# What happens to light after it enters an eye

What happens to the light [energy] after it enters an eye and hits the rods and cones? I presume the energy becomes electrical, and it must be near 100% perfect, else our eyes would heat up? Or am I missing something?

The motivation of this question is solar panel technology.

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Nice question ! – user346 Jan 2 '11 at 18:58

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Lot of that light reflects back to where it came from. Think about "red eye" effect when shooting people with lighting on. Also, eye is of course getting a little warmer (to the point that you can evaporate the retina with laser) but the effect should not be terribly huge to cause damage usually. As for the efficiency of conversion to electric signals, I am not sure. Let's whether someone else has something to say. – Marek Jan 2 '11 at 19:01

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@Marek: Actually very little of that light is reflected, which is why your pupils appear dark. Red eye in photographs occurs because the camera flash, while brief, is very intense. Your eyes do not have time to react, so they are more dilated than they would be under continuous illumination of that intensity. In this case, enough bounces back for the camera to detect it, but it isn't typically very much. If it were then everybody would look red-eyed all the time. How creepy would that look! :) – Colin K Feb 4 '11 at 5:14
– Marek Feb 4 '11 at 9:11

## 5 Answers

There is some heating that takes place, but the amount is pretty trivial, because there just isn't that much light reaching the back of your eye. A back-of-the-envelope sort of estimate would be to say that the light of the Sun reaching the Earth's surface amounts to about a kilowatt of radiation per square meter. Your pupils have a radius of maybe a millimeter, probably much less in bright sunlight. So, if you're staring directly at the Sun (which, hopefully, you're not really doing), you're getting at most a few milliwatts delivered to the back of your eye. That's not going to tax the temperature regulation systems in your body, given that a living human generates about the same heat as a hundred-watt light bulb.

If you dramatically increase the amount of light delivered to your eye, say by accidentally catching a high-power pulsed laser in the eye, you can overwhelm the body's ability to carry away the heat, and do real damage. The pulsed-laser lab next to the office where I did my undergrad thesis research had a sign on the door explaining in gruesome detail what would happen if you were to catch a full YAG laser pulse in the eye, which involved the boiling retina basically blasting your eyeball out of your skull. Which is why you wear safety glasses in those labs, and knock before entering any optics lab.

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The head is quite well cooled, which is necessary: the brain clocks in at ~20W. A few extra mW from the eyes indeed don't matter. The laser danger is from in localized heating; the body as a whole could handle several W extra. (which is what your typical tabletop laser will produce) – MSalters Jan 4 '11 at 14:38
Other signs at laser labs say: "Refrain from looking into the laser beam with your 'remaining' eye" – Lagerbaer Feb 4 '11 at 4:59

Your reasoning is correct. Our eyes do heat up. The constant flow of blood is likely what carries away the heat.

Elaborate explanation: all biological processes are ultimately thermodynamical processes. Any "operation" - such as a single cycle of ATP production by ATP synthase, or the conversion of incident photons into metabolic energy (photosynthesis) or into neuronal impulses (vision) - can be thought of as an engine with a certain efficiency $e$. This is traditionally given by the ratio of the amount of useful work $W$ the engine performs in a single cycle to the total amount of energy $E$ fed into it:

$$e = \frac{W}{E}$$

and since $E = W + Q$, where $Q$ is the waste heat released into the environment, we have:

$$e = 1 - \frac{Q}{E}$$.

The second part of your question as I interpret it is: "What pressures of natural selection lead to the evolution of vision?". The simplest answer I can think of is the exposure of photo-reactive compounds to a constant bath of photons (from the Sun) over millions of years lead to the evolution of different mechanisms to exploit this energy, one of those mechanisms being that of vision. Of course, this is not really an answer. Maybe somebody with greater knowledge of evolutionary biology can shed more light on this aspect.

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Well since this is the most complete answer, I'll try to explain the last question of Jonathan here. There are biological solar panels and they are called chlorophylls. They are responsible for photosynthesis(although chlorophyll is not the only substance capable of doing so, it is the most common). Now why do WE not have chlorophylls? It is simply because photosynthesis itself isn't that efficient in supplying a mobile and agile system like an animal's. Even plants do not use it directly to generate energy directly, they rather use it to produce glucose, which they oxidate later on. – Cem Jan 3 '11 at 1:03
@Cem I think the gist of the problem lies, in your words, in the fact that "photosynthesis itself isn't that efficient". Evolution is associated with the creation of mechanisms with each one being more efficient than its predecessors. The interesting question is how a system switches between states with different efficiencies during the course of evolution. – user346 Jan 3 '11 at 1:16
@space_cadet: Something being more efficient than its predecessor does not mean that it is efficient enough. This is called a fallacy actually. Furthermore, please read the whole sentence: isn't that efficient in supplying a mobile and agile system like an animal's. To generate enough energy purely with photosynthesis, you'd need to be travelling around with a few meter squares of green panels hovering above your head, which I assume would reduce your mobility. – Cem Jan 3 '11 at 20:03
@cem I thought I was agreeing with what you said. And I agree with your observation about the size of photosynthetic surfaces for "mobile plants". To clarify, the efficiency of a system is always measured w.r.t its operating environment. For instance, humans are efficient (in that they stay alive) in an oxygen rich environment, but inefficient (i.e. not alive) in an oxygen-poor background. So when we say that one mechanism is more efficient than another that aspect has to be kept in mind. – user346 Jan 3 '11 at 21:39
Yeah exactly. I think I misinterpreted your first comment and thought that you were implying something totally different. Your second comment clarified much for me. And yes, exactly as you say, efficiency is completely dependent on the situation. – Cem Jan 4 '11 at 9:54

You are correct, that the light becomes an electrical charge from our eyes, and the resulting signal is processed by a very complex system. The rest of it will heat up our eyes, but realize that the same thing is true of light hitting our body anywhere.

The signal generated from our eyes is minimal, I wouldn't use the word solar panel in the slightest to describe the affect at all.

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"The second part of your question as I interpret it is: "What pressures of natural selection lead to the evolution of vision?"

algae grows at sunny spots. having a means to detect these sunny spots gives a greater chance of having more food and thus more offspring.

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In humans (and vertebrates) the retinal structure evolved basically inverted in that the photons have to pass through layers of neurons and blood vessels before they hit the rods and cones. Wolves and some other vertebrates have a reflective membrane(the tapetum lucidum) behind the photoreceptors that reflect the photons back through the rods and cones and provide a double pass amplification (causing "eyeshine"). Humans lack this. The retina has ten layers which process signals from the photoreceptors, whose surface membranes contain retinol, which is isomerised by the photon energy, which affects ion channels, and may result in an action potential (necessarily leaving out all sorts of steps) which is preprocessed by retinal ganglion cells and information compressed and conducted along the optic nerve axons to the visual processing areas of the brain. Vision has been extensively studied and is immensely complicated. Much of the energy takes part in chemical reactions and in multiple action potentials (electrochemical) in the preprocessing in the 10 layer retina. Francis Crick (physicist, of DNA fame) and Christof Koch have written much about it--Koch's book, The Quest for Consciousness is fairly technical neuroscience focussing on the visual system.

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""and provide a double pass amplification "" Think about the meaning of "amplification"! – Georg Feb 4 '11 at 10:27
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