By surfing through the web, I've found this rule of thumb for faraday cages:

"One rule of thumb often used for Faraday cages to prevent transmission is that the holes need to be no larger than 1/10 of the wavelength of the signal."

Does anyone have an explanation for where this rule of thumb came from? Is there a theoretical explanation for this using formulas and concepts?

Thanks!

 

Hello there. welcome to AAC!
Take for instance those little holes in the window of your microwave oven. they are called Frequency Selective Surface and are designed to block the microwave energy from the microwave oven front glass window.
FSS layer which consists of array of circular ring
patch made up of copper conduction plate and is printed on the front glass window of your microwave oven and yes by all means it is a faraday cage. The calculations our time consuming and cumbersome there's not just one formula,I wish there was.
Equivalent circuit method for analyzing frequency selective surface with ring patch in oblique angles of incidence are made by computational computers ,algorithms, programs.The FSS screen possesses band-pass response at 10 GHz and high attenuation at 2.45 GHz band. Approximate frequency at which you cook your food with.