In an electromagnetic wave propagating in a medium:

The correct answer is Option (3) → Electric energy density is equal to the magnetic energy density

In an electromagnetic wave the fields are linked by \( E = vB \), where \( v = 1/\sqrt{\mu\varepsilon} \) is the speed of the wave in the medium. Compare the electric and magnetic energy densities:

    \( \displaystyle u_E = \frac{1}{2}\varepsilon E^{2} \qquad u_B = \frac{B^{2}}{2\mu} \)

Putting \( B = E/v \) and \( v^{2} = 1/\mu\varepsilon \):

    \( \displaystyle u_B = \frac{E^{2}}{2\mu v^{2}} = \frac{E^{2}\,\mu\varepsilon}{2\mu} \)

    \( \displaystyle u_B = \frac{1}{2}\varepsilon E^{2} = u_E \)

The energy of the wave is shared equally between its electric and magnetic fields.

NCERT (2026-27):
Exercises – …