How does one graphically explain the relationship between wavelength and grid apertures in a microwave oven window?

the electromagnetic waves in a typical microwave oven have a wavelength of 12 cm. These electromagnetic waves reflect off metal surfaces: the sides and back of a microwave oven are therefore made of metal.

The microwave oven window also consists of a metal plate, with small grid openings as you indicate. Because these openings (about 1 mm? ) are much smaller than the wavelength, the electromagnetic waves are also reflected in here.

Compare it with letting a tennis ball (~8cm diameter) collide with a metal plate with holes. As long as the holes are small enough, the tennis ball will bounce as if it were a full metal plate. Once there are holes in the plate that approximate the size of your tennis ball, it can fall through or bounce less.

quote below on the wikipedia page (link in this answer):

“The oven cavity in which the food is prepared must be a Faraday cage to ensure that the radiation does not escape to the environment. Therefore, the glass door of the microwave oven is fitted with a conductive metal grille to realize the shielding through the door. The Microwave radiation (wavelength of about 12 cm, which is much larger than the meshes of that grid) cannot pass through, but light, also a form of electromagnetic radiation but with much shorter wavelengths, can.”