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Suppose a human is lying on a beach. He/she starts to feel warm after exposing his/her skin to the sunlight. I assume that feeling is due to the ability of the human body of "measuring" the increasing in temperature of the skin.

Now I want to understand what are the physical processes involved in this increasing in temperature. Imagine a group of photons impinging on the skin in a certain interval of time. I tried to list the possible interactions from a particle physics perspective between photons and the human tissue and I concluded that the possible interactions may be:

  1. Photoionization
  2. Compton scattering
  3. Rayleigh scattering
  4. Pair production

The first 3 seems to be reasonable, but the fourth one requires an energy threshold too high: there are no incident photons that may have that energy. I conclude that by looking at the spectrum of sunlight that actually reaches the earth's surface below the atmosphere. So I think that the pair production does not play a role in this situation.

Are there any other interaction processes between photons and tissue molecules involved in the increasing of temperature of the human tissue?

After listing the processes I wonder what actually increases the temperature: is the temperature increasing because the photons-molecules interactions lead to a transition of molecules to excited vibrational states? or maybe transitions to excited rotational states?

I thought that another possibility is that the photons interactions are increasing the kinetic energy of the water molecules in the skin or maybe are increasing the lattice vibration of other tissue (skin, bones or others). Are this processes happening simultaneously? One of this processes (for example transition to rotational excited states) is dominant over the others ?

I'm looking to a qualitative answer, without going into too much details of the Biology of the human body. I just want to create an approximate picture of this situation in my mind. I want to create a mental "video" from the instant in which a photon or a group of photons impinges in the skin to the moment in which tissue molecules are affected and the temperature starts rising up.

I thank in advance anyone who answers this question.

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    $\begingroup$ You are missing good old fashioned photoabsorption in your list, which is in my opinion the most relevant process here. I am fairly sure that human skin is typically not ionized to large degree by normal light. $\endgroup$
    – Hans Wurst
    Apr 8 at 12:30
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    $\begingroup$ This is really a biology question, but there's enough "crossover" that I'm not in favor of closing here. $\endgroup$ Apr 8 at 15:49
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You forgot garden-variety absorption! Here, light promotes electrons from lower energy states to higher energy states. However, skin is made of many small particles, so scattering is important as well.

Here’s the mental video:

  1. Sunlight illuminates the skin.
  2. The light encounters a disordered dielectric interface consisting of mostly transparent wavelength-scale particles.
  3. Massive scattering (ultimately driven by refraction and diffraction) ensues, and the light penetrates some depth into the skin layer.
  4. Meanwhile, there is continuous photoabsorption due to the interaction of the light with the skin molecules. This excites the electrons to higher, non-equilibrium energy states.
  5. The electrons relax back to their ground states, but in doing so transfer their excess energy to molecular vibrations (heat).
  6. These vibrations from all the cells within the absorption depth merge and spread through inter-molecular jostling (not a technical term) until they excite the nerves in the skin.
  7. The nerves send a signal to the brain, which informs the person that the skin is now warm.
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    $\begingroup$ Nice sequence. Note that asecond mechanism for "feeling heat" is when the epidermis is physically damaged, i.e. sunburn, but that shows up rather later on. $\endgroup$ Apr 8 at 15:50
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    $\begingroup$ The really cool thing about this is that this same absorption process (photons knocking on electrons) is the beginnings of photosynthesis. Except that instead of merely generating heat it triggers a chemical reaction that knocks carbons out of CO2 $\endgroup$
    – slebetman
    Apr 9 at 0:47

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