What is "radiation"? And--if you "nuke" food in a microwave oven, can it become radioactive?

The word "radiation" in common use has (unfortunately) more than one meaning. And to answer the 2nd question--no, microwave radiation and nuclear radiation (involving radioactivity) are very, very different.

"Radiation" is essentially something which spreads radially in all directions, along straight lines. Examples are light and its close relatives, infra-red (IR) radiation (emitted by hot objects) and ultra-violet (UV) radiation.

In the middle 1800s James Clerk Maxwell concluded that light was an electromagnetic wave. That suggested such waves could be created by purely electrical means and in 1886 Heinrich Hertz did so, producing radio waves. The microwave radiation in your oven is also produced electrically, it is just a short-wave radio wave, developed in World War II for use in radar. When physicists talk of "radiation" they usually mean "electromagnetic radiation," the large family covering light, IR, UV, radio and microwaves.

Then came the year 1895. Two discoveries stirred up the world of physics and seemed to upset Maxwell's neat scheme.

It was already known that in a glass tube containing very rarefied air and two separated electrical contacts, if a high voltage was connected to those contacts, an electrical current flowed and a faint glow was produced. [In 1897 J.J. Thomson found the current was carried by tiny pieces of matter, electrically charged, which were named "electrons."] One of the discoveries of 1895, by the German Konrad Roentgen, was that the tube also emitted something that could fog photographic plates, even ones kept in tight dark boxes. Obviously some radiation was produced, and Roentgen named it "X-rays", "X" for "unknown."

The same year the Frenchman Henri Becquerel found that uranium ores also emitted some "radiation" able to darken photographic materials. It thus seemed for a while that Nature may have many other new types of radiation--in fact, some claims were made (and later retracted), while science fiction writers had fun with stories about "death rays."

Gradually a clear picture emerged. X-rays were part of the electromagnetic family, with very short wavelength and high penetration. Radiation emitted by radioactive materials consisted of 3 kinds: of these the gamma-rays were

similar to X-rays, but usually with even shorter wavelength and greater penetrating ability. The other two were fast particles, atomic bullets--fast electrons (negative charge), and "alpha particles", fast positive nuclei of helium.

Those particles should really not be called "radiation," but it is too late to educate the public, especially since many detector instruments (such as Geiger counters) respond both the particles and to X- or gamma rays. When James Van Allen's instrument on the "Explorer" satellites I and III discovered high energy particles magnetically trapped in near-Earth space, they were called the "radiation belt" and the name stuck. The best we can do is call the kind which comes from radioactivity "nuclear radiation."

But you never find it in microwave ovens.

Our thanks this week to David P. Stern, a GSFC physicist, retired as an emeritus, who has created several large web courses on space, astronomy, physics, astronomy and more (even math). See http://www.phy6.org/prospect.htm

If you wonder why light and radio are thought of as "electromagnetic waves," see his web page http://www.phy6.org/stargaze/Sun5wave.htm and perhaps also its neighboring web pages, in the course "From Stargazers to Starships."