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Why does light allow us to see things?

  1. May 27, 2012 #1
    I don't understand, light is just a form of energy.

    Why is it that I can see the things when light hits me in the eyes?
  2. jcsd
  3. May 27, 2012 #2
    That's what your eyes do... They sense energy in the form of EM radiation within a certain frequency range.

    It's just a sensor doing its job.
  4. May 27, 2012 #3
    But is it that the photons carry electrical information about the object it just hit and deliver it to our eyes?
  5. May 27, 2012 #4
    No, aside from the photon's frequency (color, since we're talking about vision), there's no transmission of information, electrical or otherwise. Photons just bounce around until they're absorbed by something... In this case that'd be a rod or cone in the back of your eyeball.
  6. May 27, 2012 #5
    So the EM radiation changes depending on what it hits and that's how we see if something is red/blue.

    And is something actually a colour or is it that, it's only that colour because of the frequency of the EMR? What is it about the object that changes the frequency of the EMR for it to appear that colour, is my question.

    Thanks btw I'm new to this I appreciate you helping out someone who doesn't know physics.
  7. May 27, 2012 #6
    Yeah, that's about right.
    It's just the frequency of the light. When a photon hits something, it'll either be absorbed, reflected, or pass through, depending on the properties of what it hits. Stuff that looks red, for instance, reflects "red" light and doesn't reflect the rest. You can get some pretty weird effects by shining colored lights on things that only reflect (or absorb) very narrow bands of light.

    As to the actual mechanism that determines how a photon interacts with any given molecule, I don't know. I suspect it has something to do with the distances between electrons as compared to the wavelengths of the photons, but that's really just speculation on my part.

    You're welcome :smile:
  8. May 29, 2012 #7

    The whole rainbow of colors hits the object that you're looking at. The object doesn't change the frequency of the radiation, it just absorbs some colors and reflects the rest. If it reflects all the colors, then it looks white. If it absorbs all, then it looks black. Different combinations of the different rainbow colors produce the entire array of colors that you see.

    Each individual photon will just tell you the color at a certain point. However, what you see is many many photons bouncing off of many surfaces, and put together they form a picture. Indeed, what you see on your monitor right now is simply a collection of dots lighting up in different colors. Looked at from far away, they form a coherent picture.
  9. May 29, 2012 #8
    There is other information photons/EM waves gives us with regards to sight. For, example, intensity is an important piece of information light gives us that we can sense and use to know what the object the light came from was. Direction is another piece of information (also a factor in intensity) that light carries with it that we can interpret to understand what the light came from.
  10. May 29, 2012 #9
    The light source around you is typically "white" light containing a wide spread of frequencies.

    The object you see absorbs some frequencies and reflects others.

    In the retina of the eye there are rods (for black/white night vision) and three types of cones (for color vision).

    The three different kinds of cones respond to different bands of frequency range, so they act as if they are sensitive to three colors. Combinations of response are processed to indicate all the colors you see.

    The actual mechanism of photo-reception is a molecule in the retina that resides in stacked layer structures within the rods and cones; this molecule is called "retinal". This is a long molecule that is looped back and forth through a semipermeable membrane in these layered structures - it passes through the membrane seven times so there are six loops, three on one side, three on the other side. These loops have parts that comprise functional groups (particular arrangements of atoms) and these loops can move with respect to each other when certain things happen.
    Retinal has two forms or shapes - one called "trans" and one called "cis", these being the difference between a very particular section of the molecule holding straight or that section being bent.
    There is an ongoing active biochemical process that uses energy to force the retinal to be bent. It is like setting a spring; when a photon is absorbed at a very particular atom in the molecule, the spring is released and the bent shape straightens out. The loops through the membrane can change their shapes depending on their structure and on what particular part of the length of the molecule is embedded in the membrane wall at that time - both can change, and change each other... That physical movement of the "spring" being released by the absorption of the photon (actually takes about six photons arriving at the same place at the same time for this to happen...) within the molecule laced through the membrane causes the shift in position of other functional groups on the loops and changes their influence on other things going on in the membrane area - the result is a depolarization of the charge that is usually maintained across the membrane (similar to how the nerve cells propagate an impulse), and the result of this is an amplification process ending in the proper full depolarization and initiation of a neural impulse associated with that sensor structure (rod or cone) out and through the optic nerve.
    Then the process of bending the "spring" back into position to be ready to go again is resumed. The process of releasing the "spring" is called "bleaching" because retinal is a pigment. Unbleaching the retinal is the process of setting the spring back into position to be ready to go again.

