Final answer:
In a plane electromagnetic wave propagating in a vacuum, half of the average energy density is stored in the electric field, and the other half is stored in the magnetic field, each accounting for 50% of the total.
Step-by-step explanation:
In an electromagnetic (EM) plane wave propagating in a vacuum, energy is stored both in the electric field and the magnetic field. The energy density due to the electric field is given by uE = ½ε0E2, and similarly, the energy density due to the magnetic field is given by uB = ½B2/μ0. However, because of the relationship between the electric field and the magnetic field in a vacuum (B = E / c), the energy densities of both fields are equal, where c is the speed of light in a vacuum.
Therefore, for an EM plane wave propagating in vacuum, half of the total average energy density is stored in the electric field, and the other half is stored in the magnetic field, making the fractions equal to 1/2 or 50% each.