|MLA Citation:||Bloomfield, Louis A. "Question 1208"|
How Everything Works 20 May 2018. 20 May 2018 <http://howeverythingworks.org/print1.php?QNum=1208>.
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.