A Faraday cage is generally used to block electromagnetic interference (EMI) from sensitive electronic equipment. It is an enclosure formed by a solid or fine meshed conductor.
Michael Faraday in 1836 discovered an electrical charge resided on the outside of an enclosed conductor and did not create any electric field inside. Faraday actually built a room encased with a metal foil. He used high-voltage static electric generators induce a charge on the foil and inspected the electric field inside with an electroscope and found no field inside.
A Faraday cage is explained in physics as a result of Gauss’s Law, which states that an electric field of an electric charge is proportional to the enclosed charge only and inversely proportional to the square of the radius. Therefore, if the charge resides on the outside conductor, the only electric field exists outside and no electric field exists inside. But it is also true that if a charge existed some distance outside the conductor, the outside charge would induce and equal charge on the conductor.
Because the material of a conductor by definition allows the electrons to flow freely, the charge quickly redistributes into an even charge density over the surface and the electric fields from the surface charges cancel out internally. Because a Faraday cage has both an outside conductor and prevents any buildup of internal electric field or charge, Faraday cages are an excellent means of lightning protection.
This is the reason that the best advice is to stay inside your car in a thunderstorm which has a body of metal and safely conducts the extreme current of a lighting bolt around you and does not allow any conduction of current through you inside. It is also the reason that any metal-bodied aircraft can usually survive lightning strikes safely in air. However, the new use of non-conducting composite materials prevents this benefit, and aircraft designers have to incorporate a conducting mesh impeded in the composite to allow conduction.
A Faraday cage’s electric field protection becomes limited when it comes to external electromagnetic radiation, which is just oscillating electric and magnetic fields. The oscillating electric field also induces a flow of charge on the conductor like the static charge, except as the frequency increases, the electrons have a limited response speed compared to the changes in the electric field. The net result is some electromagnetic field is attenuated and passes through if the conductor is not thick enough. This is material dependent and is known as the skin depth, so if the conductor is thicker than the skin depth, all the electromagnetic field is absorbed.
If the conductor is meshed, the holes allow the electromagnetic field to pass through if the wavelength is smaller than the diameter of the hole in the mesh. Mesh holes are generally sized to prevent radio frequency or microwaves from passing through. This is the reason that a Faraday cage is used for electromagnetic interference shielding since EMI is generally radio or microwave wavelengths. Visible light is a much higher frequency in the electromagnetic spectrum than radio frequency or microwaves that we can see through the mesh easily and a mesh is the standard method to block microwaves from passing through the glass door on a microwave oven.
