Visualizing standing waves in a microwave
Here’s a fun little experiment that anyone can conduct at home with a chocolate bar and a microwave oven.
Remove turntable from microwave (the plate that rotates)
Take a chocolate bar on a plate and place it inside microwave.
Heat for for 30-60 seconds on high.
You will notice that the chocolate would have melted in some regions and not in others (see image above). But don’t worry this is supposed to happen
A microwave works by setting up a standing wave inside it.
The size of the oven is chosen so that the peaks and troughs of the reflected microwaves line up with the incoming waves and form a “standing wave”.
The above is a 1D analog of a standing wave, but a 2D standing wave looks like so:
And there are nodes and anti-notes in three dimensions throughout the entire oven.
At the anti-nodes is where the wave oscillates the most
And therefore a molecule placed at the anti-nodes will rub against each other more rigorously than the ones at the nodes.
More the rubbing, more the food the gets heated up.
This is why the chocolate in our image is melted in some regions (the anti-nodes) whether remains intact in others (the nodes).
If you take a ruler and measure the distance between two successive anti-nodes and plug it into the frequency-wavelength relationship, one can obtain the speed of light.
But the key insight that one can gather from this experiment is the visual feel for how long the wavelength of a microwave actually is!
It’s a lot of fun to do this experiment on your own.
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You can do more with some better but less delicious equipment.
You can get a couple of neon bulbs from an electronics shop and place them in a grid inside the microwave to view the standing wave while the microwave is in action
This tells you how the heat is distributed at the bottom of the microwave oven. The lit bulbs are anti-nodes and unlit ones are the nodes .
You can also try (NOT recommended) to do this by placing a light bulb inside a cup of water instead of a neon bulbs.
As you can clearly see the bulb only lights up in the anti-node regions of the standing wave while the remaining regions are the nodes.
You can take this a step further if you have an infrared camera .
Mark Rober had this brilliant idea of using the infrared camera inside a microwave in order to ensure that food that is being microwaved is cooked evenly and completely on the inside.
And ElectroBOOM then took this to the next level by placing a cardboard box inside a microwave oven and looking at the heat map using a infrared camera.
Source: ElectroBOOM
That last gif my dear friends was the most satisfying physics animation that I had ever seen for a really long time ! It clearly illustrates the 3D standing wave heat map that is produced inside a microwave oven.
Although these aren’t the only ways to visualize the standing microwave pattern inside a microwave oven, but these are the ones that I was able to test them out with equipment that you can probably find at home or at school/university.
You are welcome to suggest more thought experiments, alternate methods or edits to this post, I would highly appreciate it!
Have a good one!
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