If \( U_E \) = electric field energy density and \( U_m \) = magnetic field energy density. Then,
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → \( \displaystyle\frac{U_E}{U_m} = 1 \)
In an electromagnetic wave, the electric and magnetic fields carry equal energy densities. The two densities are
\( \displaystyle U_E = \frac{1}{2}\varepsilon_0 E^{2} \qquad U_m = \frac{B^{2}}{2\mu_0} \)
The field amplitudes are related by \( E = cB \), and the speed of light is \( c = 1/\sqrt{\mu_0\varepsilon_0} \). Substituting,
\( \displaystyle U_E = \frac{1}{2}\varepsilon_0 c^{2} B^{2} = \frac{1}{2}\varepsilon_0 \cdot \frac{1}{\mu_0\varepsilon_0} B^{2} = \frac{B^{2}}{2\mu_0} = U_m \)
\( \displaystyle \frac{U_E}{U_m} = 1 \)
NCERT (2026-27):
Exercises – …
