Which of the following is not true about electromagnetic waves?

The correct answer is Option (2) → The electric and magnetic field vectors are parallel to each other and perpendicular to the direction of propagation of the wave.

Option (1) The energy in an electromagnetic wave is divided equally between the electric and magnetic fields. — True. The energy densities are \( \varepsilon_0E^{2}/2 \) and \( B^{2}/2\mu_0 \). With \( B=E/c \) and \( c^{2}=1/\mu_0\varepsilon_0 \) the two are equal, so the electric and magnetic fields carry equal shares of the energy.

Option (2) The electric and magnetic field vectors are parallel to each other and perpendicular to the direction of propagation of the wave. — Not true. The two fields are perpendicular to the direction of propagation. They are also perpendicular to each other, so they are never parallel. In a wave travelling along z, \( \vec{E} \) lies along x and \( \vec{B} \) along y.

Option (3) These waves do not require any material medium for propagation. — True. They are self-sustaining oscillations of electric and magnetic fields, so they travel through free space or vacuum.

Option (4) The electric and magnetic field vectors attain their maxima and minima at the same place and the same time. — True. The fields vary as \( E_x=E_0\sin(kz-\omega t) \) and \( B_y=B_0\sin(kz-\omega t) \), with the same phase, so they peak and fall to zero together.

NCERT (2026-27):
Nature of Electromagnetic Waves – “It can be shown from Maxwell’s equations that electric and magnetic fields in an electromagnetic wave are perpendicular to each other, and to the direction of propagation.”