On seeing and especially in the dark

Our retina detects the change in the electromagnetic field due to the jiggling of the atoms surrounding us as I understand the seeing process. In the total absence of a light "source", however, why can't we detect the jiggling of atoms in our insides, for example (or, in this case, even when there's one)? Perhaps we do but our brain hides such images? What about bodies outside us? I'd assume that there's some kind of atomic motion in the table in front of me.

• electromagnetic waves have a frequency. The optical frequencies that excite the retina are a very small portion of the electromagnetic waves. en.wikipedia.org/wiki/… . The motion of atoms in matter fall into the infrared region and the cones of the retina do not detect them. Also seeing colors is with combinations of frequencies. Biology is more complicated than simple physics – anna v Oct 8 '19 at 10:46

What your retina detects are photons: quanta of light. These photons are indeed (loosely speaking) created by the jiggling of atoms (as well as by other processes, but this is an important one).

Each photon has a given energy (and this corresponds to the frequency of the light by the relation $$E = h\nu$$ where $$E$$ is the energy, $$\nu$$ is the frequency and $$h$$ is a constant known as Planck's constant). What range of photon energies a given bit of matter emits depends on how fiercely the atoms in it jiggle, and how fiercely the atoms jiggle corresponds to the temperature of the object.

In particular the hotter an object is the more vigorously its atoms vibrate and the higher the energy of the photons it emits are. In fact this is not strictly true: objects of any temperature can emit photons of all energies, but the rate at which they emit them depends on the temperature, and it becomes very small indeed for high-energy photons.

Your retina is only sensitive to photons within a certain range of energies: the range which corresponds to visible light. Visible light has a frequency between about $$430\,\mathrm{THz}$$ and $$770\,\mathrm{THz}$$, which corresponds to a photon energy of between about $$2.8\times 10^{-19}\,\mathrm{J}$$ and $$5.1\times 10^{-19}\,\mathrm{J}$$, or in more useful units, about $$1.8\,\mathrm{eV}$$ and $$3.2\,\mathrm{eV}$$: $$1\,\mathrm{eV}$$ is the energy required to move an electron through a potential difference of a volt.

Well, if you assume that you are surrounded by objects at about room temperature, the question is now, how many photons do such objects emit in the right range of energies that your retina could detect them. The answer turns out to be about one photon every minute per square meter of area.

To have a chance of being able to see something your eyes need to be receiving a large number of photons every second (how large is something I don't want to get into because I'm not actually sure, but let's say at least thousands and actually much more than that probably). This is hugely more than the rate at which of photons are being emitted by objects around you, let along the tiny fraction of them which make it into your eyes, and this is why you can't see in the dark.

The reason you can see in the light is that the Sun is very much hotter than room temperature, and it emits a huge flux of visible-light photons. These photons reflect from all the objects around you and then end up in your eye.