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I obviously do not understand how a wavelength greater in length than a cell can be detected by a cell.

As I understand it, infrared ranges from 300GHz to hundreds of THz.

As an example, if an infrared wave is 1 THz, it is 0.3mm in length. How does this get even noticed by a cell?

I would think it doesn't see the wave simply because the wave is longer (larger?) than the cell.

Qmechanic
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Gary La
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    What sort of infrared sensitivity are you asking about? How does a portable radio a few cm in size receive an AM broadcast signal with a wavelength of 300m? – John Doty May 17 '23 at 00:25
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    Where did you read that nerve cells can detect infrared light? The cells in your eye don't detect wavelengths longer than ~780 nm. – The Photon May 17 '23 at 00:33
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    Or are you asking why you feel warm when stand in sunlight? In that case the nerves don't detect the IR light. Water in your body absorbs the IR, and then your nerves detect that your flesh is warmed up. You might want to ask how can a water molecule absorb light with long wavelengths, but that's a different question. – The Photon May 17 '23 at 00:37
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    Somewhat related. How light interacts with small things - If we repeatedly divide a colorful solid in half, at what point will the color disappear? – mmesser314 May 17 '23 at 03:44
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    https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3316879/ describes a mechanism by which the question reduces to "how is it possible for a water molecule to absorb infrared EM radiation". – zwol May 17 '23 at 14:02
  • In the mammalian eye, there's a light-sensitive molecule (retinal) whose light absorption curve depends on the "opsin" protein it's bound to. Retinal is very roughly 2 nm from one end to the other along its long axis, so again the question reduces to "how do small molecules manage to absorb longer wavelengths of light". – zwol May 17 '23 at 14:12
  • I assume the radio's antenna is designed according to the wavelengths it needs to accept. – Gary La May 18 '23 at 11:14

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The cells don't react directly to radiation in that wavelength range. Temperature sensitive cells can react to the heating caused by this radiation, though. It doesn't take a lot of power to set off our temperature detectors. I would estimate that one can easily "feel" 1W per square decimeter of skin or less in an otherwise "quiet" thermal environment. The reason for that sensitivity is biological: the human body produces $100W$ of heat and has approx. $2m^2$ of surface area. It is of vital importance to us to maintain our body temperature with that heat loss of $100W/200dm^2=0.5W/dm^2$. A few tenths of a $W/dm^2$ of heat loss more or less will lead to hypo- or hyperthermia.

FlatterMann
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    What I do not understand is how is it possible that my molar tooth that is sensitive to cold can feel its touch within a fraction of a second; after all it is not made of diamond... What is the heat conduction coeff of the enamel? – hyportnex May 17 '23 at 01:21
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    @hyportnex I can feel your pain. Literally. – FlatterMann May 17 '23 at 03:22
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You can very well ask how does an infrared photon get absorbed by a hydrogen atom - which has a size of just about 0.05 nm.

The condition you used: $\lambda > L$ where $L$ is the cell's length, accounts for just scattering alone (which you seem to know). But there's just one photon. If you send more photons, there will be some absorption or scattering by the same cell. And there's a multitude of scattering effects out there; elastic (Rayleigh) and inelastic scattering (Raman effects) - which transfers energy to the cell.

Karthik
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@hyportnex: The secret are optical phonons. They are oscillations, in any lattice cell with at least two different ions oscillating against eachother. The very stiff coupling yields frequencies in the infrared range of light, only weakly k-dependent, and the dell dipole moments in a line are coupling to light perfectly, hence the name optical phonons.

Even if they don't play a role energetically at 37C by the Boltzmann factor $e^{- \hbar \omega/kT}$, they are perfect signal transmitters.

Roland F
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