Is Light/Radiant Energy considered Kinetic Energy or Potential Energy?

I have started studying Thermodynamics, and the concept of internal energy was introduced, and defined as the sum of kinetic and potential energies of a system.

Another important definition was that:

ΔU = q + w , where

q = heat absorbed/released by the system

w = work done on the system by surroundings/work done on surroundings by system

U = Internal Energy of a system

There are lots of chemical reactions, such as combustion reactions, where light is produced, but there aren't many examples in thermodynamics concepts that deal with light energy. Where does light fit into this concept of internal energy?

If light is produced by a chemical reaction, is that a form of work done by a system on the surroundings?

If light is absorbed, is that a form of work done on the system by the surroundings?


2 Answers 2


Absorbing a photon will increase the potential energy of a molecule, and possibly a little kinetic energy change if rotational and vibrational energy is changed in the process. The translational kinetic energy is not changed immediately after absorption (there if no external force to do this) but can be imparted into products after reaction when bonds break. Green photons have $\approx 25000\,\mathrm{cm^{-1}}$ vs thermal energy of $\approx 210\,\mathrm{cm^{-1}}$ so a lot of energy can be released on reaction. If light is emitted then this can simply heat up the surroundings by being absorbed by unreactive molecules or some work could be extracted if these photons are absorbed by, say, a photodiode/solar panel. (Excited molecules can also release energy by non-radiative means but this just heats up the surrounding molecules which energy then diffuses away into other molecules))

  • $\begingroup$ Thank you so much for your explanation. So light energy absorption or release can be categorised as both Heat and Work in the First Law of Thermodynamics equation? For example, when the absorption of photons increases the temperature of the system, would that be categorized as Heat? Or when a reaction like combustion produces photons, would those photons be categorised as Work? $\endgroup$
    – Nefeli
    Commented Feb 7 at 3:17
  • 1
    $\begingroup$ heat and work clearly on emission, as to doing work on absorption , this seems to be more difficult, perhaps ionising a molecule producing electrons that could then be used to do work, or storing energy in a reaction as in isomersation that could be released later to do work. $\endgroup$
    – porphyrin
    Commented Feb 7 at 9:43

Electromagnetic radiation hereafter called EM, is energy. EM consists of individual photons or possibly of coupled photons. A photon has a specific energy, specific momentum and a spin or polarization. Bundled photons also have an entropy based on their frequency distribution, directionality, and source. Photons also have size in 3 dimensions and a questionable 4th dimension.

EM interacts with matter in at least three common methods. The First is Black Body Radiation, BBR. BBR is inextricably involved with the kinetic energy and momentum of molecules and of the photons. Molecular collisions with the repulsion and attractions of electrons induce the emission and absorption of photons. The energy distribution of the photons at thermal equilibrium is equal to the energy distribution of the molecular KE. When two substances have small differences in their thermal and BBR envelopes, heat energy will transfer from hot to cold by conduction, if in contact, and by radiation. When and where the energy envelopes do not overlap other things happen: chemical reactions, plasma formation, or even transparency. Example of the first: letting the water evaporate when cooking, the second: an atomic bomb, the third: outer space is transparent to infrared emitted by the Earth and the Earth's atmosphere is transparent to the visible light from the Sun. Further thought indicates that air at the same temperature as the ground is not transparent to emitted BBR. The reason BBR is lost to space is the continual lowering of atmospheric density with decreasing gravity; eventually molecular collisions cease, and the driving force is the entropy increase as the radiation is lost to space. In conclusion: radiation received or emitted by BBR raises or lowers the heat content of a system and is measured by the temperature change and the heat capacity. The energy transfers are quantum mechanical, however, the QM energy differences that involve molecular velocities and BBR are too miniscule to be resolved; they are described by continuous probability functions.

The second common interaction of radiation with matter is a Quantum Mechanical overlay over the black body radiation. These are the direct absorption and emission of photons into molecules resulting in transitions in the rotational, vibrational and electronic energy levels of the molecules. These transitions cause the "bumps and valleys" in the spectra of molecules. The function of measuring a background spectrum in spectroscopy is to remove the common BBR and isolate the overlay. The absorption or emission resulting in a QM transition does not cause an immediate change in temperature; it is not heat. It is a change in Chemical Potential Energy that can last for femtoseconds to hours or even days for phosphorescence [or hopefully forever in our computer storage]. The excited states can be converted into heat thru collisional deactivation, cause a chemical reaction with its change in heat and chemical potential energy, or reradiate with subtle consequences in molecular recoil [doppler effect] or interaction with other QM states. The initial interaction is work. The beauty of Thermodynamics is that it is mostly concerned with STATE Functions that are measured after everything is calmed down. Watching the fireworks can be exciting too, hence the study of kinetics.

The third common interaction is the variation on the Compton Effect. Photons can literally bounce off molecules in a process called scattering. There is an interchange in momentum with a small change in energy resulting in a change in the frequency of the photon and in the KE of the molecule. This is black body radiation that failed because the energy differences did not overlap. It appears to be an effect of momentum exchange and the size of the molecules and photons. This effect is weak. It results in the scattering of light, the blue sky; refraction of light, the reflection of light, the bouncing of some radio waves off the ionosphere, the Raman effect. The interchange of energy is small, much of the change is an entropy change, but energy interchange can be substantial if radiation is intense, just stick your hand in a laser beam [better yet don't!].

In conclusion: Black Body Radiation is heat where the energy envelopes overlap. Otherwise, there is everything from chemical reaction to chaos to transparency. Initial absorption of radiation resulting in changes in rotational, vibrational, or electronic energy levels is work but can change into heat, chemical potential energy or reemission with subtle changes in energy and entropy. Finally scattering results in entropy and energy changes in both the radiation and the substance depending on physical constraints. Thermodynamics studies the changes in the State Functions. Kinetics tries to study the actual processes or mechanisms.

  • $\begingroup$ Thank you so much for this answer! Even though the concepts are very complicated I appreciate all the details and explanations! $\endgroup$
    – Nefeli
    Commented Feb 12 at 3:29

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