Energy Density Of The Wave

The instantaneous energy density of an electromagnetic wave splits into an electric part u_E = \frac{1}{2}\varepsilon_0 E^2 and a magnetic part u_B = \frac{B^2}{2\mu_0}. Because the wave satisfies E = cB with c = 1/\sqrt{\mu_0\varepsilon_0}, substituting B = E/c into u_B gives u_B = \frac{E^2}{2\mu_0 c^2} = \frac{1}{2}\varepsilon_0 E^2 = u_E. Hence at every instant, and therefore on time average, the electric and magnetic energy densities are exactly equal, each carrying half of the total. The option that electric energy is twice the magnetic ignores the E = cB constraint. The reverse claim that magnetic dominates likewise violates the field relation. Saying the electric contribution is negligible is simply false, since the two halves are identical. This equal partition of energy is a key qualitative result emphasised in the NCERT Electromagnetic Waves chapter and explains why total energy density can be written compactly as u = \varepsilon_0 E^2 averaged appropriately. The equality also has a deeper meaning: it shows the electric and magnetic fields are not independent reservoirs but two aspects of a single travelling disturbance that continuously regenerate one another. As a consistency check, equal energy densities follow naturally from the symmetric way Maxwell's equations couple the two fields, so any answer claiming dominance of one field over the other would break that symmetry.