Since a mirror can reflect (bounce it in a different direction) heat, doesn't that mean it can reflect flame because it is also heat, I know that heat is anything hot, and flames are hot. If yes does it reflect it in a specific direction and if no why not?

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    $\begingroup$ A mirror reflects light, not heat. Light carries with it energy so it can be used to "reflect heat", but only in this form. $\endgroup$ Commented Feb 4, 2017 at 14:49
  • $\begingroup$ So heat can only be reflected when it's inside the light? I am sorry for this stupid question, I knew I had something wrong :/ $\endgroup$ Commented Feb 4, 2017 at 14:51
  • $\begingroup$ heat is (infrared) radiation, where light is usually a different range of frequencies from heat. The mirror reflect radiation, although FYI not all requencies are reflected equally well. $\endgroup$ Commented Feb 4, 2017 at 14:55
  • $\begingroup$ So I guess I have been misunderstanding the definition of heat :/ But what about: " where light is usually a different range of frequencies from heat." I know that frequency means the amount of number something is repeated during a time period, though I don't think that makes sense in your sentence. I guess you mean they are bouncing off at different time periods? $\endgroup$ Commented Feb 4, 2017 at 15:04

2 Answers 2


So there's not really a physics concept of "heat" as an object. Heat isn't an object. It is a name we use for certain transfers of energy. When you wake up and you put your feet on the floor and then you go "ow! that is cold!" we say that it is because of a "heat transfer", this is a transfer of thermal energy from your feet to the floor. So when it's moving we sometimes call that thermal energy "heat."

You might say "oh, CR Drost is just making a big fuss out of nothing, he wants me to say 'thermal energy' instead of heat." But no, I am actually saying something important, because 'thermal' is also a very important word for physicists, and it refers to where the energy is located. The idea is that since energy is conserved, you can pretend that it is a "stuff" and it lives in the "degrees of freedom" of a system. This sounds like a very scary idea but in practice it just means the 1D coordinates that we use to describe a system's motion. If a system has many moving parts, like your body has many atoms, then there are many degrees of freedom there.

Now the idea of thermal energy in particular is that it is located 'everywhere'. It is kind of like pressure, which is a force that is located everywhere in a fluid. Thermal energy is a layer of energy which is not 'stored up' in any particular degree of freedom but is instead 'spread out' among all of the ways that a system can move. So when we speak of "heat transfer" we mean that you lost some of your everywhere-energy to something else's everywhere-energy because its degrees of freedom had less energy, on average, than yours do. It's like if you had two basins full of water (its height representing this amount of "energy stuff") to different levels and you put a siphon tube between them, and the water flows from the higher-level basin to the lower-level basin, the water that got transferred is equivalent to the "heat energy" transferred from your feet to the floor.

Now one of the ways we share our thermal energy with the world is called "thermal radiation." Our eyes cannot see human thermal radiation, but we can see this stuff for example when we poke a fireplace with an iron poker, and pull the poker out, and see that it is "red hot." That red/orange light we see is the poker's thermal radiation. The Sun's light is also a thermal radiation. Candle flames are usually thermal radiation too. Radiation just means "electromagnetic field that travels out to infinity." Thermal radiation comes because your thermal energy means that the electrons in you are bouncing around randomly, and whenever electrons "bounce" or accelerate they give off little "photons," packets of electromagnetic field. Inside of you these photons get absorbed by the rest of your atoms, but at your skin these photons can of course radiate out to infinity.

To be clear, this was not the way that heat was transferred from your feet to the ground. Or, at least, it was not the main way. Because your feet were radiating the same amount out both before and after you touched the ground, but you felt a sudden cold sensation when your feet touched, and that heat flow did not come from the thermal radiation, but just from the coupling of the degrees of freedom: now when your foot-atoms are vibrating they started vibrating the floor-atoms with them, which allowed their degrees of freedom to share energy.

But I am telling you about thermal electromagnetic radiation because mirrors reflect electromagnetic radiation. And so the only way I know of for a mirror to "reflect heat" is for it to reflect thermal radiation! And that's not all that there is to "heat" or "thermal energy" or anything, it's this small part.

A mirror probably can reflect the thermal radiation of a candle, but a candle is also a flowy process by which a liquid of parrafin wax (or whatever) is vaporized into a gas which rises into this "flame" region, where the atoms are buzzing around so intensely that they rip apart the wax molecule into little pieces. It turns out that this wax molecule "wants" to be in little pieces, in an oxygen-rich atmosphere. I use this word "wants" in the same sense that a mousetrap which is set "wants" to be sprung: it takes a certain vibration to rip the atoms apart, but once they're far enough apart they start to attract so strongly to those oxygen atoms that they just fly out of the wax molecule, leaving with more energy than the initial jiggle that it took to break them apart. It's the same with a mousetrap, a little jiggle from a mouse and snap, all of this energy is released.

Candles also of course have a heat transfer due to convection, as you'll notice if you ever put your hand in the stream of air flying up out of the candle: that stuff is hot!

Anyway the point is that if you try to literally shove this flame into a mirror all of these atoms don't go into the mirror and the candle will only operate less efficiently, not more. Mirrors cannot reflect "flame" but only the electromagnetic radiation that the candle emits. Some of this is light. Some of this, like your thermal radiation and the floor's radiation, has too long of a wavelength to be visible to your eyes, it's the same stuff as light but it's not "visible." Some electromagnetic radiation is even too short of a wavelength for your eyes to see, which is how "black lights" work. Mirrors can potentially reflect these sources of radiation. But they cannot reflect physical processes like flames or light bulbs, just the radiation that comes out of them.


Since a mirror can reflect(bounce it in a different direction) heat,

There exist heat reflectors, true, but they are not mirrors in the sense of coherent images. A mirror reflects the rays of the object coherently so the pattern remains . Heat reflectors reflect like a lot of point sources. Any heat reflected by the usual mirror will also be incoherent infrared radiation, because the wavelengths of visible light which make up the flame image, are much smaller than the infrared wavelengths of reflected heat, and the mirror is a mirror designed for visible light.

doesn't that mean it can reflect flame because it is also heat,

The image of the flame will be there,but not its heat concentrated as in real space.

I know that heat is anything hot, and flames are hot. If yes does it reflect it in a specific direction and if no why not?

The light frequencies of the flame will be reflected in a coherent image, from the silver back side of the mirror. . The infrared frequencies hitting the average mirror will be reflected incoherently from the glass infront, which will be opaque to the infrared frequencies , depending where the photons hit.

Special mirrors are designed for infrared frequencies, for example this link. I suppose they would reflect the heat localized as an image, as this video shows in the test, where you can see the source and the infrared is caught in the detector.

  • $\begingroup$ "because the wavelengths of visible light which make up the flame image, are much smaller than the infrared wavelengths of reflected heat" Does that mean the wave of energy of heat of a flame is too small that it can't be reflected? And why is it so small? $\endgroup$ Commented Feb 4, 2017 at 16:08
  • $\begingroup$ It depends on the surface , the way it has been designed and if there is a protective coating, as is the usual glass of everyday mirrors. It is not small, it is more spread out and it requires different materials to be reflected as an image with concentrated heat, as the video shows. You have to study physics in many courses to really understand how light works. The electromagnetic spectrum is here en.wikipedia.org/wiki/Electromagnetic_spectrum . the visible is a small part of the range in frequencies/wavelengths $\endgroup$
    – anna v
    Commented Feb 4, 2017 at 16:38

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