Statement-1 : Electromagnetic waves with frequencies smaller than the critical frequency of ionosphere cannot be used for communications using sky wave propagation.
Statement-2: The refractive index of the ionosphere becomes very high for frequencies higher than the critical frequency.

Options

A. Statetment-I is True, Statement-II is True , Statement-II is a correct explanation for Statement-I
B. Statement-I is True, Statement-II is True , Statement-II is NOT a correct explanation for Statement-I
C. Statement-I is True, Statement-II is False
D. Statement-I is False, Statement-II is True

Answer

To analyze the statements provided in the question, we will break down the concepts of sky wave propagation and the behavior of electromagnetic waves in the ionosphere.

### Step 1: Understand Sky Wave Propagation
Sky wave propagation refers to the transmission of radio waves that are reflected off the ionosphere back to the Earth. This method is commonly used for long-distance communication.

**Hint:** Remember that sky wave propagation relies on the ionosphere reflecting certain frequencies of electromagnetic waves.

### Step 2: Define Critical Frequency
The critical frequency is the maximum frequency at which a radio wave can be transmitted vertically and still be reflected back by the ionosphere. Frequencies above this critical frequency will pass through the ionosphere and not be reflected.

**Hint:** Think of the critical frequency as a threshold that determines whether a wave will be reflected or transmitted through the ionosphere.

### Step 3: Analyze Statement 1
Statement 1 claims that electromagnetic waves with frequencies smaller than the critical frequency cannot be used for communication using sky wave propagation. This statement is **false**. In fact, waves with frequencies below the critical frequency can be reflected by the ionosphere and used for communication.

**Hint:** Consider how waves behave when they encounter the ionosphere at frequencies below the critical frequency.

### Step 4: Analyze Statement 2
Statement 2 states that the refractive index of the ionosphere becomes very high for frequencies higher than the critical frequency. This statement is **true**. When the frequency of the electromagnetic waves exceeds the critical frequency, the ionosphere behaves as a medium with a high refractive index, causing the waves to pass through rather than reflect.

**Hint:** Reflect on how the refractive index changes with frequency and its impact on wave propagation.

### Step 5: Conclusion
Based on the analysis:
- Statement 1 is **false**.
- Statement 2 is **true**.

Thus, the correct option is that Statement 1 is false and Statement 2 is true.

**Final Answer:** Statement 1 is false, Statement 2 is true.