# Is infrared really equivalent to heat? **URL:** \ **Category:** Factual Questions\ **Created:** [January 7, 2004, 7:30am UTC](https://boards.straightdope.com/t/is-infrared-really-equivalent-to-heat/223174 "2004-01-07T07:30:45Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![Aeschines](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/aeschines/32/16251_2.png) [@Aeschines](https://boards.straightdope.com/u/Aeschines)\ **Post date:** [January 7, 2004, 7:30am UTC](https://boards.straightdope.com/t/is-infrared-really-equivalent-to-heat/223174/1 "2004-01-07T07:30:45Z")
I’ve heard heat described as molecular motion within a body, and I’ve also heard that infrared radiation is itself heat. Is heat within a body, or between molecules, mediated by infrared photons? I also heard that the background microwave radiation gives the universe a temperature (ie, heat) of 3 degrees K, but microwaves are lower in frequency than infrared light. Clue me in here, friends. ---
**Author:** ![Achernar](https://avatars.discourse-cdn.com/v4/letter/a/e274bd/32.png) [@Achernar](https://boards.straightdope.com/u/Achernar)\ **Post date:** [January 7, 2004, 7:38am UTC](https://boards.straightdope.com/t/is-infrared-really-equivalent-to-heat/223174/2 "2004-01-07T07:38:05Z")
Most sources of heat give off light in a certain pattern called blackbody radiation. In reality, a blackbody gives off light in _all_ frequencies, but it gives off most light in a certain frequency range. What that range is depends on its temperature. You’re right that 3K blackbodies peak in the microwaves. 300K blackbodies, like animals, peak in the infrared. 3000-30000K blackbodies, like stars and incandescent light bulbs and campfires, peak in the optical. So we can see them easily. So when someone uses an infrared camera as a heat detector, they’re looking for things that are a few hundred Kelvin, like people or appliances. ---
**Author:** ![Urban\_Ranger](https://avatars.discourse-cdn.com/v4/letter/u/e9c0ed/32.png) [@Urban\_Ranger](https://boards.straightdope.com/u/Urban_Ranger)\ **Post date:** [January 7, 2004, 7:42am UTC](https://boards.straightdope.com/t/is-infrared-really-equivalent-to-heat/223174/3 "2004-01-07T07:42:20Z")
A blackbody is an idealised object, it does not exist in reality AFAIK. ---
**Author:** ![Desmostylus](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)\ **Post date:** [January 7, 2004, 7:45am UTC](https://boards.straightdope.com/t/is-infrared-really-equivalent-to-heat/223174/4 "2004-01-07T07:45:20Z")
> [@](#): > > \*Originally posted by Aeschines \* > **Is heat within a body, or between molecules, mediated by infrared photons?** No. Propagation of both sound and heat in solids occurs by kinetic interactions between particles (i.e., vibrations). There’s an artificial thing called a _phonon_ that can be used to describe such interactions. ---
**Author:** ![Achernar](https://avatars.discourse-cdn.com/v4/letter/a/e274bd/32.png) [@Achernar](https://boards.straightdope.com/u/Achernar)\ **Post date:** [January 7, 2004, 7:54am UTC](https://boards.straightdope.com/t/is-infrared-really-equivalent-to-heat/223174/5 "2004-01-07T07:54:34Z")
> [@](#): > > \*Originally posted by Urban Ranger \* > A blackbody is an idealised object, it does not exist in reality AFAIK. Sure, blackbodies exist. They may not be perfect, but nothing ever is. Some are closer to the ideal than others. The cosmic microwave background mentioned in the OP is pretty darn close. But even stars are not _that_ far off. ---
**Author:** ![umop\_ap\_sdn](https://avatars.discourse-cdn.com/v4/letter/u/dbc845/32.png) [@umop\_ap\_sdn](https://boards.straightdope.com/u/umop_ap_sdn)\ **Post date:** [January 7, 2004, 8:06am UTC](https://boards.straightdope.com/t/is-infrared-really-equivalent-to-heat/223174/6 "2004-01-07T08:06:12Z")
Infrared is a very wide region of the spectrum - between .76 microns (µ) to about a thousand microns wavelength. It is generally broken down into 3 regions - near IR, thermal IR, and far IR. The thermal infrared (8-14µ) is associated with blackbody radiation around room temperature and body temperature. CCD cameras or infrared film will pick up some of the near IR, but imaging in the thermal IR requires some expensive equipment. Heat is the amount of random motion of molecules. Infrared itself is not heat per se but usually has some degree of heating effect on matter. ---
