Wow, the sand is making such beautiful patterns on the vibrating plate! But you can’t see them all the time. It depends on how fast the plate is vibrating.
A speaker under the plate sends out sounds. Sounds are actually vibrations, so the sounds made by the speaker make the plate vibrate. The higher the pitch, the faster the plate vibrates. The number of vibrations per second is also called the frequency and is expressed in Hertz (Hz).
The plate does not vibrate equally in all places. At certain frequencies, the plate vibrates very strongly in some places and not at all in others. This creates beautiful patterns. How? Because the sand is pushed away from the places which are vibrating most (the ‘antinodes’) and collects in the non-vibrating places (the ‘nodes’). This combination of nodes and antinodes is called a standing wave.
Oh yes, we call those patterns ‘Chladni patterns’, because – you guessed it – the German Ernst Chladni discovered them.
Standing waves in a microwave oven
Why does a microwave oven rotate your food?
You heat up your food with microwaves, but standing waves are also created in the microwave oven. This means that you will find nodes and antinodes in particular places. Where there are antinodes, the water particles in your food vibrate the most vigorously and get hot. At the nodes, the water particles don’t vibrate and the food remains cold. A pancake which is still cold in patches: not so nice. That’s why the plate revolves. It means that everything heats up evenly.