Which of the following cannot be polarised?

Options

A. Radio waves
B. Ultraviolet rays
C. Infrared rays
D. Untrasonic waves

Explanation

To solve the question "Which of the following cannot be polarised?", we need to understand the concept of polarization and the types of waves involved.

### Step-by-Step Solution:

1. **Understand Polarization**:
- Polarization is the process by which the vibrations of a wave are restricted to a particular direction. This phenomenon is typically observed in transverse waves, where the oscillations occur perpendicular to the direction of wave propagation.

2. **Identify Types of Waves**:
- There are two main types of mechanical waves: longitudinal waves and transverse waves.
- **Longitudinal Waves**: In these waves, the particle displacement is parallel to the direction of wave propagation. Examples include sound waves and ultrasonic waves.
- **Transverse Waves**: In these waves, the particle displacement is perpendicular to the direction of wave propagation. Examples include light waves and waves on a string.

3. **Determine Which Waves Can Be Polarised**:
- Transverse waves can be polarized because their oscillations can be restricted to a particular plane.
- Longitudinal waves cannot be polarized because the oscillations occur in the same direction as the wave propagation, leaving no room for restriction to a plane.

4. **Identify the Correct Answer**:
- Since the question asks which of the following cannot be polarised, we conclude that longitudinal waves, such as ultrasonic waves, cannot be polarized.

5. **Final Answer**:
- Therefore, the correct answer is that ultrasonic waves (which are longitudinal waves) cannot be polarized.