**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\ **Post date:** [January 7, 2004, 8:10am UTC](https://boards.straightdope.com/t/is-infrared-really-equivalent-to-heat/223174/7 "2004-01-07T08:10:18Z")
> [@](#): > > \*Originally posted by Urban Ranger \* > \*\*A blackbody is an idealised object, it does not exist in reality AFAIK. \*\* Yeah but … a box with a small hole in it and a rough interior takes in most of the energy that enters the hole and transmits very little back out. The hole is close to an ideal black body in that it absorbs most of the energy falling on it. An ideal electrical resistor doesn’t exist either. Nor an ideal electrical inductance nor an idea capacitance. All real electrical resistors, inductors and capacitors are actually a combination of all three with one property emphasized. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [January 7, 2004, 4:05pm UTC](https://boards.straightdope.com/t/is-infrared-really-equivalent-to-heat/223174/8 "2004-01-07T16:05:36Z")
From [http://www.amasci.com/miscon/miscon4.html#heat:](http://www.amasci.com/miscon/miscon4.html#heat:) > [@](#): > > Infrared light is invisible light. When any type of light is absorbed by an object, that object will be heated. The infrared light from an electric heater feels hot because the light is EXTREMELY BRIGHT LIGHT. Just because human eyes cannot see the light which causes the heating does not mean that the light is made of some mysterious entity called “heat radiation.” When bright light shines on an absorbtive surface, that surface heats up. > > And this is no benign misconception. Those who fall under its sway may also come to believe that _visible_ light cannot heat surfaces (after all, visible light is not “heat radiation.”) Misguided science students may wrongly believe that warm objects emit no microwaves (since only IR light is “heat radiation”), even though hot objects actually do emit microwaves. Or they may believe that the glow of red hot objects is somehow different than the infrared glow of cooler objects. Or they may believe that IR light is a form of “heat,” and is therefore fundamentally different than any other type of electromagnetic radiation. > > In his book “Clouds in a Glass of Beer,” Physicist C. Bohren points out that this “heat” misconception may have been started long ago, when early physicists believed in the existence of three separate types of radiation: heat radiation, light, and actinic radiation. Eventually they discovered that all three were actually the same stuff: light. “Heat radiation” and “actinic radiation” are simply invisible light of various frequencies. Today we say “UV light” rather than “actinic radiation.” Yet the obsolete term “heat radiation” still lingers. Since human beings can only see certain frequencies of light, it’s easy to see how this sort of confusion got started. Invisible light seems bizarre and mysterious when compared to visible light. But “invisibility” is caused by the human eye, and is not a property carried by the light. If humans could see all the light in the infrared spectrum, we would say things like this: “of COURSE the electric heater makes things hot at a distance, it is intensely BRIGHT, and bright light can heat up any surface which absorbs it.” ---
**Author:** ![constantine](https://avatars.discourse-cdn.com/v4/letter/c/3bc359/32.png) [@constantine](https://boards.straightdope.com/u/constantine)\ **Post date:** [January 7, 2004, 5:04pm UTC](https://boards.straightdope.com/t/is-infrared-really-equivalent-to-heat/223174/9 "2004-01-07T17:04:30Z")
> [@](#): > > \*Originally posted by Aeschines \* > \*\*I’ve heard heat described as molecular motion within a body, and I’ve also heard that infrared radiation is itself heat. > > Is heat within a body, or between molecules, mediated by infrared photons? > > I also heard that the background microwave radiation gives the universe a temperature (ie, heat) of 3 degrees K, but microwaves are lower in frequency than infrared light. > > Clue me in here, friends. \*\* **Aeschines** maybe your questions have already been answered, but here goes. Putting aside the