Refractive index of lonosphere is

Options

A. zero
B. More than one
C. Less than one
D. One

Answer

### Step-by-Step Solution:

1. **Understanding the Ionosphere**: The ionosphere is a layer of Earth's atmosphere that is ionized by solar and cosmic radiation. It is located above the troposphere and extends from about 30 miles (48 km) to 600 miles (965 km) above the Earth's surface.

2. **Refractive Index Concept**: The refractive index (n) of a medium is defined as the ratio of the speed of light in a vacuum (c) to the speed of light in that medium (v). It is given by the formula:
\[ n = \frac{c}{v} \]
A refractive index less than 1 indicates that the speed of light in that medium is greater than in a vacuum.

3. **Refractive Index of the Ionosphere**: The density of the ionosphere is much lower than that of the lower layers of the atmosphere. Therefore, the refractive index of the ionosphere is less than 1.

4. **Total Internal Reflection (TIR)**: Since the refractive index of the ionosphere is less than that of the lower atmosphere, when electromagnetic waves (like radio waves) travel from a denser medium (lower atmosphere) to a rarer medium (ionosphere), total internal reflection can occur. This phenomenon allows radio waves to be reflected back to Earth, facilitating sky wave propagation.

5. **Mathematical Representation**: The refractive index of the ionosphere can be mathematically expressed as:
\[ \mu = \sqrt{1 - \frac{80.6 \cdot N}{f^2}} \]
where:
- \(N\) is the free electron density (approximately \(10^{11}\) electrons/m³),
- \(f\) is the frequency of the waves in kilohertz.

6. **Conclusion**: Since the term inside the square root (1 - ...) will yield a value less than 1, it confirms that the refractive index of the ionosphere is indeed less than 1.

### Final Answer:
The refractive index of the ionosphere is less than 1.

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