A photon is emitted from a source and reflected off an object (or objects) until it hits the human eye. The color of the object we see depends on the photon wavelength. If photon travels with constant speed how does the human eye know how far the object from which the photon was reflected?

  • 3
    $\begingroup$ Why do you think the human eye "knows" that? $\endgroup$
    – ACuriousMind
    Sep 30, 2014 at 19:31
  • $\begingroup$ Who says it does? $\endgroup$
    – Kyle Kanos
    Sep 30, 2014 at 19:31
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    $\begingroup$ It doesn't. That's why we need two eyes for binocular vision. $\endgroup$
    – CuriousOne
    Sep 30, 2014 at 19:32
  • $\begingroup$ I agree with CuriousOne. If you're interested, there's more about this at en.wikipedia.org/wiki/Binocular_vision. Good luck! $\endgroup$
    – CoilKid
    Sep 30, 2014 at 19:40

2 Answers 2


To have depth perception two eyes are needed. Our two eyes are some distance apart which causes the photons from an object to arrive at slightly different angles. The brain then reconstructs the depth field from these differences.

Similarly, we can figure out how far nearby stars are by using images made by a telescope at two different times of the year, since when the earth is (for example) at two opposite positions of its orbit around the sun, it acts like two "eyes" that are a large distance apart.

  • $\begingroup$ So why I see with 1 eye closed? $\endgroup$
    – mhgnhg
    Sep 30, 2014 at 21:08
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    $\begingroup$ You cannot see depth with one eye , except for the "simulated" depth that your brain tries to invent based on shadows, color, sharpness and previous experience. $\endgroup$
    – Jasper
    Sep 30, 2014 at 21:33
  • $\begingroup$ The brain can combine multiple cues to determine distance - also the focus distance, whether other objects are overlapping, size, or for large distances, the haziness. So you can tell distance with a single eye under the right circumstances. $\endgroup$
    – Demis
    Oct 4, 2017 at 7:53
  • $\begingroup$ and how does brain now that those two photons come from the same object? $\endgroup$
    – Our
    Mar 16, 2020 at 12:00

Binocular vision has already been discussed, but it left out an important aspect.

A single eye is sensitive to distance. The shape of the lens changes to focus on near/far objects.

enter image description here

The reason this is needed is that our pupil has finite size and cannot be modeled as a pinhole. The same physics is going on here as in a lens of a camera focusing on an object. The rays coming from objects will be focused at different points depending on their distance to the eye; only those objects whose rays are focused on the retina are in focus.

So, in principle, one could use the shape of the lens to determine distance to an object.

  • $\begingroup$ Thx for answer. What I have problem to understand is when I close 1 eye (so without binocular vision) I can pretty well tell that for example "that car is somewhat 50m away" or "that building is like 300m away" but how do I know if object is bigger and more far away or just smaller and closer. $\endgroup$
    – mhgnhg
    Sep 30, 2014 at 22:33
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    $\begingroup$ @mhgnhg, you already have some idea how big a car or a building is, so your brain just estimates the distance based on how big it looks compared to how big it guesses it really is (and other clues available in what you see). $\endgroup$
    – The Photon
    Sep 30, 2014 at 22:41
  • $\begingroup$ Google "Forced Perspective" or look at hongkiat.com/blog/force-perspective-photos $\endgroup$
    – DJohnM
    Sep 30, 2014 at 22:47
  • $\begingroup$ @ThePhoton Yes but even when you see new object you haven't seen before you can pretty well tell it (I know there are pictures where you cannot tell difference that's why I said pretty well) - if I would make from clay object you haven't seen before and throw it into air you could tell in approximation how far it is from you $\endgroup$
    – mhgnhg
    Sep 30, 2014 at 22:57
  • $\begingroup$ @mhgnhg: I think a few trivial optical illusions would cure you from imagining that you can tell depth with one eye. The example of a falling object doesn't count because your brain knows, again, how to estimate the height based on the time it takes for something to fall. $\endgroup$
    – CuriousOne
    Sep 30, 2014 at 23:26

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