issue of what happens at absolute zero (which is complicated) consider any collection of molecules (e.g. a glass of water, a star, or a person trying to sneak through a dark room at night). The “heat” of this collection of molecules is a measure of the energy expressed as random motion of the molecules making up the body. When a hotter body of molecules (e.g. a gallon of hot water) comes into contact with a body of cooler molecules (e.g. a glass, a countertop, the air in the room) then the temperature of the two collections moves into equilibrium. The more energetic molecules in the hot body (note that “body” here can mean a non-solid like several cubic feet of air and or a gallon of waterwater) winds up giving energy away to the colder collection. This process is known as “conduction.” How rapidly this transfer of energy takes place depends on the conductivity of the materials. Just as a body can absorb or give off energy by “conduction” it can also absorb or give off energy by “radiation” i.e. light. When you turn the burner on an electric range, it starts to heat up, and as it does so, it starts emitting more radiation, i.e. light. At first the vast bulk of the radiation it gives off is not in the visible spectrum. It’s in the infrared (i.e. “less than red”) part of the spectrum. As it gets hotter, it starts to emit light in the red (the lower end) of the visible spectrum. So, with all that said, let me try to answer your questions again. 1. Strictly speaking, heat refers to energy in the form of random molecular motions in a collection of molecules, so no form of light, whether infrared, microwaves, visible spectrum, or x-rays, is “heat.” 2. Hot bodies (i.e. any body not at absolute zero) emit (aka radiate) energy in the form of light. 3. Bodies at a certain temperature emit most of their light in the infrared spectrum. Such bodies include most human bodies. So, if you want to invent a device that can detect people, dogs, etc. walking around at night in the dark, it makes sense to make the device sensitive to the infrared spectrum. ---
**Author:** ![bbeaty](https://avatars.discourse-cdn.com/v4/letter/b/c0e974/32.png) [@bbeaty](https://boards.straightdope.com/u/bbeaty)\ **Post date:** [January 7, 2004, 7:47pm UTC](https://boards.straightdope.com/t/is-infrared-really-equivalent-to-heat/223174/10 "2004-01-07T19:47:17Z")
> [@](#): > > \*Originally posted by Aeschines \* > \*\*I’ve heard heat described as molecular motion within a body, and I’ve also heard that infrared radiation is itself heat. > \*\* “Heat” is the molecular motion within a body, plus the electromagnetic waves within a body, plus the temporarily excited states of the body’s atoms/molecules. Also, there’s nothing particularly special about infrared. The connection betweeen “heat” and IR light is a widespread myth. Because of its non-zero temperature, all matter glows with EM radiation. This EM radiation has a broad spectrum, and the peak of the spectrum depends on the temperature of the matter. For example, white-hot objects emit mostly visible light. Cold objects emit mostly microwaves. Perhaps the misconceptions about IR light are caused by the fact that objects _at room temperature_ emit mostly IR light… and some authors wrongly leap to the conclusion that “thermal radiation” is inherently the same as “IR light.” This might be the norm for the environment within a classroom, but it’s not true in general. If any object absorbs light, the object heats up, and there’s no special frequency where the light becomes “heat.” And if you look at a 100 watt lightbulb, every bit of the light which strikes your eye is “thermal radiation.” > [@](#): > > \*\*Is heat within a body, or between molecules, mediated by infrared photons? \*\* Not IR photons, but simply photons. As “heat” travels through an object, the energy can be transferred from molecule to molecule both by acoustic interaction (mechanical vibrations via chemical bonds or via collisions, ‘phonons’), or by optical interaction (EM waves crossing the space between atoms, or ‘photons’.) An atom or molecule within a warm substance can absorb a light wave and emit a wiggle, absorb a wiggle and emit a light wave, or just pass on the incoming wiggle or light wave to a neighboring atom.