Which one of the following waves is purely longitudinal?

Options

A. Sound waves in air.
B. Radio waves traveling through air.
C. Light waves traveling through vacuum.
D. Waves on a plucked guitar string.

Answer

To determine which wave is purely longitudinal, we need to understand the characteristics of different types of waves.

### Step-by-Step Solution:

1. **Understanding Wave Types**:
- Waves can be classified into two main types: transverse waves and longitudinal waves.
- **Transverse Waves**: In transverse waves, the motion of the particles is perpendicular to the direction of wave propagation. An example of this is waves on a string or water waves where particles move up and down while the wave travels horizontally.
- **Longitudinal Waves**: In longitudinal waves, the motion of the particles is parallel to the direction of wave propagation. An example of this is sound waves, where particles compress and rarefy in the direction the wave travels.

2. **Identifying Examples**:
- **Transverse Waves**: Examples include water waves, seismic S-waves, and electromagnetic waves (like light and radio waves).
- **Longitudinal Waves**: The primary example is sound waves, which consist of compressions and rarefactions in the medium through which they travel.

3. **Analyzing the Options**:
- Given the options, we need to identify which one represents a purely longitudinal wave.
- Sound waves are the most common example of purely longitudinal waves, as they involve the back-and-forth motion of particles in the same direction as the wave travels.

4. **Conclusion**:
- The wave that is purely longitudinal is the sound wave. Other options like transverse waves and electromagnetic waves do not fit this classification.

### Final Answer:
The wave that is purely longitudinal is **sound waves**.