Your microwave oven spins around and around when it heats up food.
But you don’t need your food to spin when you use an oven, or a toaster, or slow cooker. So what is unique about this way of cooking?
Why does the microwave spin?
Your microwave oven heats up food by using microwaves which are a type of radiation, technically short length radio waves.
This radiation is produced by the magnetron and bounces around inside the microwave, reflecting back in off the metal inner walls, until it gets absorbed by food or water.
These waves tend to focus on certain points where they are higher in energy, while also having points where the energy is much lower.
The heat is not evenly distributed like it would be with a convection oven, for instance. Radio waves are a few inches in wavelength which makes the difference between the higher energy waves noticeable in the size of a microwave.
All microwaves are fitted with a “wave stirrer” that helps distribute the waves around the oven, but it’s impossible to make it perfect.
All microwave ovens need to spin (or rotate) to allow these high energy focus points to be spread around the food. This distributes the heat evenly throughout the food.
What happens if the microwave doesn’t spin?
If a microwave doesn’t spin it’s not the end of the world.
Maybe you’ve tried cooking something without a turntable or where the turntable wasn’t on properly so wasn’t turning? If you had, you’d notice that your food still gets cooked. You might not even notice a difference.
Even in the best of conditions, and with a rotating turntable, the microwave heats up certain parts more than others.
If you eat the food straight from the microwave you might notice annoying cool spots and hot spots. To avoid this, follow the good microwave practice of stirring your food (if possible) and leaving to stand for 2-3 minutes upon finishing. This gives the food the best chance to spread the heat out through conduction.
Actually, microwaves don’t even have to rotate. The original microwave ovens from decades ago didn’t even rotate at all. The rotating turntable was introduced by some smart cookie to reduce hot and cool spots. Everyone else soon followed suit.
A visual demonstration of why your microwave spins
You cannot see the waves in a microwave as that particular type of electromagnetic radiation (radio waves) is completely invisible. You can however try this neat experiment to see exactly what’s going on in your microwave instead.
Simply choose a food that can be easily melted while still holding its form as it melts, such as chocolate or cheese, put it on a plate and see how it melts.
This is the results from my microwave:
[can’t do this right now because I can’t get to the store because of damn coronavirus. will update when I can]
Interestingly, you can use this to measure the speed of light! You can see a great video on that here.
You can actually measure the waves formed in a microwave that isn’t turning food. Take a microwaveable tray that fits in the bottom of the microwave. Line it with one layer of marshmallows . you want the marshmallows to cover the tray evenly so take the time to lay them side by side. Put it in the microwave and turn on for a few seconds.
You will see where the waves have made some spots start to cook or melt while others are untouched. Measure the distance between the cooked sections and you can get a good idea of where the hot spots are in that microwave.
These types of ovens are why microwaveable foods often instruct to turn them after a certain amount of time so everything cooks evenly. Microwaves with a rotating tray take care of that automatically.
One other use of the turntable
An interesting second use of the glass turntable that I discovered while doing my research for this article is here. Basically, the waves that bounce around inside your microwave can be quite dangerous to your microwave apparatus, particularly the magnetron.
If there is no food or water, or very little, these microwaves will bounce around until they get absorbed, eventually by the magnetron, potentially causing it damage and breaking your microwave.
The glass turntable serves as an absorbent sponge that can absorb these microwaves if there is little food or water to absorb them, keeping your microwave safe. You can test this by seeing how hot the glass turntable gets when there is no food inside the oven.
What happens if my microwave spins the other way?
A small percentage of microwaves will stop and reverse the direction of rotation of their turntable partway through cooking. The reason for this is nothing to do with heat distribution or anything else I’ve talked about, but just a peculiarity of the design.
Basically it’s to do with cheap motors that want to use as little energy to start moving in a certain direction. After a certain point, it’s less energy to start reversing the way it was turning. A fuller, and much better, explanation can be found here.