Introduction

In chapter 1 we briefly mentioned different types of electromagnetic radiation, one of which involved a type of energy you can find in most kitchens – microwaves . As a quick recap, microwaves are a form of electromagnetic energy comprising wavelengths that fall between infrared and radio waves, meaning that in general terms their wavelengths are relatively long, and they exert relatively low energy compared to other types of energy (e.g. UV) ( Fig. 21.1 ).

Diagram of the electromagnetic spectrum (EMS) showing the relationship between wavelength, frequency and energy.

Now, it’s probably no surprise to hear that one of the most easy-to-understand practical uses of microwave radiation is for the generation of heat (thermal energy) in a microwave oven. This means that every time you convert an item of food from a chilled substance into its molten lava counterpart in the microwave, you’re technically doing a scientific experiment that utilises interference properties of electromagnetic radiation (see chapter 10 ). This is possible because the microwave energy is absorbed by fats, sugars, proteins and water, causing them to heat up. The microwaves also interfere with each other causing the formation of hot spots in the food in locations where constructive interference takes place. This can be likened to the ‘bright maxima’ we discussed in chapter 10 , but instead of bright light, it produces hot spots. If your microwave contains a turntable, then the turntable serves to dissipate these hot spots, enabling food to be cooked more evenly, and if your microwave doesn’t contain a turntable, then instead it will contain a built-in, rotating motor which serves the same purpose.

The hot spots generated by the constructive interference are related to the wavelength of the microwaves, as anywhere two peaks or two troughs align, there will be maximum heat produced ( Fig. 21.2 ). This means that the distance between the hot spots will equate to half of the wavelength.

Diagram showing the amplitude profile and wavelength of a microwave (black line). As the amplitude increases, the energy also increases, meaning that both the peak and the trough of the microwave profile will produce the highest amount of intensity (heat). This means that the microwave will produce two hot spots (shown in red dashed lines) whose distance will equate to half a wavelength (distance between peak to trough).

The experiment

In order to find the wavelength of the microwaves in our microwave ovens, we purposely want to create hot spots within the oven – thereby cooking something incredibly unevenly. . . . This means we will need to remove the turntable from the microwave before we begin.

Please note: if your microwave does not contain a turntable (or if the turntable can’t be removed), then unfortunately you won’t be able to do this experiment unless you borrow someone else’s microwave.