    When you get up in the middle of the night and switch on the light - you have bleached your retina, same as when you glance at the sun and see purple spots - those are bleached regions of the retina...

    At this point what you have is neural impulses from the rods and cones making their way down the fibers of the optic nerve... the retina itself does a tremendous amount of preprocessing of the signals from the rods and cones - it compares the response of adjacent sensors to detect edges, various angles of lines and shapes, movement, etc... the retina of the eye has ten physical layers of neural integration... same as the cortex surface of the brain - in many ways the retina is actually an extension of the cortical surface of the brain through the optic nerve out to the eyes themselves... wild stuff for sure.

    There are multiple processing nuclei (areas where special nervous structures are dedicated to various processing functions) that extend on up to higher levels... no one really knows what happens at the higher levels.
  11. May 29, 2012 #10
    Doesn't intensity come from how many of them there are? Hmm... maybe I'm thinking of something else.

    AFAIK, photons don't directly carry directional information. You can guess a few things about their past based on where they came from, but it'd just be a guess. Kinda like if you saw a guy walking out of a store's parking lot you could guess that he just went shopping there, but he could've just as easily been out a walk and simply decided to take a short-cut through the lot.

    Photons do carry information about their polarity (or a lack thereof), but I didn't mention it because the human eye generally can't detect that part. I think maybe bees can... There was some insect or animal or something that someone thought could maybe sense polarization, but I can't remember if it was ever proven.
  12. May 29, 2012 #11
    Actually, light does provide information about direction... in so far as the eye uses a lens and aperture to project onto the retina. The specific locations on the retina correspond geometrically with specific directions in front of the eye... so that when you have something sensed at a location on the retina, that corresponds to a point in your visual field, which also corresponds to a particular direction axis through the aperture, which also corresponds to the line between that point on the retina through the aperture to the direction from which the light originated.
  13. May 29, 2012 #12
    Yeah, but directionality, in the sense of the word that you're using, results from the interaction between the light and your eye (specifically, as you pointed, with the lens); it's not normally information that the photon actually carries. We're talking about how the eye perceives color here, not cutting-edge, causality-violating, quantum entanglement experiments.
  14. Jun 12, 2012 #13
    It's weird the thought that, what we see isn't actually how the universe is, just how our eyes interpret it.

    Like if our eyes evolved differently we'd have a completely different view of the universe init?
  15. Jun 12, 2012 #14


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    There is a huge distinction between sensing light as it arrives on your retina and 'seeing' the object from which it came. The whole experience of seeing involves analysing the pattern of signals from the retina and then forming a model / map of the space around you in your brain. The optics bit is the most 'trivial' step in the chain (pretty damned clever, nonetheless).
  16. Jun 12, 2012 #15


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    Sure, if we could see into the UV or IR range we could see different patterns, spectra, etc. But the fundamentals of the universe wouldn't change. A square would still be a square. Deducting 3 from 4 would still equal 1. Etc.
  17. Jun 12, 2012 #16
    The information (frequency, direction, intensity, phase, etc.) is still carried by the light from source to eye. All of the information of an object that we sense doesn't magically appear in our minds and it isn't created during interaction at the eye or by whatever is processing the light. After the information is processed, then we can talk about things like perception, but you cannot exclude all the variables of information that light transfers from source to sensor besides frequency. For example, if light only transfers an information value of frequency, modern fiber optics technology would be impossible. When talking about radio communication, frequency is only one component of an EM wave that you can modulate to encode information in.
    Last edited: Jun 12, 2012
  18. Jun 12, 2012 #17


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    Your eyes just have to assume that the light entering them comes, originally, from the direction it enters the eye (spectacles, telescopes and halls of mirrors all fool your brain about this). The way the brain makes sense of what it gets and translates it into an awareness of the space around you is amazing. Of course, blind people have an equally good model of their immediate surroundings, based on touch. We ALL have an excellent memorised picture of the inside of our mouths and we rely on our tongues for all the positional information.
    It's strange that we make such assumptions about the 'obviousness' of the space around us. Just try writing a computer program to do all that! Definitely state of the art.

    @DragonPetter: your point about modulation of light is correct for modern communications but our eyes have only a very crude measure of what we call colour (not wavelength) and we can only spot the very lowest frequency of amplitude modulation. It's a great example of 'just good enough for the job' as with most of evolution.
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