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My understanding: Electromagnetic radiation is carried via photons - which enter our eyes and activate receptors depending on the amount of energy the photons have when hitting them.

At what point do we "lose" photons which are above or below our visible light spectrum?

  • Does the cornea at the front of the eye deflect those photons (They don't enter the eye at all)?
  • Does it enter our eye, but the degree of refraction cause it not to hit the retina?
  • Does it enter our eye and hit the retina, but the retina somehow doesn't pick it up?
  • Does it enter our eye and hit the retina, the retina picks it up, but somewhere in our image processing the input is ignored when passed on to the brain?
  • Something else?

I am looking for the answer that is more pertaining to the physics of the eye and electromagnetic radiation, but if the Biology Stack Exchange is a more appropriate place for this question, please let me know.

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    $\begingroup$ I think the Wikipedia entries on the visual system are pretty comprehensive: The absorbances of the photoreceptive cells just fall off quickly outside the visible spectrum - so it's "the retina somehow doesn't pick it up". $\endgroup$
    – ACuriousMind ♦
    Commented Dec 28, 2015 at 20:34
  • $\begingroup$ But then what happens to the EM wave? If it doesn't absorb, does it continue (pass) through the cells? $\endgroup$
    – DoubleDouble
    Commented Dec 28, 2015 at 20:44
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    $\begingroup$ Depends on its wavelength. An X-ray will mostly just pass through, infrared will probably just be absorbed by other things - there's no generic answer to that question. $\endgroup$
    – ACuriousMind ♦
    Commented Dec 28, 2015 at 20:46
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    $\begingroup$ The cornea absorbs ultraviolet out to ionizing energies. Infrared gets to the retina but does not trigger any of the molecular transitions that start a nervous response. This is why at modest energy densities infrared lasers are more dangerous than ultraviolet: they both pose hazards from the inability to see them to avoid them, but corneal transplants are available and a burned retina is not fixable. $\endgroup$
    – dmckee --- ex-moderator kitten
    Commented Dec 29, 2015 at 0:58
  • $\begingroup$ I think the combination of comments have answered my question. But unsure how to proceed since these are comments and not answers. $\endgroup$
    – DoubleDouble
    Commented Dec 29, 2015 at 18:56

1 Answer 1

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Short answer:

  1. Proteins or photopigments involved in vision have different sensitivity to light
  2. The transduction paths are different or the paths to produce/mantain the proteins/photopigments.

Generic answer:

The proteins or photo pigments responsible for sensing light usually undergo a conformation change/chemical change induced by light, but this change depends on the wavelength of the incident light. There are many different proteins that are light sensitive and the reasons why this reaction happens on a specific wavelength interval and what kind of reaction happens depends on the protein. However, not only different proteins react differently to light, but also what happens next may be different. After the protein suffers a change because of light (whatever that is) a long chain of chemical reaction begins which will end up as an electrical signal. Also, usually proteins or photopigments have to be recycled after being activated/bleached, which is done by parallel biochemical pathways. Mistakes on these paths will also after the ability of seeing a specific colour.

Example:

There are many kinds of vision mechanisms in the animal kingdom. The colours (sets of wavelength intervals) that each animal sees highly depends on the set of proteins that these animals have and the transduction path after the proteins are activated. We humans, for example, are good at seeing green and other colours that help us to identify food that we usually eat [1]. Interestingly, even small changes on the aminoacid sequence of the proteins change its sensitivity for light. For example, some people have cones (some of the cells responsible for vision in humans) with photo pigments that are sensitive at 530 nm and some people are sensitive at 562 nm. This difference is caused by only 3 aminoacids substitutions in a protein [2]. Another example involving the recycling pathway of a photopigment can be found at [3]. It was observed that some mutant Drosophila flies had their vision impaired because they could not keep the concentrations of the visual pigment Rhodopsin.

[1] Surridge, Alison K., Daniel Osorio, and Nicholas I. Mundy. "Evolution and selection of trichromatic vision in primates." Trends in Ecology & Evolution 18.4 (2003): 198-205.

[2] Neitz, Maureen, Jay Neitz, and Gerald H. Jacobs. "Spectral tuning of pigments underlying red-green color vision." Science 252.5008 (1991): 971-974.

[3] Ostroy, SANFORD E. "Characteristics of Drosophila rhodopsin in wild-type and norpA vision transduction mutants." The Journal of general physiology 72.5 (1978): 717-732.

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