If heat is just our physical perception of infrared radiation (IR), is there a material transparent to heat, as is glass with respect to visible light?
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1$\begingroup$ it isn't; and there is $\endgroup$– Carl WitthoftCommented Aug 9, 2021 at 17:39
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4 Answers
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You wrote 2 different things:
Heat is just our physical perception of infrared radiation (IR).
Is there a material transparent to heat, as is glass with respect to visible light.
The first thing is wrong, so I'll start with trying to clear the confusion: Heat is a general name for energy transferred not due to applying work. We don't sense heat, we sense Temperature. Now, heat is transferred by 3 mechanisms: conduction, convection and radiation. Most of the time we encounter conduction and convection, while radiation is way weaker (of course except for the sun, because radiation is the only way heat can travel in the vacuum of space). Furthermore, for everyday temperatures, the radiation emitted by bodies losing heat is indeed in the IR region, so that's why you confused heat with IR.
- Are there any materials transparent to IR radiation? Yes, air for example is transparent in the wavelengths of $8-12 \mu m$, which allows us to build thermal cameras operating in these wavelengths.
answered Aug 9, 2021 at 13:00
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1$\begingroup$ "We don't sense heat, we sense Temperature". Is this true? A piece of metal at 10C feels way cooler to me than a piece of wood at 10C... $\endgroup$ Commented Aug 9, 2021 at 13:37
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2$\begingroup$ @MariusLadegårdMeyer You sense the temperature of your fingers, not the material. :P $\endgroup$– DKNguyenCommented Aug 9, 2021 at 14:12
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2$\begingroup$ @MariusLadegårdMeyer as DKNguyen said, you measure your finger's temperature. Metals conduct heat better than wood because of their free electrons, meaning you need to wait way longer for you finger to reach the temperature of a piece of wood than the temperature of metal. You still sense the temperature of your finger, it just doesn't change much when touching a heat insulator $\endgroup$ Commented Aug 9, 2021 at 14:53
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Radiated heat is typically 5.5um to 14um which is considered long IR (not near IR like cameras or some night vision stuff). Optical windows made of NaCl (yes, salt) are transparent there while or other materials like Caesium Iodide are transparent from the visible to far, far beyond that:
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IR spectroscopy is usually concerned with vibrational modes of molecules. One requirement is that the dipolemoment varies during the vibration or mathematically $$ \frac{\partial \mu}{\partial q} \neq 0 $$ where $q$ is the normal coordinate of the vibration. Because of the requirement monoatomic gases such as He or dimers such as H$_2$ or N$_2$ won't have a vibration spectrum.
Gases such as H$_2$O for example however do have a varying dipolemoment and as such have transitions in the IR energy regime. This explains the greenhouse effect.
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One example would be a vessel filled with either $N_2$ or $O_2$. These gases, being diatomic and symmetrical, have essentially no dipole moment and they thus don't respond to IR radiation.
This is usually a point covered when studying greenhouse gases or IR spectroscopy, since our atmosphere is largely nitrogen and oxygen. See here and here.
I think I came across a paper a few years ago that showed collisions can create assymetries in these molecules, that can make them slightly absorbent of IR radiation. If I can find it, I'll link it here.
Edit: This is not the paper I was thinking of, but it makes the point. See the beginning of chapter 3.
