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.