Ask the Expert Archive: Microwave (MW) and Combo Ovens

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Are microwaves radiation?
How then do microwaves differ from other forms of electromagnetic (EM) energy?
How do microwaves cause a material to heat?

Archive of Questions and Answers

Are microwaves radiation?
Technically, yes. Microwaves are electromagnetic (EM) energy, which can be defined as radiation, but definitions sound scarier than reality. Radio waves, analog television signals, light from a light bulb or from the sun are all forms of EM energy. Sunbathers are technically "irradiating" themselves. However, in common language, radiation is generally associated with the energy given off by radioactive materials. Microwaves have nothing to do with radioactivity.

How then do microwaves differ from other forms of electromagnetic (EM) energy?
Light from the sun is composed of a range of wavelengths of EM energy including infrared, visible and ultraviolet (UV). Microwaves have much longer wavelengths (on the order of centimeters) than the bulk of the EM energy given off by the sun (sub-micron wavelengths). As the wavelengths of EM energy become smaller, the energy per photon becomes more intense (for an in-depth understanding look up Plank's Law). At the UV wavelengths, the energy is substantial enough to become ionizing. This is why there is such concern to overexposure to the sun: the UV energy can cause cellular damage that can ultimately lead to health problems such as melanoma.

All EM energy with wavelengths in the UV range and shorter are referred to as ionizing radiation.
All EM energy with wavelengths longer than UV is referred to as non-ionizing radiation.
Microwaves are non-ionizing radiation. The relatively long wavelengths of microwave allow penetration into certain materials resulting in volumetric heating, a great advantage in certain commercial and industrial heating applications.

How do microwaves cause a material to heat?
Microwaves cause a thermal response in certain materials. The exact physical mechanisms can be mathematically complex to describe, but basically the impinging time-harmonic EM wave excites molecules, or parts of molecules, and the result is heat. Traditionally, the materials to be heated have to be dielectric (basically, a non-conductor). Plastics, for example, are dielectric materials. Some plastics do not respond to microwaves (i.e., Teflon, polyethylene), and others respond very well (PVC). This has to do with the molecular structure. Some non-responsive materials can be doped with receptors to enable heating by microwave energy. Conductive materials typically are not heated by microwave energy, since the incident EM is reflected from the surface. That is why metals are used to fabricate MW ovens (the metal walls keep the microwave energy contained). Recently, some researchers have developed techniques for microwave heating of conductive materials, such as carbon-fiber composites and sintered metal parts, which is possible since these materials are not perfect conductors.