In the blackbody radiation experiment, the inside of the body is metal so its considered a conductor inside, shouldn't the electric field be zero, so we would be left with only magnetic field instead of E.M
For the electromagnetic waves, the electric field parallel to the wall must go to zero if the wall is a perfect conductor. An Electromagnetic wave , which is a self propagating transverse waves of oscillating electric and magnetic fields. In case of a metal wall,
the metal has electrons free to move through the entire solid. This is why metals can conduct electricity. It is also why they are shiny. These unattached electrons oscillate together with large amplitude in response to the electrical field of an incoming light wave. They themselves then radiate electromagnetically, just like a current in an antenna. This radiation from the oscillating electrons is the reflected light. In this situation, little of the incoming radiant energy is absorbed, it is just reradiated, that is, reflected. We in this case, get standing waves.