Which one of the following is the property of a monochromatic, plane electromagnetic wave in free space?

Options

A. Electric and magnetic fields have a phase difference of `pi//2`
B. The energy contribution of both electric and magnetic fields are equal
C. The direction of propagation is in the direction of `vecBxxvecE`
D. The pressure exerted by the wave is the product of its speed energy density

Explanation

To solve the question regarding the properties of a monochromatic, plane electromagnetic wave in free space, we will analyze each of the given options step by step.

### Step 1: Understand the Properties of Electromagnetic Waves
Monochromatic plane electromagnetic waves have specific characteristics. They consist of oscillating electric (E) and magnetic (B) fields that are perpendicular to each other and to the direction of wave propagation.

### Step 2: Analyze Each Option
1. **Electric and magnetic fields have a phase difference of π/2**:
- This statement is **false**. In a plane electromagnetic wave, the electric and magnetic fields oscillate in phase, meaning they reach their maximum and minimum values simultaneously. Therefore, this option is not a property of the wave.

2. **Energy contribution of both electric and magnetic fields are equal**:
- This statement is **true**. The energy density of the electric field is given by \( U_E = \frac{1}{2} \epsilon_0 E^2 \) and for the magnetic field, it is \( U_B = \frac{1}{2} \frac{B^2}{\mu_0} \). In free space, the relationship between electric and magnetic fields is such that their contributions to the total energy density are equal. Hence, this option is a property of the wave.

3. **Direction of propagation is in the direction of B cross E**:
- This statement is **false**. The direction of propagation of an electromagnetic wave is given by the cross product of the electric field and the magnetic field, which is \( E \times B \). Therefore, this option is incorrect.

4. **Pressure exerted by the wave is the product of its speed and energy density**:
- This statement is **false**. While electromagnetic waves can exert pressure (radiation pressure), it is not defined simply as the product of speed and energy density. Thus, this option does not represent a property of the wave.

### Conclusion
Based on the analysis, the only true property of a monochromatic, plane electromagnetic wave in free space is:
- **The energy contribution of both electric and magnetic fields are equal.**

### Final Answer
The correct option is **B**.

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