enter image description here

I have never been able to understand how we see the world with light from different angles. If a photon hits one point on an object and reflects, it enter your eye and you see that point on the object. However, if you reposition your viewing point, how will you be able to see that same point on the object? For that matter, how do you see the entire object at all? It doesn't make sense to say that light reflects off the point randomly because the surface is not changing and any light hitting the same point from the same source should always reflect at the same angle.

  • 1
    $\begingroup$ Light reflects off of the walls, and whatever else is nearby. This acts as another source of light. $\endgroup$ – garyp Jan 26 '17 at 15:01
  • $\begingroup$ I assume that there are no walls. The light source should be the only source. $\endgroup$ – Hexiang Chang Jan 26 '17 at 15:02
  • 1
    $\begingroup$ Scattering is something difficult to fight, sometimes. Also light sources usually do not output photons in a coherent manner (and in general not one at a time), as it happens for laser sources, for instance. So the way your diagram is expressing your intuitive understanding is confusing you (and it is not really necessary to think in terms of single photons). $\endgroup$ – Vendetta Jan 26 '17 at 15:47
  • 1
    $\begingroup$ Your diagram does not illustrate what you are asking. $\endgroup$ – sammy gerbil Jan 27 '17 at 7:21
  • $\begingroup$ I could be wrong and I apologise if this is an oversimplification. However, I just feel somewhere that maybe you're thinking there's only one single beam of photons that hits the mirror and reflects and, given the angle that it comes to you at, it should only be seen by one eye? If that's what your thought is, then maybe if you imagine and drew several hundred (thousand or even million) lines from the source of the light, hitting the mirror, and then reflecting accordingly, it'd probably explain why both our eyes receive the visuals. $\endgroup$ – Dhruv Saxena Jan 28 '17 at 23:25

This is because light scatters from the surface rather than reflecting in only one direction. This kind of scattering is often called 'diffuse reflection', as opposed to 'specular reflection' which is what a mirror does (approximately: even a mirror will scatter some light of course). Really good mirrors can be rather hard to see if illuminated by point sources of light against a very dark background, unless you are looking at such an angle that you see the specular reflection, and even then you may think you are seeing another point source of light.

Wikipedia has quite a good article on diffuse reflection, and in particular this quote is notable:

The visibility of objects, excluding light-emitting ones, is primarily caused by diffuse reflection of light: it is diffusely-scattered light that forms the image of the object in the observer's eye.

  • $\begingroup$ But even if the surface encourages diffuse reflection, light would still always reflect the same way off of any particular point. Unless light somehow splits into all directions at the one point. $\endgroup$ – Hexiang Chang Jan 26 '17 at 15:11
  • $\begingroup$ @HexiangChang: that is essentially what happens, yes: surfaces are rough at wavelengths comparable to that of light, and also not completely opaque (so some light scatters from points underneath the surface). See the Wikipeda article which I've now cited as well. $\endgroup$ – tfb Jan 26 '17 at 15:21
  • $\begingroup$ Although this is slightly OT, it may be useful: at most radio frequencies the ground is a strong diffuse reflector, while sea water is almost entirely specular. This is why it is very easy to make a radar that can find boats and ships at very long range - the radar signal reflects forward and away from the aircraft and only reflects back when it hits something else. In contrast, looking for aircraft below you is much much more difficult because you see the diffuse reflection of your own signal, which is often stronger than that from an aircraft. $\endgroup$ – Maury Markowitz Jan 26 '17 at 15:43
  • $\begingroup$ @Maury Markowitz "This is why it is very easy to make a radar that can find boats and ships at very long range "; you probably have never been involved in real radar design, clutter (reflection from the environment) is a notoriously difficult problem in all situation. $\endgroup$ – hyportnex Jan 26 '17 at 19:12
  • $\begingroup$ Bowen was getting reflections off ships with AI Mk. I at about 12 miles when ground reflections made aircraft invisible at any distance higher than their altitude. $\endgroup$ – Maury Markowitz Jan 26 '17 at 22:00

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.