So we see cause photons hit our eyes. Cool. But what about after that? Do they just stop and lose their energy? Are they absorbed by us?
And generally, what happens to photons that “run out of energy”? Having no mass, they shouldn’t be converted to heat right?
In: Planetary Science
They get absorbed by whatever particle (usually an electron [or electron cloud] around an atom). When flying through space one never “runs out of energy” for the same reason celestial bodies don’t slow down, there’s nothing to stop it. When something stops it, it gets absorbed.
They deliver their energy to the molecules they interact with, causing a slight change in shape that triggers the nerve response we eventually interpret as sight. As with anything that’s causing work to be done in the technical sense, there is waste heat. The photon ceases to exist when its energy is fully absorbed, although you could argue it lives on in some sense in whatever heat emissions come later.
To the second part, photons never “run out of energy” they instead just become harder and harder to detect… dimmer in a sense. They shift further to the red end of the spectrum, becoming longer and longer waves, but they never just magically go away.
Photons are quantum mechanical objects. They cannot be “half there” or even “99.9% there”. They are also massless and have momentum (momentum = frequency times planks constant).
Once they interact with something like the photo sensitive chemicals in the rods or cones of your eyes their energy/momentum is absorbed and they cease to exist.
So, you cannot see photons in the sense that you cannot see light in a dark room unless it’s shining at you. If you see light from a flashlight shining across a room in front of you at night what you are probably seeing the rare photon bouncing off of dust or being absorbed and remitted by the dust (scattered).
Now I lied a little. all QM objects can kind of be 1/2 there. If their field equation of probability is at 50% … there is a 50-50 chance a particle (photon on this case) is present. Only by checking, via attempting to detect it, would you know for sure. It’s pretty fuzzy wazzy, and beyond ELI5.
The photons being absorbed by the cells in your eyes, giving up their energy is exactly how they stimulate your retina. The energy is what tells your nerves to send the signal to your brain to tell you what you see.
Photons cannot run out of energy. They either continue on with their energy or they get absorbed, and give their energy to whatever they hit, at which point, they stop existing.
Depending on what they hit, they are usually turned into heat, but they could also excite an electron in an atom, either breaking apart a molecule (like silver nitrate in an old school camera) or being reemitted as a lower energy photon (fluorescence and phosphorescence)
Yes, they get absorbed by us, by the light sensitive molecules in our eyes.
The energy they carried moves an electron to a higher (“excited”) energy state, and then a bunch of other stuff happens that ends up with a signal going to the brain.
>Having no mass, they shouldn’t be converted to heat right?
They have energy, which is the important part.
Photons can get converted into heat, but it depends on the wavelength. Infrared photons transfer energy to entire molecules, causing them to vibrate directly as heat. That’s why infrared light feels warm and is often called “heat” as well.
Other wavelengths of light don’t end up as heat directly, but can end up converted to heat eventually as other effects happen in atoms and molecules, unless the energy is used up otherwise, like in a solar cell.
They bounce off them, so that others may see your eyes. Otherwise your eyes would be black holes in your face.
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