • $\begingroup$ I guess the issue is the flashlight I was using doesn't really produce parallel rays as you said --- I was under the impression the cone shaped reflective material around the bulb is designed to make all rays parallel, but I guess that's not true. At least not true for all the light. But some of the rays are indeed parallel, aren't they? --- When you point a candle towards the wall you don't get a beam at all, but pointing a flash light at a wall it does create a circle beam. $\endgroup$
    – Cosmo
    Jul 30, 2023 at 0:31
  • $\begingroup$ Yes, the mirror and lens in a flashlight concentrates the light, but there's still plenty of divergence so you can see that the flashlight is "lit" even quite a bit off-axis. $\endgroup$
    – BowlOfRed
    Jul 30, 2023 at 3:43
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Imagine you are in a space ship ten times as far away from the sun as we are now, just past the orbit of Saturn. You have a window pointing towards the sun and a fog generator in your craft for some reason. The sunlight scattering off the fog would make the beam passing through the window visible from the side, and you would see a beam the width of the window with almost no noticeable divergence.

But if you looked at the sun through the window you would see a very bright point of light (or at most a tiny spot) as all of those (almost-)parallel rays were focused onto a point on your retina. You can make the rays as nearly parallel as you want by moving farther from the sun; it just becomes harder to see the beam scattered off mist.

Any time you see something as having some width, that means that the rays entering your eye from that thing are not parallel. If you have a very parallel beam (which rare), and it appears from the side that way, what you are seeing is non-parallel rays of light being scattered from particles in the path of the beam.

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  • $\begingroup$ How does this explain the fact that we can see the sun as a spot with some width? The suns rays are, to a very good approximation, parallel. $\endgroup$ Sep 20, 2023 at 14:48
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    $\begingroup$ They are parallel to exactly as good an approximation as the angular size of the sun. The sun is just half a degree across in the sky. That's big enough to perceive as a disk, but, still, two lines with a half-degree angle between them are pretty close to parallel. $\endgroup$ Sep 20, 2023 at 17:57

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