Microwave ovens, more commonly known as microwaves, have become an essential appliance in every household as a convenient way to cook and heat up food. The mundanity of the microwave and its apparent simple operation are contrasted by the ingenious physical phenomena that underpin the operation of these devices, providing efficient means of cooking to all.


The physical concept of a microwave is to use electromagnetic radiation in the microwave band to induce kinetic rotation in polar molecules. This results in an increase in average kinetic energy and, hence, an increase in thermal energy as well.


The device used to produce the microwaves is called the magnetron. Originally invented as the cavity magnetron, they were vacuum tubes that were used in radar systems during World War II. A stream of electrons would be created via thermionic emission, and these would interact with a magnetic field in cavity resonators. This interaction would create a stream of microwave radiation at a specified resonance frequency that could be determined by the strength of the magnetic field and the configuration of the cavity resonators. The microwaves created are in the form of standing waves. For this reason, the plate that the food is on must spin in order to ensure there aren’t cold spots at the nodes of the standing wave.


Once these microwaves have been created, you want them to interact with polar molecules present in the food. Water, or H2O, is the most desirable, as its configuration of positively charged hydrogen and negatively charged oxygen makes it a magnetic dipole that is present in almost all foods. As the standing microwave oscillates, it exerts a torque on these magnetic dipoles, which attempt to align themselves with the electric field. This causes them to rotate and increase friction forces between the neighbouring molecules, heating up the substance.


Discovered in 1945 by Percey Spencer, an engineer who worked for Raytheon, he noticed that a chocolate bar that had been in his pocket had melted after being exposed to microwaves created by an active radar set he was working on. He would go on to realise the significance of this discovery, leading to the commercial development of the first microwave oven in 1947. The microwave was one of the most revolutionary inventions of the 20th century, utilising physical phenomena to make cooking convenient and efficient for the masses. An estimated 1.1 billion microwaves are in use today emphasising the importance of this invention, all utilising the principles of electromagnetism to make warm food for all!



Finan, C.C. (2023). Microwave Ovens. Oops! Accidental Inventions.

Detz, R. J. and van der Zwaan, B. (2020) ‘Surfing the microwave oven learning curve’, Journal of Cleaner Production, 271, p. 122278. doi: 10.1016/j.jclepro.2020.122278.

Woodford, C. (2018). How do magnetrons work? [online] Explain that Stuff. Available at: https://www.explainthatstuff.com/how-magnetrons-work.html.

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