# What happens to light after it hits the retina? **URL:** \ **Category:** Factual Questions\ **Created:** [June 8, 2003, 2:04pm UTC](https://boards.straightdope.com/t/what-happens-to-light-after-it-hits-the-retina/180455 "2003-06-08T14:04:05Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![Scarf-Ace](https://avatars.discourse-cdn.com/v4/letter/s/bc79bd/32.png) [@Scarf-Ace](https://boards.straightdope.com/u/Scarf-Ace)\ **Post date:** [June 8, 2003, 2:04pm UTC](https://boards.straightdope.com/t/what-happens-to-light-after-it-hits-the-retina/180455/1 "2003-06-08T14:04:05Z")
I mean the light is made up from some kind of particle, when these hit the retina electrical signals travel through the optic nerve and our brain does the rest. But what about the light particle that hits the retina? Where does it go? Is it absorbed like with skin? I imagine we must do something with it since our eyeballs would get clogged up with a load of old light otherwise. ---
**Author:** ![USCDiver](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/uscdiver/32/354_2.png) [@USCDiver](https://boards.straightdope.com/u/USCDiver)\ **Post date:** [June 8, 2003, 2:09pm UTC](https://boards.straightdope.com/t/what-happens-to-light-after-it-hits-the-retina/180455/2 "2003-06-08T14:09:07Z")
Rods and cones (the receptors in the eye) have special molecules that change shape when they absorb a photon. This shape change opens ion channels in the cell that initiates an action potential (technobabble for electrical signals in the brain). Does that answer your question? ---
**Author:** ![ParentalAdvisory](https://avatars.discourse-cdn.com/v4/letter/p/53a042/32.png) [@ParentalAdvisory](https://boards.straightdope.com/u/ParentalAdvisory)\ **Post date:** [June 8, 2003, 3:14pm UTC](https://boards.straightdope.com/t/what-happens-to-light-after-it-hits-the-retina/180455/3 "2003-06-08T15:14:25Z")
Why does it have to go anywhere? It just constantly gets replaced. How are particles getting into my eye? Isn’t it just a reflection? F it, I don’t know. ---
**Author:** ![PosterChild](https://avatars.discourse-cdn.com/v4/letter/p/e274bd/32.png) [@PosterChild](https://boards.straightdope.com/u/PosterChild)\ **Post date:** [June 8, 2003, 3:27pm UTC](https://boards.straightdope.com/t/what-happens-to-light-after-it-hits-the-retina/180455/4 "2003-06-08T15:27:07Z")
This may be what you’re looking for. The photons act like particles in some circumstances, but are energy and the energy gets absorbed by the channels in the retinal cells (cones or rods). The absorbed energy causes a conformational change which allows ions across the membrane, etc. The energy itself, I think, is disipated as heat through conduction, radiation (I’d guess there’d be discountable convection). PC ---
**Author:** ![mnemosyne](https://avatars.discourse-cdn.com/v4/letter/m/c4cdca/32.png) [@mnemosyne](https://boards.straightdope.com/u/mnemosyne)\ **Post date:** [June 8, 2003, 5:13pm UTC](https://boards.straightdope.com/t/what-happens-to-light-after-it-hits-the-retina/180455/5 "2003-06-08T17:13:19Z")
Light is sometimes a particle, sometimes a wave, as defined by quantum mechanics (which I do not know enough about to elaborate on). A classical drawing of a photon hitting the retina usually shows a squiggly arrow (the light) entering the eye through the pupil and hitting the back of the eye, at the retina. This “particle” of light is really just a form of energy, and should not be thought of as an object hitting a wall, which is what that drawing usually implies. Once the light energy hits the retina, the energy is transfered to the photoreceptors found in the rods and cones. The photoreceptors are chemicals - fairly large molecules which undergo a conformational change at one or more locations when energy is added to them. This change in conformation places the molecule at a higher energy state than is usual for it, and as it “relaxes” to it’s ground state, it gives off energy which is transfered to a nearby nerve ending, which then carries the nerve signal to the brain in order for it to be processed correctly. So in effect, that photon ends up in your brain. ---
**Author:** ![Scarf-Ace](https://avatars.discourse-cdn.com/v4/letter/s/bc79bd/32.png) [@Scarf-Ace](https://boards.straightdope.com/u/Scarf-Ace)\ **Post date:** [June 13, 2003, 10:55am UTC](https://boards.straightdope.com/t/what-happens-to-light-after-it-hits-the-retina/180455/6 "2003-06-13T10:55:40Z")
Kewl! ---
**Author:** ![Colophon](https://avatars.discourse-cdn.com/v4/letter/c/f05b48/32.png) [@Colophon](https://boards.straightdope.com/u/Colophon)\ **Post date:** [June 13, 2003, 11:32am UTC](https://boards.straightdope.com/t/what-happens-to-light-after-it-hits-the-retina/180455/7 "2003-06-13T11:32:48Z")
Out of interest, the molecules that change shape are called rhodopsin (also called visual purple) in the rods and iodopsin (three variants sensitive to different wavelengths of light) in the cones. Here’s a scheme that shows the shape change that takes place. [http://www.biology.ucsc.edu/classes/bio70/images/rhodopsin.gif](http://www.biology.ucsc.edu/classes/bio70/images/rhodopsin.gif) It’s a cis-trans isomerisation reaction, i.e. a double bond between two carbon atoms changes configuration, so the carbon chain goes from being “bent” to being “straight” (obviously these terms are relative). The reverse reaction recycles the rhodopsin molecules so they can “fire” again. Vitamin A helps with this reversal, which is why your mum told you that carrots help you see in the dark… ---
**Author:** ![Quitesane](https://avatars.discourse-cdn.com/v4/letter/q/8dc957/32.png) [@Quitesane](https://boards.straightdope.com/u/Quitesane)\ **Post date:** [June 13, 2003, 11:51am UTC](https://boards.straightdope.com/t/what-happens-to-light-after-it-hits-the-retina/180455/8 "2003-06-13T11:51:45Z")
I wonder how much weight gain i can contribute to the photons(energy) absorbed by my brain each year. :dubious: ---
**Author:** ![Dragline](https://avatars.discourse-cdn.com/v4/letter/d/edb3f5/32.png) [@Dragline](https://boards.straightdope.com/u/Dragline)\ **Post date:** [June 13, 2003, 1:26pm UTC](https://boards.straightdope.com/t/what-happens-to-light-after-it-hits-the-retina/180455/9 "2003-06-13T13:26:06Z")
That little hole in the inner part of the eyelid is the Photon Drain. It lets them back out to roam freely in the world.