Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The physics of sight?

  1. Mar 8, 2015 #1
    This is probably a pretty elementary question, but it is something that has always puzzled me. And that is how does the physics of sight work? I am not refereeing to how the eye works specifically, but more how the information from the shape and colour of an objected is transmitted to the eye.

    I assume from my limited knowledge that light is reflected from the object into the eye and the retina does the rest. But how is the light changed when it is reflected from an object? Is it the wave or particle nature or light that transmits the information. How does the various frequencies of light prior to being reflected effect what we see? And why are there no gaps in our sight?

    Anyway, just thought I'd ask and see if anyone wouldn't mind giving a bit of an explanation. Thanks:)
  2. jcsd
  3. Mar 8, 2015 #2


    User Avatar
    Gold Member

    There are. It's called the 'blind spot' , one in each eye but not in the same place. This is where the optic nerve joins the eye and is slightly medial of the fovea. Look at a small dot (an LED is good) and, with the right eye look a bit to the left about 10o, or vice versa with the left eye look to the right of the dot. Or see https://faculty.washington.edu/chudler/chvision.html for a better explanation!
    The brain conveniently ignores this, unless you carefully bring it to its attention. But as always, we see what the brain thinks it sees, not what the physicists say is there!

    Geometric optics projects an image on the retina, which I think the Physicists put down to waves, but again, the brain maps this to what it thinks should be there.
    Interestingly (?) the image on the retina is obviously inverted and we see the world the right way up, but this may not be 'hardwired'. Experimental psychologists have worn 'spectacles' which invert their vision to make the retinal image the other way up and after a week or two of confusion have found they they begin to see the world as usual. https://en.wikipedia.org/wiki/George_M._Stratton
    Colour and brightness are detected by rod and cone cells in the retina. They contain photosensitive pigments, which are electrically changed by absorbing matching photons and can then initiate nerve impulses. Colour and brightness are frequency and amplitude of light waves. (I suppose some people might call that, the energy of photons and the number of photons.)
    Again the brain takes the information from these cells via the optic nerves and makes up its own mind what colour and brightness it thinks it is seeing. There are plenty of illusions and effects to show this as well.
    Because you have only three type of cone cell, all combinations of colours must map to three coordinates, so colour mixtures which could be distinguished physically, by spectrometer for eg, can be indistinguishable to the eye. Some people have different pigments in their optic receptors and have anomalous colour vision, so that they don't distinguish colours in the same combinations that most people do.

    Some light is absorbed, so brightness is changed. Some colours are absorbed more than others, so the combination or balance of colours in the light are changed. A surface which looks yellow in white light is absorbing blue light and could be reflecting a mixture of red and green as well as yellow. If it were illuminated with a blue - green light light (appearing cyan), it would look green, because it did not reflect blue and the light did not contain red and yellow.

    Some of what you are asking is straight physics and I can't say much about that. But the psychology of visual perception is quite a complex business. I hate to sound like a physicist, but there really is a lot of basic stuff to learn about before you can really understand what you are asking. However, unlike physics, it is easy to understand once you try. (And maths is not required!)
    Try something like Richard Gregory's "Eye and Brain" for an easy introduction.
  4. Mar 8, 2015 #3


    User Avatar
    Gold Member

    And just to add an easy one to try yourself. The cone cells in your eye are more sensitive than the rods, but instead of 3 types of cone cell, there is only one sort of rod cell. So in very low light only the rods are working and we no longer see colour. The light entering your eye is still coloured, but you have no idea what colour it is. You only register the brightness.
  5. Mar 8, 2015 #4
    Thanks very much for the explanation's and the links. I didn't know about the blind spot, interesting test! Why does it work only on one side? (E.g. if I look at the plus with my right eye I can see the dot disappear. But if I look at the dot with my right eye, the plus is always there.) And never really thought about the psychology of visual perception, which sounds really interesting so will definitely look that up. Thanks.

    Yes, I suppose I was also interested in the physics too. In particular the properties of light and how it effects sight. I was interested in how light carries the information, e.g. is it the wave or particle nature. As well as a few other physics type questions. So will continue to search but finding it difficult.

    Anyway, thanks again.
  6. Mar 8, 2015 #5


    User Avatar
    Gold Member

    Blind spot: Looking at the eye from above, the optic nerve joins the eyeball at the back, slightly on the nasal side, rather than in the centre. See diagram http://www.allaboutvision.com/resources/anatomy.htm
    The fovea, shown at the centre, is on the optical axis of the eye. When you look at an object, the eyeballs swiwel to point at the object and the centre if the image forms there. This is the area where the cones densest (like more megapixels) and gives you your detailed fine vision. Further away there are less cones and more rods.

    If you imagine walking along the retina (back of the eye, where the image is formed by the lens) and looking out through the lens & pupil, when you are standing on nose side of the eyeball, you are looking to the side of what the eye is looking at. See the diagram here http://www.forbestvision.com/accommodation-and-convergence/ . In the right eye, standing by your nose, you are looking to the right. In your left eye standing by your nose, you are looking left. The blind spot is on the nose side in each eye.
    So with your right eye, the blind spot appears to the right of what the eye is looking at. Vice versa left eye.

    Ok. I've found the right diagram now, http://www.doobybrain.com/2008/02/25/the-human-eye-has-a-blind-spot/ [Broken]

    Strangely, all the wiring for your eye (& blood vessels) runs over the inside surface of the retina. So you can actually see it, if you make your brain stop ignoring it. The simple method is to use a pencil torch or similar, close your eye, put the torch on the side of your eyelid and wiggle it about. It helps if you look towards the opposite side - say I'm doing the right eye and holding the light on the right side of the eyelid, then look left. You should be able to make out an irregular squiggly pattern a bit like a map of river basins all converging. (I don't know if that's a good description, it's just how it looks to me.)
    Last edited by a moderator: May 7, 2017
  7. Mar 8, 2015 #6


    User Avatar
    Gold Member

    As for the Physics, I don't think it matters how you regard light. The physical properties of the eye were largely understood in classical wave terms.
    The absorbtion of light by pigments to initiate nerve impulses would now, I think, be regarded as a quantum effect by chemists. But psychologists were happy simply to understand that it happened.
    See http://math.ucr.edu/home/baez/physics/Quantum/see_a_photon.html for some remarks on photons in the eye.

    The information is carried by the path of the light, its amplitude and its frequency. These are the same or have comparable analogues, whether you think of it as waves or particles or quantum entities.

    As a personal note, I might say that I don't actually believe in particles in this context so your question in that area is not meaningful to me. To me a particle means something like a billiard ball or grain of sand or even a PM10 soot particle and I simply don't believe that subatomic entities (what everyone calls subatomic particles) and photons, etc go around behaving like miniature footballs or ball-barings etc. Quantum entities have well known and defined properties and behaviour, often represented in wave functions. They do not need to be particles nor waves: they are what they are. Sometimes it may help to make analogies with particles or waves, but sometimes it may not. When we use the water analogy for electricity, we know it's not really like that in all respects. Well light is not like particles in all respects nor like waves in all respects, light is like quantum entities called photons.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: The physics of sight?
  1. Light & Sight (Replies: 58)