Question Video: Recognizing a Source of Infrared Radiation Physics

Video Transcript

Which of the following could be a source of infrared radiation? (A) Alternating electric currents, (B) decaying atomic nuclei, (C) direct electric currents, (D) thermal motion of atoms and molecules, (E) none of the answers is correct.

We see that each one of the options (A) through (D) is a candidate for being a source of infrared radiation, a particular type of radiation in the electromagnetic spectrum. As we consider which of these four options could be a source of IR, infrared radiation, let’s start out at the top with option (A), alternating electric currents.

When alternating electric currents are used to generate electromagnetic radiation, what is typically produced from this source is either microwaves or radio waves. This is because the frequency of oscillation of these currents is low enough that it produces these particular types of radiation. We see that not only option (A) talks about electric currents but so does option (C), this time in the form of direct electric currents, that is, current that always points in the same direction.

Even though direct currents do always point the same way, we can effectively turn them into alternating currents by switching the direct current on and off over and over again. It’s by this mechanism that radio waves are created. What we’re seeing is that both of these options, alternating as well as direct electric currents, do act as sources of electromagnetic radiation but not sources for infrared radiation. Instead, they’re typically used to create microwaves and radio waves. So we’ll cross these off our list of options.

Moving on to option (B), decaying atomic nuclei, this is a process where an atomic nucleus breaks or splits apart into smaller pieces. That’s called fission and, in the process, releases electromagnetic radiation. When a break like this happens though, the radiation typically emitted is gamma radiation, that is, the emission of gamma rays. So, once more, this option is a source for a particular type of electromagnetic radiation, but not the type we’re interested in, infrared radiation. We’ll cross off option (B) too.

This brings us to option (D), the thermal motion of atoms and molecules. Here’s what this option means. Everyday objects, such as chairs or tables or cups or plants or really anything, will probably be at around room temperature, that is, about 20 degrees Celsius or 70 degrees Fahrenheit. Just by virtue of their temperature, these objects will have enough thermal energy that atoms and molecules that make them up are in thermal motion. Thanks to this thermal motion, a certain type of radiation is emitted. And this indeed is infrared or below-red radiation. Our eyes aren’t sensitive to this particular type of radiation. We can’t see it, but nonetheless it’s there. And it’s created by the thermal motion of atoms and molecules.

Answer option (D) can be a source of infrared radiation. Therefore, option (E), that none of the answers is correct, is itself not correct. Our final answer is that the thermal motion of atoms and molecules can be a source of infrared radiation.