which of the following in not true for electromagnetic waves ?

Options

A. They transport energy.
B. They have momentum.
C. They travel at different speeds in air depending on their frequency.
D. They travel at different speeds in medium depending on their frequency.

Answer

To determine which statement is not true for electromagnetic waves, we will analyze each option provided in the question.

### Step-by-Step Solution:

1. **Understanding Electromagnetic Waves**:
Electromagnetic waves are waves that consist of oscillating electric and magnetic fields and can propagate through space. They transport energy and momentum.

2. **Analyzing Each Option**:
- **Option A: They transport energy**:
- This statement is true. Electromagnetic waves, such as light from the sun, carry energy. For example, sunlight provides heat energy to the Earth.

- **Option B: They have momentum**:
- This statement is also true. Electromagnetic waves exhibit both particle and wave properties, which means they carry momentum. Photons, the particles of light, have momentum despite having no mass.

- **Option C: They travel at different speeds in air depending on their frequency**:
- This statement is false. In a given medium (like air), electromagnetic waves travel at a constant speed regardless of their frequency. The speed of light in air is approximately \(3 \times 10^8\) m/s, and it does not change with frequency.

- **Option D: They travel at different speeds in medium depending on their frequency**:
- This statement is true. When electromagnetic waves pass through different media, their speed can change based on their frequency. This phenomenon is observed in dispersion, such as when light passes through a prism and separates into different colors.

3. **Conclusion**:
The statement that is not true for electromagnetic waves is **Option C**: "They travel at different speeds in air depending on their frequency."