Early TV dinners were less labor-intensive than cooking from scratch, but they weren't the ultimate in immediate gratification, since it still took 25 minutes or so to re-heat them in the oven. That eventually would change, thanks to scientists' invention during World War II of the magnetron, an electron tube that improved the capability of radar systems to spot Nazi aircraft.
A Raytheon engineer named Percy LeBaron Spencer was working on improving radar technology when he noticed that the microwaves given off by the magnetron had melted a candy bar in his pocket. In 1945, Raytheon filed a patent for a microwave cooking process, and eventually licensed the technology to the Tappan Stove Company. In 1955, Tappan introduced the first home microwave oven. But it wasn't until the mid-1970s, when the ovens became smaller and more affordable, that they started to catch on. By 1986, a quarter of American homes owned microwave ovens, and 90 percent have them today [source: Ganapati].
The potential of microwave ovens wasn't lost upon frozen dinner manufacturers. But the old-fashioned aluminum trays were a no-go for microwaving, since electric current would be conducted through the metal, and give off sparks that possibly could start a fire. In 1986, Campbell Soup's Frozen Food Division, which owned Swanson, retired the aluminum tray for all 61 varieties of its frozen dinners, and replaced them with containers made of polyethylene terephthalate, or PET -- the same plastic used in soda bottles [source: Kircheimer].
While some people fear a link between plastic and cancer, Harvard Medical School says not to worry. The U.S. Food and Drug Administration closely regulates the packaging of microwavable food, and requires manufacturers to demonstrate that the amount of plastic that leaches into the food is less than 1/100th of the amount per pound of body weight shown to harm laboratory animals over a lifetime [sources: Harvard Family Health Guide].