When a sky wave is reflected on to the ground

Options

A. frequency of the reflected wave is different to that of incident wave
B. there is a phase difference of `pi//2` is introduced to the reflected wave
C. the reflected wave is out of phase of incident wave and reach the receiving antenna along with the direct wave from transmitting antenna causing interference.
D. the waves are not reflected by the ground.

Answer

### Step-by-Step Solution:

1. **Understanding Sky Waves**: Sky waves are radio waves that are transmitted into the atmosphere and can be reflected back to the Earth by the ionosphere. This is an important concept in communication systems, particularly in long-distance radio communication.

**Hint**: Remember that sky waves are affected by the ionosphere, which can reflect them back to the ground.

2. **Reflection of Waves**: When a sky wave is reflected off the ionosphere, it does not change its frequency. The frequency of the wave remains constant before and after reflection.

**Hint**: Focus on the properties of waves; frequency is a characteristic that does not change upon reflection.

3. **Phase Change upon Reflection**: When a wave reflects off a boundary (like the ionosphere), it experiences a phase shift. For sky waves reflecting off the ionosphere, this phase shift is typically π (180 degrees), meaning the reflected wave is out of phase with the incident wave.

**Hint**: Recall that a phase shift of π indicates that the wave is inverted upon reflection.

4. **Interference of Waves**: The incident wave (the original wave sent from the transmitter) and the reflected wave (the wave that bounces back from the ionosphere) reach the receiving antenna simultaneously. This can lead to interference, which is the combination of the two waves.

**Hint**: Consider how two waves can interact; they can constructively or destructively interfere depending on their phase relationship.

5. **Analyzing Options**: Given the options:
- The frequency of the reflected wave is different from that of the incident wave (Incorrect).
- There is a phase difference of π/2 introduced to the reflected wave (Incorrect).
- The reflected wave is out of phase with the incident wave (Correct).
- Waves are not reflected by the ground (Incorrect).

The correct option is that the reflected wave is out of phase with the incident wave.

**Hint**: Carefully evaluate each statement against the principles of wave behavior during reflection.

### Final Answer:
The correct statement is that the reflected wave is out of phase with the incident wave, leading to interference at the receiving antenna.