# What is the difference between heat and light?

What i know is that light is an electromagnetic wave. Is heat also an electromagnetic wave of certain wavelength other than light ? Why the light wave cause heat in an object when it falls on the object ? and why does an electric heater emits light ? It means both of these are very closely related but i am not understanding the relation between the two?

• light is visible heat – user83548 Jun 7 '16 at 20:47
• @brucesmitherson: Heat is well defined as an internal form of energy in the first law of thermodynamics. One should not mistake that with infrared radiation that is being given off by warm bodies. – CuriousOne Jun 7 '16 at 20:54
• Heat is any energy transfer that is not macroscopically ordered (or should I return my PhD?) – user83548 Jun 7 '16 at 20:54
• @brucesmitherson: Don't be snapped in. Language always leaves space for multiple levels of communication in which words are used at different levels of precision. A "heat-seeking-missile", for instance, is not really seeking heat but it is honing in on IR radiation sources, still, I think one can agree that for the purpose of the description the choice of words is appropriate. For the sake of the OP we should still try to maintain a somewhat precise level of language. – CuriousOne Jun 7 '16 at 21:01
• @brucesmitherson Appealing to a degree is a weak form of argumentation. First you state that heat is a subset of light, then you refer to it as energy transfer. You should be more precise and tell that light can be transferring energy, but it is not a transfer of energy. – Andrii Magalich Jun 7 '16 at 21:01

In physics, heat is energy that spontaneously passes between a system and its surroundings in some way other than through work or the transfer of matter. When a suitable physical pathway exists, heat flows spontaneously from a hotter to a colder body.The transfer can be by contact between the source and the destination body, as in conduction; or by radiation between remote bodies; or by conduction and radiation through a thick solid wall; or by way of an intermediate fluid body, as in convective circulation; or by a combination of these.

(Source: Wikipedia)

So, to put it simply, we talk of "heat" whenever there is transfer of energy without work or transfer of matter. This is what is stated in the first law of thermodynamics:

$d U = \delta Q - \delta W$

which holds for a closed system (we rule out the transfer of matter).

The variation of the internal energy of a closed system which is not due to work is what we call "heat".

The word "light" usually refers to visible light, that portion of the electromagnetic spectrum whose wavelength is between $\sim400$ and $\sim700$ nm. But we can more generally talk about "electromagnetic radiation" and consider the whole spectrum.

As stated in the Wikipedia article I quoted, heat can be radiation between two bodies. This just means that the "$\delta Q$" in the first law of thermodynamics can be due to electromagnetic radiation.

Every body at a temperature $T$ different from $0$ will emit electromagnetic radiation (thermal radiation). This is because the atoms inside the material are moving, and this causes accelerations and decelerations of the charged particles the atoms are made of, and therefore electromagnetic radiation.

Therefore, an object at temperature $T$ will emit electromagnetic radiation in the surroundings (which are at temperature $T_e$) and absorb radiation from them. At the same time, the surroundings will emit and absorb electromagnetic radiation in the same way. The higher the temperature, the more the radiation.

This process will stop only when $T_e=T$, because then the absorbed radiation will be equal to the emitted radiation. So, the object and its surroundings will start at different temperature and end up having the same temperature. Since temperature and internal energy are related, this will mean that the internal energy of the bodies has changed. Seeing as how there has been no work or transfer of matter, we call heat this change of energy, which was due to electromagnetic radiation.

This one of the possible connections between heat and electromagnetic radiation.

Update: clarification

Of course there is also EM radiation which isn't thermal, i.e. it is not originated by the acceleration/deceleration of charges due to random motion. An example is given by transition between atomic energy levels, which is the mechanism behind the laser. This kind of radiation can of course be used to transfer energy without work/transfer of matter, so it is a kind of heat. The EM radiation is always the same: what changes is the mechanism originating it.

It is also clear that you can have heat without EM radiation. For example you put two objects in contact and transfer energy by conduction.

To sum up: heat is a change in energy without work/transfer of matter. This transfer can be realized via EM radiation or via other means (conduction, convection etc.). Also, EM radiation can originate in different ways: thermal motion, electronic transitions etc.

• I read a statement "electromagnetic radiation can be categorized into two major groups: "thermal" and "nonthermal" radiation." does it mean that some of the electromagnetic waves in electromagnetic spectrum can cause heat while others do not cause heat ? – Alex Jun 8 '16 at 10:13
• No, it simply means that there is also radiation that is non thermal, i.e. it does not originate from acceleration/deceleration of charges due to random motion. An example are transitions between energy levels, which is basically the mechanism of the laser. – valerio Jun 8 '16 at 10:20
• If you want, I can add this remark to the answer for greater clarity. – valerio Jun 8 '16 at 10:22
• so sun emits both electromagnetic and thermal radiation ? electromagnetic radiations that cause light etc and thermal radiation that cause heat ? so in sun acceleration of charges is causing electromagnetic radiation and burning of some chemical is cause causing thermal radiation ? – Alex Jun 8 '16 at 10:28
• Thermal radiation is a kind of electromagnetic radiation. It is the part of EM radiation due to random motion of charges. The radiation is always the same: what changes is the mechanism that is behind it. In the Sun you have also radiation generated by the process of nuclear fusion. – valerio Jun 8 '16 at 10:30

Also they are closely related in empirical life, they are different and the concepts come, I would claim, from different theories.

Light is electromagnetic radiation in the visible spectrum which is produced by moving charges according to Maxwell's equations.

