# B How does radiation become heat?

#### iamconfused

I'm confused on how something can absorb visible light and release heat. Specifically, I'm confused how the Earth absorbs visible light and emits IR + heat. I'm in a class (about global warming) for non-science majors and don't have much of a background in science, so I would appreciate responses similar to an "explain like I'm 5" response. Here is what I have come to learn so far so you can see where I might be getting lost....

1) The temperature of an object is related to how much the atoms are moving around (kinetic energy). Atoms and molecules in things that are hotter move around more than in things that are colder.
2) There are three ways of transferring heat. Conduction, convection, and radiation. For now I'm going to ignore convection. Conduction, which is like when a hot object with atoms moving around a lot bumps into a cold object and makes the atoms/molecules in the colder object move around more, similar to how slow bumper cars will move faster if fast bumper cars bump into them. Radiation is electromagnetic waves...
3) electromagnetic waves can be absorbed either electronically, vibrationally, rotationally, or translationally (and for the purpose of my question and my sanity, I'm going to ignore rotational and translational). So just looking at electronically and vibrationally.....
4) Molecules can absorb radiation vibrationally if there is a dipole when the molecule is vibrating, and if the vibration is "oscillating the electric field around it" at the same frequency as the radiation. This is usually infrared radiation that can make a vibrational transition. When this radiation is absorbed, this makes the molecule vibrate more, and if there are other molecules nearby, it will bump into them and make them move more, which means heat was transferred (because the light went into make things vibrate more).
5) Molecules/atoms can also absorb radiation electronically, which means that an electron will jump up to a higher excited state (if the atom/molecule has a transition at the exact wavelength/frequency as the light that was being absorbed). This is usually UV or Visible light that can make an electronic transition.

So here is where I'm getting lost... The Earth apparently absorbs visible light and gives off heat + IR. I don't understand why an object wouldn't absorb visible light and not just release visible light.

As an example, when I'm shown an energy diagram in class for electronic transitions, I'm assuming that if an electron jumps to n=1 to n=4, then it will just emit light when it goes from n=4 back to n=1. I don't know if this is true, but if it absorbs from n=1 to n=4, could it come back down by releasing a photon from n=4 to n=3? and then n=3 to n=2? etc? This is the only way I could think that something could absorb visible and release IR (less energetic light than what came in). And if this is what is happening, why? Why wouldn't the Earth absorb visible light and give off visible light instead?

And then where does heat come from? How does absorbing visible light electronically make the atoms in a substance move around more?

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#### Drakkith

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And then where does heat come from? How does absorbing visible light electronically make the atoms in a substance move around more?
My understanding is that the vast majority of visible light is not absorbed 'electronically', but partitioned into translational, vibrational, and rotational modes. Then, as you pointed out yourself, the atoms/molecules bump into others., Thus the high-energy of a single visible light absorption event is dispersed, transferring that energy around and dispersing it into many other lower-energy particles.

After this, these lower-energy particles can then radiate this energy away as infrared radiation.

Also note, if you didn't know already, that electronic transitions are only one way for an atom or molecules to give off light. The charged particles that make up atoms and molecules are accelerated (which includes deceleration) when they bump up and interact with each other, which is how the almost all of the IR radiation is created. In fact, this is how almost all of the black body (thermal) radiation of any frequency is created, even for very hot objects like stars.

That's my understanding at least. Someone correct me if I'm wrong please.

#### iamconfused

The charged particles that make up atoms and molecules are accelerated (which includes deceleration) when they bump up and interact with each other, which is how the almost all of the IR radiation is created.

That's my understanding at least. Someone correct me if I'm wrong please.
Interesting, could you expand on how the charged particles accelerating/decelerating leads to the creation of IR?

#### Drakkith

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2018 Award
Interesting, could you expand on how the charged particles accelerating/decelerating leads to the creation of IR?

The basic idea is that a charged particle being accelerated must give off EM radiation, and this radiation increases in frequency and intensity as the magnitude of the acceleration increases. Hence why higher temperatures (and thus more energetic particles that undergo larger accelerations) lead to an increase in the frequency and intensity of the thermal radiation they produce.

#### Orodruin

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Interesting, could you expand on how the charged particles accelerating/decelerating leads to the creation of IR?
It is the general source of any EM radiation. Not just IR.

#### DrClaude

Mentor
I don't like how this discussion is somehow focussing on accelerating electrons, since I think this is quite irrelevant for the situation at hand.

