A free electron is placed in the path of a plane electromagnetic wave. The electron will start moving -
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → along the electric field
A charge in an electromagnetic wave feels the Lorentz force \( \vec{F} = q\vec{E} + q\,\vec{v}\times\vec{B} \). The electron starts from rest, so \( \vec{v} = 0 \) and the magnetic part is zero at the start. Only the electric force \( -e\vec{E} \) acts, and it lies along the line of the electric field (opposite to \( \vec{E} \), since the charge is negative).
Option (2) along the electric field — Correct. The force \( -e\vec{E} \) is along the electric field line, so the electron starts moving along it.
NCERT (2026-27):
Magnetic Field, Lorentz Force – “… The force on an electric charge q due to both of them can be written as
\( \displaystyle \mathbf{F} = q\,[\,\mathbf{E}(\mathbf{r}) + \mathbf{v}\times\mathbf{B}(\mathbf{r})\,] \equiv \mathbf{F}_{\text{electric}} + \mathbf{F}_{\text{magnetic}} \qquad (4.3) \)
This force was given first by H.A. Lorentz based on the extensive experiments of Ampere and others. It is called the Lorentz force. … (iii) The magnetic force is zero if charge is not moving (as then |v| = 0). Only a moving charge feels the magnetic force.”