Final answer:
The energy density of the electromagnetic field in a medium is the sum of the energies of the electric and magnetic fields per unit volume.
Step-by-step explanation:
The energy density of the electromagnetic field in a medium is given by the sum of the energies of the electric and magnetic fields per unit volume. The energy density from the electric field is represented by εoE², where εo is the permittivity of free space and E is the electric field. The energy density from the magnetic field is given by (1/2)B²/μo, where B is the magnetic field and μo is the permeability of free space.