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Is there a physical explanation for why we can't see certain colors?

  1. May 12, 2013 #1
    Why can't low frequency or high frequency light enter my eye just like ordinary light? I imagine an ordinary camera should be able to detect light if it enters the lens, no matter the frequency. Am I under a misapprehension about cameras? Why can't the eye do the same?

    I know this is a physics forum, but could the explanation be that we evolved to see visible light because it is more useful than lower/higher frequencies? Perhaps the answer is in biology. But whatever, I'm sure you guys will set me straight.
  2. jcsd
  3. May 12, 2013 #2


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    All light does enter your eyes; only what we term "visible light" causes a reaction.
    You are correct about it being a biological phenomenon. The rods and cones in the retenae of your eyes undergo a chemical change upon exposure, which triggers a neurological signal. The specific chemicals involved determine what frequencies are noticeable. There might be some evolutionary advantage to what those are for humans. Birds and insects can see well into the UV band.
    This might be better suited to the Biology subforum. I'll just report it to the Moderators to see if they want to move it. Please don't repost it there.
  4. May 12, 2013 #3

    D H

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    Thread moved to Biology forum.

    High UV light cannot enter your eye; the cornea and lens absorb it. This is why you should wear sunglasses on a bright, sunshiny day. That absorption of UV can result in permanent damage to the cornea and lens. UV light that is just above the visible range can get to the retina but there's nothing in the human retina that is sensitive to that light.

    Infrared light enters your eye and reaches the retina. However, there's nothing in the retina that is sensitive to infrared electromagnetic radiation. This is why IR lasers are so dangerous. Visible lasers can cause damage to your retina, but your blink reflex mitigates that damage. Because you can't "see" IR light, an IR laser will cause damage that is not mitigated by the blink reflex.
  5. May 12, 2013 #4
    This thread should set you straight:


    Check out post #6, I put a cool picture up there:approve:
  6. May 12, 2013 #5


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    I was unaware of that. My apologies for misleading Acesuv.
  7. May 12, 2013 #6


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    Our eyes contain proteins called rhodopsins that can detect light because they contain a chromophore molecule (retinal) that can switch between two different shapes. The activation energy, the amount of energy that it takes to switch the chromophore between these two states, is on the order of a few 100 kJ/mol, which corresponds to the energy of photons in the visible range (different rhodopsin proteins contain different chemical environments surrounding the chromophore which tunes the absorption to a specific wavelength).

    If we wanted to engineer a rhodopsin to be sensitive to much lower frequencies, what activation energy would we want our chromophore to have? For example, if you wanted to detect radio frequencies (around 100 MHz), each photon would contain about 0.04 kJ/mol. Therefore, the activation energy required to change the shape of the chromophore molecule would have to be about 0.04 kJ/mol.

    Do you see the problem here? At human body temperature (37oC, 310K), the amount of thermal energy available is about 2.6 kJ/mol. This means that thermal energy alone will be enough to activate our hypothetical radio-sensitive rhodopsin! The protein would not actually be able to sense radio waves because it would always be on regardless of whether or not radio waves were present.

    If you look at calculations like these, you'll see that it is not an accident that animal vision is limited to a small range of the EM spectrum ranging from the near IR to the near UV. At frequencies significantly below the visible region, you get to the point where thermal energy becomes more energetic than the photons and a chromophore would not be able to distinguish thermal energy from the absorption of photons. At frequencies significantly above the visible region, you get into the range of ionizing radiation – photons so energetic that their energies are comparable to the activation energies for breaking chemical bonds.
  8. May 13, 2013 #7
    The human lens normally absorbs most UV-B and some UV-A radiation. When I had cataract surgery my natural carbon-based lenses were replaced with silicone lenses that are more transparent to the UV. Most of the UV-A and some UV-B gets through these lenses (and my optical humors) and stimulates my retinas, but not in a good way. The cones and rods ignore it, but the epithelial cells respond and the result is discomfort, like when you are in sunlight that is strong enough to cause a sunburn.
    I need to wear UV protection outdoors now except on the cloudiest of days.
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