It is better to think of matter here as a bunch of dipoles than can oscillate. Oscillating dipoles can emit and absorb EM radiation, and this is basically what is going on here.

So here is where I'm getting lost... The Earth apparently absorbs visible light and gives off heat + IR. I don't understand why an object wouldn't absorb visible light and not just release visible light.

As an example, when I'm shown an energy diagram in class for electronic transitions, I'm assuming that if an electron jumps to n=1 to n=4, then it will just emit light when it goes from n=4 back to n=1. I don't know if this is true, but if it absorbs from n=1 to n=4, could it come back down by releasing a photon from n=4 to n=3? and then n=3 to n=2? etc? This is the only way I could think that something could absorb visible and release IR (less energetic light than what came in). And if this is what is happening, why? Why wouldn't the Earth absorb visible light and give off visible light instead?
In dense material or even for isolated complex molecules, energy gets transformed from one form to another faster than emission of radiation by spontaneous emission. In the latter case, this is known as non-radiative relaxation: the molecule gets into an excited state by absorbing a photon, but then relaxes by transferring the energy to other degrees of freedom (internal conversion) or through collisions with other molecules.

And then where does heat come from? How does absorbing visible light electronically make the atoms in a substance move around more?
Once the energy has been absorbed, it spreads to the all the degrees of freedom of the system, such that (more or less) an internal equilibrium is reached. In that sense, the system now has a defined temperature. It will then emit radiation due to that temperature, which is known as blackbody radiation.

The crux to the understanding is that one must consider that there is a decoupling between the absorption and the mission, with an intermediate state where the absorbed energy is nothing more than heat.

so I would appreciate responses similar to an "explain like I'm 5" response
This is actually a bit hard to do here because the difficulty you have is due to a misconception that a 5-year-old wouldn't have, that bulk matter behaves like an isolated atom with respect to its interaction with photons.

#### Lord Jestocost

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2018 Award
My understanding is that the vast majority of visible light is not absorbed 'electronically'....
Here I have to correct you. The primary mechanism for the absorption of visible light photons (energy range ~ 1.6 - 3.3 eV ) are electronic transitions.

#### sophiecentaur

Gold Member
I don't understand why an object wouldn't absorb visible light and not just release visible light.
This, afaics, is the nub of your question. It is possible that some of the explanations given so far are too complicated and I will try to put it more simply. (Not really adding but, rather, subtracting information, to make it easier for a less scary explanation.)

When EM hits anything, the Energy can be absorbed by various mechanisms. The simplest (at least when QM is being considered) is straight absorption by individual, isolated gas atoms. The Hydrogen atom energy levels and transitions are what we learn first and it explains how (only) photons in specific spectral lines will increase the electron energy level by a specific Quantum of Energy and that precise amount of energy will be re emitted in all directions, after a short time. That process doesn't warm up the gas at all. (The message about the H atom tends to be carried into many areas of QM where it is just not appropriate; it's the first and often the last thing we get taught.)

An EM wave that hits a bigger 'condensed' object with many atoms nearby will transfer its energy by a different mechanism - into the whole of a region of the object (unless the substance is 'transparent', in which case there is very little interaction - except to slow the wave down [refractive index may not involve any loss of energy]) Most substances will actually absorb the wave energy and that energy will get spread within the substance with photons of lower energies / frequencies before it has a chance to be re-emitted with its original Energy. Also the energy is passed around by mechanical vibrations (phonons). This is the process whereby the incident light 'warms up' the substance. The Energy will eventually be radiated but at frequencies corresponding to thermal energy levels (vibrational). Temperature rises until there is a balance between incident and radiated energy.

The phenomenon of Global Warming is very complicated and relies on several mechanisms in addition to the above. When the IR wavelengths get back into the atmosphere, they have Energies that are absorbed by CO2 and Methane etc. , which are Greenhouse Gases. When the molecules of greenhouse gases absorb the energy, they actually re-emit IR of the same frequency in all directions some energy goes back towards Earth - about half, in the limit - via other molecules. This causes the surface temperature of the ground to be higher, to establish equilibrium. Hence the 'warming' effect.

#### Drakkith

Staff Emeritus
2018 Award
Here I have to correct you. The primary mechanism for the absorption of visible light photons (energy range ~ 1.6 - 3.3 eV ) are electronic transitions.
Hmm. This is the first I've heard of this. Do you have a reference that goes into more detail?

Mentor

#### Dale

Mentor
I'm confused on how something can absorb visible light and release heat. Specifically, I'm confused how the Earth absorbs visible light and emits IR + heat.
When a molecule is excited then it possesses energy which can be released through a variety of processes, not just the process that initially excited it.

