Science Questions with Surprising Answers
Answers provided by
Dr. Christopher S. Baird

How does a microwave oven heat up food even though it emits no thermal radiation?

Category: Physics      Published: May 16, 2013

A microwave oven does emit thermal radiation to heat up food. Microwave radiation is thermal radiation. For some reason, pre-college teachers and books have a mistaken notion that thermal radiation = infrared radiation. All frequencies of the electromagnetic spectrum carry energy, from radio waves, microwaves, infrared waves, visible light, ultraviolet, and X-rays to gamma rays. All frequencies of radiation heat up an object that they strike and therefore can be thermal radiation. When physicists use the term "thermal radiation", they either mean radiation that has the ability to heat up an object it strikes. Or they mean a broad spectrum of frequencies with a certain shape that depends on the emitter's temperature.

thermal spectrum at 2000 K
This plot shows the frequency spectrum of the thermal radiation that is emitted by a blackbody object that is at a temperature of 2000 K. Even though most of the thermal radiation is in the infrared band of frequencies, some of the thermal radiation takes the form of microwaves, visible light, and ultraviolet. Note that both the frequency axis and the power axis are plotted on logarithmic scales to make the curve easier to analyze. Public Domain Image, source: Christopher S. Baird.

The higher the temperature of a glowing body, the more the peak in the spectrum of its emitted radiation shifts to higher frequencies. Thus by looking at the relative strength of different frequencies in starlight, astronomers can determine the star's temperature. The word "thermal" in this way means that the radiation's spectral shape is linked to the source's temperature. In contrast, "non-thermal radiation" means light that is not correlated to the source's temperature. For instance, lasers emit light through a mechanism that is different from that of a hot filament. The laser light is therefore independent of the laser's temperature and is thus non-thermal radiation. But laser light still carries energy and is still able to heat up objects that it strikes. If you take "thermal radiation" to mean radiation that carries energy and heats things up, as many pre-college teachers do, then all radiation is thermal, independent of frequency or spectral shape. A microwave oven's radiation heats up soup in the exact same way that a campfire's radiation heats up campers: through electromagnetic radiation. The misunderstanding that only infrared radiation is thermal perhaps comes from the fact that living human bodies are at a temperature where their thermal radiation peaks in the infrared. If a soldier wants to see human bodies at night, he uses infrared goggles. But the sun's thermal radiation peaks in the visible light frequency band, and its radiation is every bit as thermal. In fact, most of the infrared portion of sunlight is absorbed by the atmosphere and does not make it to us on the surface. Sunlight has no problem heating us up, and its spectral shape is linked to the sun's temperature, despite containing very little infrared radiation.

Topics: food, heat, heating, microwave, microwave oven, radiation, sunlight, thermal radiation