# Why are only infrared rays classified as "heat rays"?

I've often heard that Infrared rays are called "heat rays". However, I feel like this term is a misnomer. Don't all the wavelengths of electromagnetic radiation carry energy?

Judging by how gamma rays are highly penetrating and are dangerous when absorbed by tissues, radiations of lower wavelengths should carry more energy, and should be able to increase the internal energy of the object that absorbed it much more than infrared rays can. This seems consistent with the conservation of energy for an isolated system: $$T_{ER} = \Delta E_{int}$$ where $$T_{ER}$$ stands for transfer of energy by electromagnetic radiation

Then why are UV rays, X-rays and gamma rays not classified as "heat rays".

• See also Why do we feel heat from infrared light but not from shorter wavelengths?, where my answer suggests a couple of ways to reliably feel the heat from visible light sources Sep 15 at 11:06
• Something that I think the existing answers aren't explicit enough about is the fact that emissivity equals absorptivity at each given frequency. Objects at everyday temperatures mostly emit at infrared frequencies (this is explained by Planck's law, which one of the answers references), therefore they will also absorb more strongly at those frequencies. Sep 15 at 22:09
• The most efficient ways to heat matter under normal conditions is too excite phonons. Only IR are in resonance with phonon vibrations. That is why this is most efficient heating mechanism even though IR phonos carry less energy. viz-UV also absorbed by matter, but by different mechanisms, e.g., causing electronic transitions. Because of nuclei' larger mass in comparison with that of electrons, after the equilibration the net temperature increase is small. XUV and higher ionize the system and therefore contribute little to its internal energy increase. Sep 16 at 3:51

Don't all the wavelengths of electromagnetic radiation carry energy?

Yes. And that photon energy $$E$$ is given by

$$E=h\nu$$

Where $$h$$ = Planck's constant and $$\nu$$ = frequency.

But not all frequencies interact with matter in the same way.

Judging by how Gamma rays are highly penetrating and are dangerous when absorbed by tissues

Very little of the energy of Gamma rays is absorbed by tissue, i.e., tissue is basically transparent to Gamma rays. They can even pass through several inches of lead. But as they pass though human tissue they energy that is absorbed can cause ionizations that damage tissue and DNA. For this reason, it is called ionizing radiation.

...radiations of lower wavelengths should carry more energy, and should be able to increase the internal energy of the object that absorbed it much more than Infrared rays can.

Yes they do, but the amount of energy that is actually absorbed depends on the frequency. Per the Hyperphysics website (http://hyperphysics.phy-astr.gsu.edu/hbase/mod3.html) regarding the interaction of radiation with matter:

"as you move upward in frequency from infrared to visible light, you absorb (the energy) more and more strongly. In the lower ultraviolet range, all the uv from the sun is absorbed in a thin outer layer of your skin. As you move further up into the x-ray region of the spectrum, you become transparent again, because most of the mechanisms for absorption are gone. You then absorb only a small fraction of the radiation, but that absorption involves the more violent ionization events"

Then why are UV rays, X-rays and Gamma Rays not classified as "heat rays"

In the case of X-rays and Gamma rays, it's because they don't interact with the skin in the same way as infrared, namely, they do not create a feeling of warmth on the skin.

The case of UV is a bit more complex. You don't directly feel UV radiation. But per the FDA.gov site,

"When UV rays reach your skin, they damage cells in the epidermis. In response, your immune system increases blood flow to the affected areas. The increased blood flow is what gives sunburn its characteristic redness and makes the skin feel warm to the touch."

Hope this helps.

• It's not that you can't feel UV directly. If it's absorbed in the skin, it will heat the skin, and you can feel that. It's that if you're absorbing enough UV to significantly heat your skin, you have other things to worry about. Sep 16 at 11:31
• @A20867 see myuv.com.au/workplaces/outdoor-worker-blog/… Sep 16 at 12:44

Infrared region is a part of electromagnetic spectrum that is mostly responsible for the radiative heat transfer in our everyday life. It is expressed by the fact that the peak of the Planck distribution at room temperature lies in the infrared range:

Planck radiation has a maximum intensity at a wavelength that depends on the temperature of the body. For example, at room temperature (~300 K), a body emits thermal radiation that is mostly infrared and invisible. At higher temperatures the amount of infrared radiation increases and can be felt as heat, and more visible radiation is emitted so the body glows visibly red. At higher temperatures, the body is bright yellow or blue-white and emits significant amounts of short wavelength radiation, including ultraviolet and even x-rays. The surface of the sun (~6000 K) emits large amounts of both infrared and ultraviolet radiation; its emission is peaked in the visible spectrum.

Electromagnetic waves with frequencies a bit higher than infrared are visible light. Therefore, although they also carry heat, they are mainly identified with the information that we get through our eyes.

Lower frequency electromagnetic waves, in radio spectrum, are less energetic and therefore less important.

