The primary reason that the holes in a microwave oven door are designed to prevent microwaves from escaping is related to the wavelength of the microwaves rather than the amplitude.
Microwaves used in microwave ovens have a relatively long wavelength, typically around 12.2 centimeters (or 4.8 inches) for the commonly used frequency of 2.45 gigahertz. The wavelength determines the physical size of the oscillating electric and magnetic fields that make up the microwave radiation.
The holes in the microwave oven door are sized and spaced in such a way that they are significantly smaller than the wavelength of the microwaves. These small holes effectively act as a barrier to the microwaves because the dimensions of the holes are much smaller than the size of the oscillating fields of the microwaves.
When an electromagnetic wave, such as a microwave, encounters an obstacle or an aperture, its behavior is influenced by the relative size of the obstacle or aperture compared to the wavelength. In the case of the microwave oven door, the holes are designed to be much smaller than the wavelength, preventing the microwaves from passing through effectively.
The amplitude of the microwaves, which represents the intensity or strength of the wave, is not the primary factor in determining whether the microwaves can pass through the holes. It is the wavelength and the size of the openings that are crucial in blocking the microwaves from escaping.