A situation of population inversion is related to

Options

A. Matter wave
B. `gamma-ray`
C. X-rays
D. LASER

Answer

To solve the question regarding the situation of population inversion, we will break down the concepts step by step.

### Step-by-Step Solution:

1. **Understanding Population Inversion**:
- Population inversion is a condition where the number of atoms or molecules in an excited state (higher energy level) exceeds the number in a lower energy state (ground state). This is contrary to normal conditions where more atoms are typically in the ground state.

2. **Energy Levels**:
- In a laser system, there are two key energy levels: the Upper Laser Level (ULL) and the Lower Laser Level (LL). The ULL corresponds to a higher energy state where atoms can be excited, while the LL is the ground state or a lower energy state.

3. **Stability of States**:
- The ground state (LL) is stable, meaning atoms tend to remain in this state. The excited state (ULL) is often referred to as a metastable state, which means that while it is not as stable as the ground state, it can hold electrons for a short duration before they return to the ground state.

4. **Condition for Population Inversion**:
- For population inversion to occur, the number of atoms in the excited state (ULL) must be greater than the number of atoms in the ground state (LL). This is an unusual condition and is essential for the operation of lasers.

5. **Emission Processes**:
- Once population inversion is achieved, two types of emissions can occur:
- **Spontaneous Emission**: This is the random emission of photons by excited atoms returning to the ground state.
- **Stimulated Emission**: This occurs when an incoming photon stimulates an excited atom to emit a second photon of the same energy, phase, and direction, leading to a chain reaction that amplifies light.

6. **Relation to Lasers**:
- The process of stimulated emission is fundamental to the operation of lasers. The coherent light produced through this process is what characterizes laser light.

7. **Conclusion**:
- Therefore, the situation of population inversion is directly related to the operation of lasers.

### Final Answer:
Population inversion is related to lasers.