So, for example, if a molecule absorbs a visible light photon then typically that corresponds to an electronic transition, where an electron goes to an unstable higher energy orbital. When that electron decays back to a stable state it may do so by emitting a single photon of approximately the same energy, or it may emit a photon of a lower energy and put the remainder of the energy into a different internal degree of freedom, such as a rotational or vibrational mode.

By the equipartition principle, on average energy is equally likely to be found in any available mode. Since for the amount of energy in a visible light photon there is often only one available electronic transition and many available rotational and vibrational modes the most common scenario is to decay into the many available modes rather than to stay in the high energy photons of the single electronic transition.

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#### Mister T

Gold Member
The Earth apparently absorbs visible light and gives off heat + IR.
What do you mean by heat + IR. Infrared radiation (IR) is heat. You seem to grasp the fact that radiation is one of the three forms of heat!

Also, you are aware that the sun emits IR, not just visible light? So even if a gas does not absorb a significant amount of visible light it can still absorb IR from the sun, raising its temperature.

#### iamconfused

What do you mean by heat + IR. Infrared radiation (IR) is heat. You seem to grasp the fact that radiation is one of the three forms of heat!

Also, you are aware that the sun emits IR, not just visible light? So even if a gas does not absorb a significant amount of visible light it can still absorb IR from the sun, raising its temperature.
I was actually quoting what my professor said in class, he wrote it on the board as
visible light —>absorption—> IR + heat.

He was trying to separate the different effects IR and visible light can have on a warming planet, so he was ignoring IR and just talking about how visible light can get absorbed and then release IR and heat.

I think I’m still sometimes confusing energy, heat, and temperature in my head. I think my intuition is a bit off here. I haven’t really been thinking of IR as heat itself but something they can heat something up.

I have been thinking of temperature as the internal jigglyness of the atoms in a substance, and heat as the ability to transfer jigglyness to other things lol. So I guess IR is heat because IR can make things jigglier? Lol sorry I know that is dumbed down pretty far down.

#### iamconfused

Thank you everyone for your replies! They were very helpful! I will have to sit and think about these more to fully grasp everything but I definitely feel I understand things a little better now.

#### iamconfused

In dense material or even for isolated complex molecules, energy gets transformed from one form to another faster than emission of radiation by spontaneous emission. In the latter case, this is known as non-radiative relaxation: the molecule gets into an excited state by absorbing a photon, but then relaxes by transferring the energy to other degrees of freedom (internal conversion) or through collisions with other molecules.

Once the energy has been absorbed, it spreads to the all the degrees of freedom of the system, such that (more or less) an internal equilibrium is reached. In that sense, the system now has a defined temperature. It will then emit radiation due to that temperature, which is known as blackbody radiation.
Thank you! I think the non radiative relaxation vs spontaneous emission part was one of the big things I was missing!

#### sophiecentaur

Gold Member
So I guess IR is heat because IR can make things jigglier? Lol sorry I know that is dumbed down pretty far down.
Here is the typical problem of naming things and trying to impose strict classification. IR is electromagnetic and it's definitely Energy. The vibrations and random motions in a substance also involve Energy which we call Heat / Temperature (average Kinetic Energy of random motion etc.). But, whilst there's no confusion about what we usually mean by Temperature, classifying Thermal Energy is a bit more fuzzy.
Would you say that the output of a transmitter which produces a monochromatic EM wave in the IR range is 'heat'? I would say "No". So how can we call the, albeit random, EM waves from a hot object as 'heat'? Why lose sleep over that question?

#### Orodruin

Staff Emeritus
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It should also be mentioned that "heat" is energy being transferred from one thermodynamical system to another. It is not in itself a property of a system.
The vibrations and random motions in a substance also involve Energy which we call Heat
Heat is not a property of a system. It is the thermal energy transfer between two thermodynamical systems.

#### sophiecentaur

Gold Member
It should also be mentioned that "heat" is energy being transferred from one thermodynamical system to another. It is not in itself a property of a system.

Heat is not a property of a system. It is the thermal energy transfer between two thermodynamical systems.
Yes - and I think that's why I used the term "we call". It's a popular catch-all word which has been used to describe and explain phenomena through the centuries. Heat has been a lot of things - including a fluid that passes between objects. Formal Thermodynamics uses the term iirc, along with the term "Energy". Both very slippery things and they only 'behave themselves' in the theoretical world, along with Photons and other Particles.

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