Light bounces off mirrors; microwaves bounce off metal.
Radar is a kind of microwave that bounces off your speeding car and cooks your goose. If what you put in the oven reflects too many microwaves back instead of absorbing them, the magnetron tube can be damaged. There must always be something in the oven to absorb microwaves.
That’s why you shouldn’t ever run it empty.
Metals in microwave ovens can behave unpredictably unless you have a degree in electrical engineering. Microwaves set up electrical currents in metals, and if the metal object is too thin it may not be able to support the current and will turn red hot and melt, as in the blowing of an overloaded fuse.
And if it has sharp points, it may even act like a lightning rod and concentrate so much microwave energy at the points that it will send off lightning-like sparks. (Those paper-covered wire twist ties are notorious because they are both thin and pointed, so beware.)
On the other hand, the engineers who design microwave ovens can devise safe sizes and shapes
of metal that won’t cause trouble, and some ovens actually do contain metal trays or racks.
Because it’s so hard to predict which sizes and shapes of metal are safe and which may cause fireworks, the best advice is never to put anything metallic in a microwave oven. And that goes for fancy dishes that have gold or other metallic trim.