Microwaves have frequencies ranging from about 1 billion cycles per second, or 1 gigahertz (GHz), up to about 300 gigahertz and wavelengths of about 30 centimeters (12 inches) to 1 millimeter (0. ...

That is half the wavelength of the microwaves that the oven produces. (Melty-bits shown in red.) Double that distance, and you’ll have the wavelength of the waves emitted by the microwave.

The screen is designed with holes that are the perfect size to keep microwaves, which have a wavelength of 4.8 inches, inside of the device while it's running.