|I know that an electromagnetic wave cannot pass through the holes in a metal cage (a Faraday cage) if those holes are significantly smaller than the wavelength of the wave. But what if it is just a constant electric field? What determines the hole size now? — KBH, Logan, Utah
If the electric field isn't changing with time, then it can't enter a metal cage no matter how large the cage's holes are. In effect, the constant electric field has an infinite wavelength and can't propagate through holes of any finite size. However, the holes don't stop an electromagnetic wave instantly—the wave does penetrate a short distance into the cage before it dwindles to insignificance. The distance over which the wave diminishes by a factor of about 3 is roughly the size of the hole through which it is trying to pass. So if your Faraday cage has holes that are 1 centimeter in diameter, the constant electric field will take several centimeters to diminish to nearly zero. If the holes are much larger than that, the electric field will penetrate far into the cage and the cage will only be an effective shield if it is extremely large. To avoid having to use a very large cage, it's better to use small holes.