Clarification
In thermodynamics/statistical mechanics heat is the energy transferred from one system to another on the microscopic level (unlike work, which is due to macroscopic changes). In this case an object is in contact with radiation. Heat rays is not a physics term, it should not be literally interpreted as "rays carrying heat". But the reason why we use this term to describe infrared radiation is the one that I give above.

Comment on the energy conservation
The radiative energy absorbed depends not only on the energy of photons of given frequency, $$h\nu$$, but also on the number of photons, $$n_\nu$$, i.e., the absorbed energy at a given frequency is $$E_\nu=n_{\nu}h\nu.$$ If we assume thermal distribution for $$n_\nu$$ and do calculation, we obtain teh above-mentioned Planck formula. This is why the peak of the Planck formula corresponds to the ferquencies where the most heat is transferred.

• But do they "carry heat"? Isn't "heating" a local phenomena experienced as a result of the interactions between the wave and the material upon which it is incident? Sep 15 at 9:06
• @CaiusJard In thermodynamics/stat.mech. heat is the energy transferred from one system to another on a microscopic level (unlike work, which is due to macroscopic changes). In this case an object is in contact with radiation. Heat rays is not a physics term, it should not be literally interpreted as "rays carrying heat". But the reason why we use this term to describe infrared radiation is the one that I give in my answer. I added this clarification to my answer. Sep 15 at 9:13
• Also worthy of note: Artificial sources of visible light (basically, light bulbs) that we experience on a daily basis are extremely weak compared to artificial sources of infrared (heaters.) An infrared heater may put out anywhere from hundreds of Watts to tens of thousands of Watts of infrared light. A modern home light bulb, on the other hand, might put out no more than one or two Watts of visible light. And, if it's a modern LED bulb, maybe not much more than that in the IR. Sep 15 at 16:43
• @SolomonSlow this is a very good point. Sep 15 at 16:52

I think "heat rays" is a very loose and informal term, so don't take it too strictly. The usage probably stems from the fact that most objects are at a temperature that primarily emits IR. To start emitting visible it has to be very hot, like steel in a forge. And because of this, night vision or "heat vision" cameras are made to detect IR.

• As untechnical as this answer is, I think it does the best job of answering the question. IR as "Heat Rays" has nothing at all to do at all with heat absorption/transmission efficiencies etc. and everything to do with Thermography: the use IR heat emissions to perceive objects at near room temperature. en.wikipedia.org/wiki/Thermography Sep 16 at 21:59

Yes, it is a bit of a misnomer, but it is informal. It's short for "the rays that we don't see, but we often feel as heat." I agree that it can cause confusion. Much of the heat we get from the sun is in the form of visible light, and so if you think of infrared as being "in charge" of transmitting heat, you'll have an inaccurate view.

Of course, other EM radiation can heat things up too, but we rarely feel heat from them, and if we do, it's a bad thing. High-energy X-rays can heat tissue, as can microwaves, but we try to avoid such exposures. So, in everyday experience, if you feel heat coming off a hot thing (but the ambient air isn't so hot) then your skin is "seeing" bright infrared.

Heat is what you feel on your skin, not any energy. probably you had x- rays at some time. the have lots of energy, but did you feel heat?

• X-ray machines don't output much energy in the form of x-rays. If they output enough energy to noticeably heat your skin, you'd have cancer. Sep 14 at 13:53
• they output enough to show your bones on a film, and you still don't feel warm, the same amount of energy from infrared does. Sep 14 at 13:59
• Those things happen for the same reason: the x-rays go right through your flesh, hardly imparting any heat. Sep 14 at 14:02
• @zucculent - and that is kind of the point - you don't feel the heat, so it isn't a 'heat ray'. (As an aside, x-rays are commonly used to treat cancer, and, yes, you can get a 'sun burn' from them.) Sep 14 at 14:30
• X-rays have very little energy even if it was all deposited on your skin where you could feel it: physics.stackexchange.com/questions/202739/… It's basically impossible to generate a sensation from x-rays without killing you. Maybe if you concentrated everything into one little spot on your arm, then cut out the cancer you just created. But if you dumped a ton of x-rays into a human, they could certainly feel the warmth. Sep 15 at 20:26

I am late to the party, but I feel that some points still need to be clarified.

IR is the most efficient heating mechanism for organic matter due to 2 ingredients:

1. It is resonantly absorbed by phonons,

2. Phononic contribution to the specific heat, and therefore to the internal energy of matter at normal conditions is dominant.

