A Brief Explanation of How Microwaves Work
If you’ve ever wondered or questioned the safety of microwave ovens, I thought it might be useful to share some knowledge I garnered during the time of my PhD when I used microwaves to synthesise new chemical compounds.
Truth be told, I’ve not looked at my thesis for more than a decade now, suffice to say, it was a little dusty as I got it out.
Microwaves were initially discovered in the late 19th century however, it’s use and application pertaining to food developed (quite by accident) during experimentation in to RADAR (detection) technology.
(From Synthetic and structural aspects of unsaturated organic ligands, PhD Thesis)
The applications of microwave heating, in the form of the microwave oven were first discovered around 1946 by Dr. Percy Spencer, an engineer working at Raytheon Corporation, USA. While he was performing RADAR related experiments on the magnetron*, he noticed that the candy in his pocket had melted. He later experimented with corn kernels, making popcorn and by placing an egg near the magnetron tube, he managed to make the egg cook and subsequently explode.
Later, Spencer designed a metal box with an opening for microwave irradiation to be concentrated within it, building the first microwave oven.
What Are Microwaves?
Microwaves are short, non-ionising electro-magnetic waves that travel at the speed of light. Based on wavelengths, they are found between infra-red and radio wave frequencies.
How Microwaves Influence Matter?
To understand how microwaves function we need consider that all matter and materials (everything in and around us) consist entirely of atoms, which are in turn made up of some charged particles like the electron and proton.
As an electro-magnetic wave, microwaves are able to ‘influence’ charged particles, mainly electrons. The electric component of the microwave is able to exert a force on the electrons of a material.
Under the influence of a source of microwaves, the electrons in a material start to migrate or rotate, and this continuously occurs until the direction of the wave is changed. As the electrons move, this also influences the atom as a whole to change direction.
As liquids and gasses, the (molecules) atoms and subsequently the electrons are much more easily influenced by this electric field compared to solids, they rotate and flip according to the direction of the microwave source.
The speed (frequencies) at which the molecules (in liquid, mainly) can rotate in alignment with the microwave fields are up to 10^6 (1 000 000) times a second. At these rates the molecules are still able to keep up with the electric field changes and do not absorb energy and therefore do no heat up. (In solid form, these rotational rates are much lower).
However, when the frequencies of the electric field is increased to 10^12 (12 zeros!) in the presence of the electromagnetic wave, the mass of the molecule is able to keep up with the changes, but the charge cannot. This is what causes the material to absorb the energy from the microwaves and subsequently heat up.
(The power on a conventional microwave ovens, e.g. 800 MHz refers to the frequency of this electric field.)
In simple terms
Focussed microwave sources cause the atoms and molecules in a material to constantly flip and rotate. These atoms and molecules constantly work to align themselves to the direction of the microwave source. (Consider that in a conventional microwave oven, the rotating plate constantly moves the food around, within a fixed direction, microwave source)
When the flipping and rotation occur at very high frequencies (very high speeds), the mass of the atoms and molecules can keep up but the charge (usually of the electron, negative) cannot, and this causes the material to absorb energy from the microwaves thereby heat up.
How is this different from conventional stove-top heating?
From an atomic and molecular stand-point, the energy taken to heat a material is similar whether on a conventional stove or in a microwave. In both cases, heat is generated when the atoms and molecules are provided with energy.
The difference lies in the original energy source, whereby on a stove-top the energy provided is heat directly, whereas in a microwave oven, the energy is initially electrical (and electro-magnetic) in nature.
Are there any unsafe instances of using microwave ovens?
Microwaves, being non-ionising and fairly low-permeable waves are generally safe to use, particularly within the closed confines of a conventional microwave oven (following manufacturers guidance).
It is however, not recommended to microwave water in its pure form (drinks etc are fine). This is because the workings of microwaves upon water (and other solvents) can cause it to ‘super-heat’. This means that its temperature can rise far above it’s boiling point (between 13-26oC higher.
This super-heating however has no external features (no rolling boil) and you would not be able to tell until something comes in to direct contact with it (e.g. a spoon is placed in the cup of water)
I hope this helps to explain and allay the fears some may have about the use of microwave ovens. Generally, I personally know that microwave ovens are safe and used the correct way, contribute an invaluable part to time-saving in the modern kitchen.
Disclaimer: I am not a microwave expert, I have simply researched the details of its workings during my PhD to better understand and optimise it’s use for chemical synthesis. Please remember to always follow the manufacturers instructions.
*a magnetron is a diode-type electron tube used to produce microwave energy