Heat is a thermodynamical concept and usually describes energy transfer that is not work (or energy transmitted through particle transport). Heat is not something inherent to a body but only a quantity transfered from one to the other. The light waves, when they fall onto an object, cause the body to heat up, i.e., to have a higher temperature. This has nothing per se to do with heat. Temperature can be understood as the speed of molecular motion - and as moving charges emit radiation, moving molecules do so too. This is why objects emit radiation. The spectrum of that emission is, in easy cases, given by the Stefan-Boltzmann-Law/Planck's law and from it you can see that warm bodies emit more electromagnetic radiation.

I hope my explanation does not have any major flaws and I could shed a bit of light on the matter.

• I read a statement "electromagnetic radiation can be categorized into two major groups: "thermal" and "nonthermal" radiation." does it mean that some of the electromagnetic waves in electromagnetic spectrum can cause heat while others do not cause heat ? – Alex Jun 8 '16 at 10:14

A simple answer is that light has energy, whereas heat is a form of energy.

It is similar to the difference between a billiard ball and the kinetic energy of the billiard ball

Electromagnetic radiation classically is described by fields, but in the underlying quantum mechanical framework it is emergent from a confluence of an enormous number of photons. Photons, elementary particles , similar to the billiard balls, have energy but are not energy.

You state:

What i know is that light is an electromagnetic wave.

True, and more so , it is an emergent phenomenon from a multitude of photons.

Is heat also an electromagnetic wave of certain wavelength other than light ?

No. Heat is a macroscopic thermodynamic energy variable which , together with energy conservation, describes thermodynamic systems.

In physics, energy is a property of objects which can be transferred to other objects or converted into different forms. The "ability of a system to perform work" is a common description, but it is misleading because energy is not necessarily available to do work. For instance, in SI units, energy is measured in joules, and one joule is defined "mechanically", being the energy transferred to an object by the mechanical work of moving it a distance of 1 metre against a force of 1 newton. However, there are many other definitions of energy, depending on the context, such as thermal energy, radiant energy, electromagnetic, nuclear, etc., where definitions are derived that are the most convenient.

So objects have energy, and light waves have energy, they are not energy.

Why the light wave cause heat in an object when it falls on the object ?

Photons, light, hitting and being absorbed by an object leave their energy in the atoms of the object, kinetic or vibrational. The kinetic and vibrational energies of the atoms in a medium define the heat energy of the medium. The photons are absorbed by the quantum mechanical levels of the medium.

and why does an electric heater emits light ? It means both of these are very closely related but i am not understanding the relation between the two?

Yes, they are related. The quantum mechanical levels of the atoms in a medium which can absorb photons ( electromagnetic waves) also emit them if in an excited state. The energy for emission in a heater, i.e raising the atomic levels so that they can relax and emit a photon, is given by the electricity supply.

• I read a statement "electromagnetic radiation can be categorized into two major groups: "thermal" and "nonthermal" radiation." does it mean that some of the electromagnetic waves in electromagnetic spectrum can cause heat while others do not cause heat ? – Alex Jun 8 '16 at 10:13
• It means that the frequencies of"thermal" are the frequencies that can excite the vibrational and rotational quantum levels of the lattice with high probability, and thus the energy the wave carries turns into heat. Non thermal means that the frequencies do not match the rotational and vibrational levels so the probability of energy transfer is small. – anna v Jun 8 '16 at 10:41

Heat and light are different but they are both forms of energy. Heat is a form of kinetic energy contained in the random motion of the particles of a material. Light is a form of electromagnetic energy.

As with other forms of energy, heat energy can be transformed into light energy and vice versa.

The particles of a material vibrate or oscillate because of their kinetic energy, like masses on springs. During this oscillation the electrical charges within the particles are also oscillating, and oscillating electrical charges emit electromagnetic radiation (EM energy), just as the oscillating electric current in an aerial or mobile phone emits radio waves or microwaves. The higher the temperature of the object, the shorter the wavelength of the EM radiation. A relatively cool object emits invisible long wavelength infra red waves, whereas a very hot object emits shorter wavelength visible light, and extremely hot objects emit X rays with very short wavelengths.

The same process acting in reverse enables materials to absorb electromagnetic radiation (radio waves, IR, visible light, etc) with the result that their particles vibrate more energetically, making the material hotter.

• I think that heat in the thermodynamic sense $\delta Q$ has nothing to do with kinetic energy which should be a part of the internal energy $U$, shouldn't it? – Sanya Jun 7 '16 at 21:13
• "Heat is a form of kinetic energy contained in the random motion of the particles of a material" I do not quite agree here - this would rather be a description of internal energy, and not heat. Heat is not something in a material - it is the transfer of something to the material. – Steeven Jun 7 '16 at 21:16
• @Sanya : But is the OP asking for a thermodynamic exposition? And how would you define internal energy? – sammy gerbil Jun 7 '16 at 22:04
• @sammygerbil I have to admit that I do not know what exactly the OP meant by the word - this is a form of blindness of thinking too much in the established physical language I guess. But I can perceive that my comment is taken as hostility and I'll be quiet. – Sanya Jun 7 '16 at 22:10
• @Sanya : You seem to be taking my questions personally as hostile. I am defending my decision to provide a dumbed-down explanation appropriate to the OP's needs, rather than a graduate-level exposition. And I am challenging you to justify why you think your approach is better suited to the OP's needs. – sammy gerbil Jun 7 '16 at 23:07

Body with higher temperature will produce the EM waves of higher energy than the EM waves produced by the body at lower temperature. All the things in the universe emits EM waves all the time. Since the body at higher temperature emits EM waves of higher energy so when they will fall on a body of lower temperature they will increase the average Kinetic energy (temperature) of the that body. This transfer of energy from one body to other body is called heat. Light is also an electromagnetic wave with a specific frequency as we know. It will also be heat for you if its emitted by a body at higher temperature than your temperature.