Visible light is also absorbed by matter, causing valence-conduction band transitions by electrons. Despite the respective photon energy is larger ($$h\nu_\mathrm{viz}\gg h\nu_\mathrm{IR}$$), the net effect is very small because of the difference in specific heats of electrons and phonons. This can be back-traced again to two points:

1. To the large mass ratio $$M/m_e\gg1$$. $$M$$ is the mass of nuclei, $$m_e$$ is the electron mass.

2. In organic materials there are almost no free mobile electrons.

Some exceptions to these arguments clarify the things further: In metals at very small temperatures electronic contribution to the internal energy is larger than phononic contribution. Why? Again because of two points:

1. There are free electrons.

2. Phonons are frozen (e.g. as predicted by the Debye model). The specific heat of electrons varies as $$T$$ (linearly with temperature, whereas the specific heat of phonons as $$T^3$$ (on the basis of Debye model for $$T\ll T_D$$, below the Debye temperature $$T_D$$).

However, you also cannot heat metals efficiently with visible light because of reflections. This has to do with typical electronic concentrations $$n$$ which is such that the dominant absorption of metals is in UV frequency range (plasmons) or even even lower due to $$d$$-$$d$$ transitions. With increasing the photon energy, very soon the ionization starts to dominate. Photoionized electrons can in principle heat the system, however, we are here in a completely different regime.

It is simply a matter of perception, and language. We sense (some of) the infrared spectrum as heat, while we sense the visible spectrum as light.

Here's an experiment you could easily try. Find an old-fashioned 60 watt incandescent bulb, which emits a lot of infrared along with the visible light. Place your hand a few inches/cm from it. You feel heat, don't you? Now take an LED bulb that emits the same lumens of visible light, and hold your hand at the same distance. I bet you don't feel any heat at all, right? That's because the bulb emits very little infrared.

If you do a search on for instance "led lamp emission spectrum", you'll find more details on just what sort of radiation LED bulbs and other light sources emit.

• The LED also emits much less electromagnetic energy. The difference you're feeling isn't so much that the LED is emitting much less infrared, it's emitting much less energy in general. If you find an actual 60 W LED (not a 60 W equivalent, but something that will output ~6000 lumen), you'll feel a similar amount of heat. Sep 16 at 9:15
• @AI0867: That's the point. It's emitting the same amount of energy in the visible part of the spectrum (measured by lumens), but very little in the infrared "heat rays". You could get the same result by putting filters - one blocking visible light, one infrared - between you and the incandescent. Block visible light, you'd feel the heat. Block IR, you'd see the light but wouldn't feel the heat. Sep 16 at 15:39
• A even more dramatic experience is fire. I was watching a water show once and near the end they had a stream of fire mixed in and I could feel the heat a hundred yards away. Sep 16 at 18:15
• Sure, you're indeed missing all the extra energy that you only notice in the form of heat, but you can also feel visible light the same way, if there's enough of it. Visible-light lasers can burn things without any infrared being involved, though they will also burn your retinas if you don't protect yourself. Sep 17 at 9:35
• @AI0867: Sure, IF the visible light is intense enough, but in ordinary conditions it isn't. That is, if we had a 60 watt LED lamp - one that output those 60 watts in the visible spectrum - would you feel the light as heat? I don't think so. An LED lamp using 60 watts would output about about 8000 lumens. They're readily available: yard lights, shop lights, light bars %c, yet I've never felt much heat from them. (And I'd bet most is waste heat...) Sep 18 at 3:23

## All light carries energy

You're completely right about that all forms of electromagnetic radiation carries energy, and you can refer to Bob's answer for the technical details. It's also quite false that only infrared radiation will heat things, but there's some truth hidden in the common misconceptions, so let's break things down.

I'm going to be talking about what how various frequencies (and their corresponding wavelength) interact with your body, and where they might come from.

My figures will be approximate, as I'm trying to convey an idea what's happening, not exact values or names.

Radio waves are really quite a broad term, ranging from waves of just a few Hz, up into the GigaHerz. Let's start at the low end.

### Very long radio waves

At frequencies up to 1 kHz, your wavelength is at least 300 km (300 Mm/s / 1000 /s). That means your body is completely insignificant compared to the wave passing over it. It barely interacts with it at all. Interacting with these efficiently requires something with a size on the order of a planet. The main natural source of them is lightning strikes.

### Still long radio waves

Jumping up several orders of magnitude, up to 1 MHz, the wavelengths are still at least 300 m. Your body still doesn't really interact with it, being more than 2 orders of magnitude smaller.

### Shorter radio waves and microwaves

Going up to 1 GHz, we're starting to enter the realm of microwave radiation, though calling them radio waves is still correct. The wavelengths can get as short as 0.3 m (30 cm), and we're not far from the frequency of the typical microwave oven (2.45 GHz, with a wavelength of about 12.5 cm).

As you go through this frequency range, they start to interact with human bodies more and more. You might have noticed how the signals of TV and radio are affected by your mere presence near the antenna. The amount of energy involved tends to be rather low though, and when they interact with you, the energy deposited is distributed all over your body, so you won't feel it on your skin.

