When I warm more than one cup of coffee or milk together in a microwave oven, some of them warm more than others. Why does this happen? Is there something wrong with our microwave oven? — ON, Istanbul, Turkey
When the microwaves bounce around inside the oven’s cooking chamber, they experience an effect called interference. Interference occurs when similar waves, or portions of the same wave, follow different paths to the same region in space. As they pass through that region, their crests and troughs ride up on top of one another and they interfere. Sometimes the crests of one wave ride on the crests of the other wave, creating enormous crests—an effect called constructive interference. However, it is also possible for the crests of one wave to ride on the troughs of the other wave, so that they cancel one another out—an effect called destructive interference.
These interference effects are quite visible in wave waves, but they also make themselves apparent in microwaves. In your oven, they lead to regions of the cooking chamber that heat quickly (regions where the microwaves experience constructive interference) and regions that don’t heat well at all (regions where they experience destructive interference). Because these fast and slow cooking regions can’t be avoided, many microwave ovens incorporate turntables to keep the food moving through the various regions inside the oven. Some ovens use rotating metal paddles to stir that microwaves around inside the cooking chamber, so that the fast and slow cooking regions move about.
Your experience with uneven heating of coffee or milk is an example of this interference problem. The solution is to move the cups occasionally while they are being heated.