Which of the following factors affects speed of electromagnetic waves ?

Options

A. Wavelength
B. Intensity
C. Power of source
D. Medium , in which wave travels

Explanation

To determine which factors affect the speed of electromagnetic (EM) waves, we can analyze the given options: wavelength, intensity, power of the source, and medium.

### Step-by-Step Solution:

1. **Understanding Electromagnetic Waves**:
- Electromagnetic waves travel through different media, and their speed can vary depending on the properties of these media.

2. **Analyzing the Options**:
- **Wavelength**: The wavelength of a wave is related to its frequency and speed (v = fλ). However, changing the wavelength does not directly affect the speed of the wave in a given medium.
- **Intensity**: Intensity refers to the power per unit area carried by the wave. It does not affect the speed of the wave.
- **Power of Source**: The power of the source determines how much energy is emitted but does not influence the speed of the EM wave.
- **Medium**: The medium through which the wave travels significantly affects its speed. The speed of EM waves is given by the formula \( v = \frac{c}{n} \), where \( c \) is the speed of light in vacuum and \( n \) is the refractive index of the medium. Different media have different refractive indices, which changes the speed of the wave.

3. **Conclusion**:
- The only factor that affects the speed of electromagnetic waves is the **medium** through which they propagate.

### Final Answer:
The factor that affects the speed of electromagnetic waves is **medium**.

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