### Microwaves

Going up to 1 THz, the wavelength shrinks down to .3 mm (300 $$\mu m$$). Used mainly for high-bandwidth wireless communication and radar, this starts to enter the range of frequencies that interacts mainly with your skin, and you will actually feel. The story goes that using microwaves for heating food was discovered by a radar engineer whose chocolate bar melted when he walked in front of an antenna. That's at a very high power level though, and you won't normally encounter those outside of a microwave oven.

The cosmic microwave background is at 160 GHz, and is a blackbody emitter at about 3 Kelvin.

### Infrared

I'm slowing down our steps a bit now, as a lot of interesting changes are starting to happen.

#### Far infrared

For our purposes, we'll define "far infrared" as everything up to 100 THz (wavelengths down to 3 $$\mu m$$). Like short microwaves, these will interact with your skin, and unlike microwaves, the black-body temperatures associated with these go up to about 80 degrees Celsius, beyond human body temperature.

As black-body emission goes up by the fourth power of temperature, this starts to involve significant amounts of energy, and this is where you start feeling the presence of warm things.

As this range also includes most of the temperatures we normally encounter, thermal cameras use it.

All in all, it's not strange that it's frequently called "thermal infrared".

#### Medium and Near infrared

Not all that much changes as the frequency increases to 430 THz (700 nm), except the black-body temperatures go up to around 4 kilokelvin and the radiative energies involved continue increasing by the fourth power of the temperature. This is the stuff you feel when near a fire, or an incandescent lightbulb.

Part of this range is used for thermal cameras that are intended to track high-temperature heat sources, typically the heat engines that power tanks, jets and rockets.

### Visible light

Going up to 750 THz, the wavelength continues to decrease to about 400 nm. Not all that much changes compared to near infrared, but there are some notable points.

• About half the solar energy that reaches the surface of the earth is in this range, similar to the amount of infrared energy.
• It's absorbed by the skin similarly to infrared. It's certainly not the case that this is an entirely different kind of radiation.
• The black-body temperatures involved are similar to those of the surface of stars, which is why the colors of stars mostly span this range.
• The photon energies are beginning to get large enough (on the order of 1 electron-volt (eV)) to do interesting things. This, along with the fact that the atmosphere is very transparent in this range, allows for things like light receptors and eyes. (infrared 'eyes' tend to use localized heating for sensing, which requires much larger "pixel sizes")

### Ultraviolet

Ultraviolet is named such because it's beyond violet: we can't see it. As the frequencies increase, more things start to change.

Up to 1 PHz (300 nm), humans may not be able to see it, but that doesn't mean other animals can't.

Beyond 1.5 PHz (shorter than 200 nm), atmospheric absorption increases suddenly, as the photon energy becomes high enough to ionize oxygen. At even higher frequencies, they will also interact with nitrogen.

As the photon energy increases, the number of molecules the photons can break increases, increasing the potential for damage and sunburn, though no ultraviolet light appears to be entirely safe.

The sun's light output in ultraviolet drops off fairly quickly, as we exceed its black-body temperature.

Ultraviolet light is still mostly absorbed by the skin, but if your skin feels warm due to ultraviolet radiation, you'll get a terrible sunburn in a hurry.

Once we exceed 30 PHz (10 nm), we cross the rather arbitrary threshold into

### X-Rays

X-rays start 'soft', meaning they don't penetrate much, and are strongly absorbed by air, but as frequencies increase, wavelengths get shorter and photon energy increases. Around 10 keV (120 pm, 12.5 EHz), penetration depth starts to exceed 1 mm, crossing into "Hard X-Ray" territory. Hard X-Rays will penetrate deeper, allowing them to do distribute their energy beyond your skin. Again, if you feel heating effects from this, you should be worrying about the lethal dose of ionizing radiation you just received instead.

### Gamma Rays

The difference between X-Rays and Gamma Rays are their origin: X-Rays are generated using electronic processes, while Gamma Rays are generated using nuclear processes. Their energy ranges overlap, but a typical Gamma Ray photon might be at 300 EHz (1 pm, 1.25 MeV). They behave similarly to hard X-Rays, but at higher energy, more so.

## Conclusion

All light carries energy, but in the typical human experience, only infrared and visible light will be felt as heating you significantly. The visible light will also typically be accompanied by infrared light, so it's not strange to assume that the heat is carried (only) by the infrared light.

However, if you walk out in the sun, and feel its 1 kW/m^2 irradiance, about half the energy heating you is actually visible light, not infrared.

Also, if you use a thermal camera, you're actually measuring a specific band on infrared light, and there are several such bands depending on what you're looking for. (animals and their environment, or the exhaust of heat engines)

It's because they are emitted by objects that we in daily life consider hot. Our skin is sensitive to these so we can avoid getting burned. Also we detect absense of heat waves as cold, so we can avoid hypothermia.