Can Infrared Light Pass Through Glass?

by James R. Barrante, Ph.D.

There seems to be some controversy, particularly by laypersons, as to whether infrared light can pass through glass.  The correct answer is, “That depends!”  Infrared radiation spans a wide region of wavelengths.  At the shorter wavelength end, near visible red, the behavior of infrared light is not that different from visible light, except, of course, humans cannot see it. This radiation, called near infrared, does pass through glass. A better way to look at it is to say that it is not absorbed by the glass. It’s energy is too large to excite atoms in molecules to higher vibrational states. If you own an electric stove, you will experience this light just before the coils begin to glow a dull red. If you doubt that it is there, put your hand near a coil. Your skin actually “sees” this light.

The middle band of wavelengths, generally referred to as thermal infrared, is infrared light produced by matter around room temperature.  It is this band of infrared that cause atoms in molecules to jiggle, and jiggling atoms generate heat.  This radiation is strongly absorbed by matter, and will not pass through glass.  It is also the radiation absorbed by CO2.  So any demonstration that attempts to show that a glass jar filled with CO2 will heat up faster and to a higher temperature than a jar filled with air by shining infrared on both jars has been staged. Oh, the gases in both jars will heat up. If you heat up any container, glass or otherwise, any gas inside the container also will heat up.

At the other end of the infrared spectrum, the far infrared, the light is significantly lower in energy, approaching that of microwaves and radio waves.  This type of radiation generally is produced by colder substances.  It is a more controllable heating radiation and is the type used in infrared heaters and saunas.

As a final reminder, infrared radiation is a form of light, not heat.  Heat is transferred by molecular collisions and is relatively slow.  Infrared radiation moves at the speed of light and is fast.  We associate infrared light with heat only when it interacts with matter and excites vibrational modes of motion of atoms in molecules.  In order for that to happen, a vibrational mode must set up an oscillating electric field in the molecule that can couple with the electric field component of the infrared wave.  While the nitrogen atoms in N2 vibrate, they are unable to create an oscillating electric field.  Consequently, N2 is not infrared active.  Carbon monoxide, CO, is a polar molecule and therefore will set up an oscillating electric field when the carbon-oxygen bond stretches.  It is infrared active.



Filed under Basic Physical Chemistry, Basic Science

12 responses to “Can Infrared Light Pass Through Glass?

  1. Whether the FAR infrared can pass through windows is not indicated here.

  2. PS — Whether far infrared rays can pass through windows is critical to know because this type of ray produces NO in the body, which has manifold heath benefits. For someone allergic to bug bites, who can’t get outside much, but who needs sunshine, whether far infrared rays can pass through windows is important information. Thanks, Margie

  3. Hiruja Mahagedara

    Thanks Doctor barrante.This article was really helpful.

  4. Nice post. I learn something more challenging on different blogs everyday. It will always be stimulating to read content from other writers and practice a little something from their store. Id prefer to use some with the content on my blog whether you don?t mind. Natually Ill give you a link on your web blog. Thanks for sharing.

  5. Martin Gregory

    A very helpful article. I just had one piece of “nitpicky clarification” to ask about, especially since you made the point yourself.

    Earlier in the article you said “jiggling atoms produce heat”. But later in the article you say that infrared radiation is light, not heat.

    I think that “jiggling atoms produce heat” is perhaps not a precisely correct thing to say. Jiggling atoms _are_ heat, and they produce light (infra red light). Right?

    • Sorry, but “jiggling atoms produce heat” is exactly correct. Heat is defined as a transfer of thermal energy. It is exactly how a solid transfers thermal energy to another solid in contact with it. Jiggling atoms do not always produce infrared light. The oxygen atoms in O2 are jiggling and produce no infrared light. The C and O atoms in CO (carbon monoxide) are jiggling and do produce infrared light. Why the difference? For jiggling atoms to generate light that jiggling motion must set up an oscillating electric field in the molecule. Remember, light is an electromagnetic wave. Because, CO is a polar molecule (has a positive end and a negative end), the stretching of the CO bond produced an oscillating electric field that generates an electromagnetic wave. Not all the jiggling motions of atoms in CO2 produce infrared light. They all transfer heat energy. As I said, infrared radiation is light, not heat.

  6. Joe

    Can infrared toaster ovens cause any damage to the eye or skin if observed closely? Are they 100% safe? I see articles about infrared and microwave cooking breaking down DNA in milk, etc. How true is that as well?

    • It depends on how close “closely” is. Keep in mind that infrared and microwave cooking devices are designed to cook things. If you get so close that you are within their sphere of cooking operation, you are going to cook your eyes or skin. My guess is that if you use these devices as directed by the manufacturer, they are safe. When things cook, their DNA breaks down. The infrared and microwave radiation in itself is non-ionizing (like X-rays), but the heat produced could be dangerous. Just use common sense when using them.

  7. Ramsey

    Can we say that Glass, Water vapour and Carbon dioxide can pass heat emissions from illuminated sources but trap (or absorb) them if coming from non-illuminated sources?

    • Not sure I understand the question (i.e., illuminated and non-illuminated sources). Glass passes electromagnetic radiation (light) over a range of specific wavelengths (e.g., visible light), and absorbs electromagnetic radiation at other wavelengths. Water vapor and carbon dioxide do the same thing but to a more limited extent. The wavelengths of electromagnetic radiation emitted from sources (particularly solids) depends on the temperature of the source. For example, the temperature of sun results in its emitting a great deal of radiation at wavelengths of visible light. Glass does not absorb visible light. The temperature of the Earth results in its emitting mainly radiation at wavelengths of infrared radiation. Glass generally absorbs infrared radiation not letting it pass through. Don’t confuse infrared light with heat. Infrared light will only produce heat if it strikes matter and the matter absorbs it.

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