A free electrons is placed in the path of a plane electromagnetic wave. The electron will start moving

Options

A. along the electric field
B. along the magnetic field
C. along the direction of propagation of the waves
D. in a plane containing the magnetic field and the direction of propagation.

Answer

To solve the problem of a free electron placed in the path of a plane electromagnetic wave, we will analyze the forces acting on the electron due to the electromagnetic wave.

### Step-by-Step Solution:

1. **Understanding the Electromagnetic Wave**:
An electromagnetic wave consists of oscillating electric (E) and magnetic (B) fields that are perpendicular to each other and to the direction of wave propagation.

2. **Identifying the Forces on the Electron**:
When a free electron is placed in the path of an electromagnetic wave, it experiences forces due to both the electric field and the magnetic field. The total force acting on the electron is given by the Lorentz force equation:
\[ \mathbf{F_L} = q \mathbf{E} + q (\mathbf{v} \times \mathbf{B}) \]
where \( q \) is the charge of the electron, \( \mathbf{E} \) is the electric field, \( \mathbf{v} \) is the velocity of the electron, and \( \mathbf{B} \) is the magnetic field.

3. **Initial Condition of the Electron**:
Since the electron is initially at rest, its initial velocity \( \mathbf{v} = 0 \). Therefore, the magnetic force component becomes zero:
\[ \mathbf{F_L} = q \mathbf{E} + q (0 \times \mathbf{B}) = q \mathbf{E} \]

4. **Direction of the Electric Field**:
The force acting on the electron is directly proportional to the electric field. Since the electron has a negative charge (\( q = -e \)), the direction of the force will be opposite to that of the electric field:
\[ \mathbf{F_L} = -e \mathbf{E} \]

5. **Conclusion**:
The electron will start moving in the direction opposite to the electric field. Therefore, the correct option is that the electron will move along the electric field (but in the opposite direction).

### Final Answer:
The electron will start moving **along the electric field** (in the opposite direction).