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Where do all the light photons go?


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Whilst staring at M42 last night, I got into thinking about all the things that have happened in man's history in the 1,500 odd years it's taken the light to reach my eye.

Then a though occurred to me - after the light photons have travelled thru space, down thru the earth's atmosphere, down my scope, bounced off the diagonals and entered my eyes, then what? Where do they do? What do they do?

Is there some light graveyard somewhere? Do they hang out with the lost socks from the laundry and all those ballpoint pens that disappear, playing pool and drinking beer? Where do they go?

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good question :)

what really amzes me is how many photons there must be given that they are going in all different directions and enough go down my scope for me to see an image - how many go down my scope? And how many don't? :shock:

Dan (counting photons)

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Photons are an example of bosons (particles which obey Bose-Einstein statistics) and as such the number of photons is not conserved in an electromagnetic interaction - quite literally photons are created and destroyed at will whenever anything electromagnetic (that includes anything to do with light) 'happens' in the Universe.

There are plenty of photons to go around, in the Universe as a whole, at any one instant. Maybe someone can come up with the figure - I think it stands at somewhere around 1090.... :shock:

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So are they absorbed by charged particles in my eyes (and if so, wont my eyes get fatter?!?) or are they destroyed once I have viewed them?

Oh, and I started thinking along those lines as well, kniclander, but the number got so big, my head began to spin!

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They are destroyed in the act of absorption by one of the molecular electrons (most likely) in the photosensitive parts of your eye. The molecule's energy is thus increased and it proceeds to shed this energy by bumping other molecules and all this bumping is termed the sensation and awareness of light.

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  • 2 weeks later...

A photon from M42 hits your eye and interacts with an electron in an atom of your retina, starting the nerve signal that results in your seeing the nebula. This process generates some heat, released from your body as more photons.

You don't see an actual photon from M42 in your telescope: the light has passed through glass (and air and eyeball), meaning the original photon has hit atoms, causing electron excitation and re-emission of new photons.

So the answer to "where do photons go?" is that generally speaking they turn into more photons (thanks to processes like the above). Sometimes the process is one-to-one (one photon is absorbed and a single one re-emitted) but if one photon gives rise to several being re-emitted then these must each have lower energy than the first. Planck's equation (E=hf) then tells us that these secondary photons have lower frequency.

So the overall trend is that high-energy, high-frequency photons get turned into lots of lower energy, lower frequency ones. This is equivalent to saying that the entropy of the universe can never decrease. Low energy photons have higher entropy.

An example is what happens to photons from the sun when they hit the Earth. Visible light frequencies warm the planet and get re-emitted as lower frequency infra-red.

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  • 2 years later...

Brilliant question with fascinating application.

Photons interact with rods and cones in the retina via Chemical Reactions

So Physics, Chemistry and Biology all taking part in the mix

Great question to explore further with the inquisitive audience

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A photon from M42 hits your eye and interacts with an electron in an atom of your retina, starting the nerve signal that results in your seeing the nebula. This process generates some heat, released from your body as more photons.

You don't see an actual photon from M42 in your telescope: the light has passed through glass (and air and eyeball), meaning the original photon has hit atoms, causing electron excitation and re-emission of new photons.

So the answer to "where do photons go?" is that generally speaking they turn into more photons (thanks to processes like the above). Sometimes the process is one-to-one (one photon is absorbed and a single one re-emitted) but if one photon gives rise to several being re-emitted then these must each have lower energy than the first. Planck's equation (E=hf) then tells us that these secondary photons have lower frequency.

So the overall trend is that high-energy, high-frequency photons get turned into lots of lower energy, lower frequency ones. This is equivalent to saying that the entropy of the universe can never decrease. Low energy photons have higher entropy.

An example is what happens to photons from the sun when they hit the Earth. Visible light frequencies warm the planet and get re-emitted as lower frequency infra-red.

/thread

Awesome answer. Stick a gold star to your fridge :D

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I was expecting to have to answer a question of photon detection in the eye in a recent exam, but it didn't come up - so frustrated, and just for completeness - here is what happens.

The photon hits the photo receptor. These are either cones of three types, tuned to red green and blue (confusingly called L, M and S cones) , or rods which just detect light monochromatically.

Incidentally in the fovea - the bit of your eye that has the highest concentration of detectors there are only cones. Cones only work above a certain light threshold, so that's why you only see B&W at night, and you see best about 10deg from what you're focused on.

Anyway - the photon hits a light sensitive molecule called an opsin (that come in different types for different wavelengths), which normally has a bit of a kink in it. Its slightly bent, but the energy from the photon is absorbed, and straightens it out into its activated form (the all-trans form). This form can then catalyse a G-protein that called transducin. One photon activated opsin can catalyse maybe 500 transducins. Transducin then activates another molecule PDE - which catalyses maybe 800-1000 molecules of cGMP (another common cellular protein). The cGMP molecules open gates in the cell that let in Calcium and Sodium ions, and this generates a nerve signal. So a single photon gets amplified many times in this cascade to generate a nerve signal.

As rods are much more sensitive to light than cones (30-100 times more so), they tend to be constantly saturated in daylight,. About 200-400 photons are enough to saturate a rod cell. When you go into darkness, the opsin pigments are no longer getting bleached out so build up to a very sensitive level over time.

After about 30 minutes of darkness, you are dark adapted, which is why astronomers know that you can see better after a while in the dark, and even better by looking off centre slightly where there are mainly rod cells.

(In actual fact the eye works backwards generating a dark current which light inhibits. So we're actually adapted to detect darkness rather than light, but it comes out to the same thing.)

Edited by JulianO
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And if you want to do a dissertation in Visual PhotoTransduction, this would be a good starting point

Visual phototransduction - Wikipedia, the free encyclopedia

Visual phototransduction is a process by which light is converted into electrical signals in the rod cells, cone cells and photosensitive ganglion cells of the retina of the eye.

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Wow, over two years since I first posed the question and I'm STILL learning stuff.

Thank you kindly to all who have contributed to this thread. I have a much, much better understanding now. Extremely interesting.

Hope the exam went ok, JulianO?

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