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 need to understand the concept of population inversion in the context of atomic physics and lasers. Here’s a step-by-step explanation:

### Step-by-Step Solution:

1. **Understanding Energy Levels**:
- In an atom, electrons occupy different energy levels. The ground state is the lowest energy level, while excited states are higher energy levels.

2. **Defining Metastable State**:
- A metastable state is an excited state where electrons can remain for a longer duration before transitioning to a lower energy state. This is crucial for achieving population inversion.

3. **Population Distribution**:
- Normally, at thermal equilibrium, more electrons occupy lower energy levels (ground state) than higher energy levels (excited states). This is described by the Boltzmann distribution.

4. **What is Population Inversion?**:
- Population inversion occurs when the number of electrons in an excited state (upper laser level) exceeds the number of electrons in a lower energy state (lower laser level). This is contrary to the normal distribution and is essential for laser operation.

5. **Role in Laser Action**:
- In a laser, population inversion is necessary to achieve stimulated emission, where an incoming photon stimulates an excited electron to drop to a lower energy level, emitting a photon of the same energy, phase, and direction. This leads to the amplification of light.

6. **Conclusion**:
- Therefore, a situation of population inversion is directly related to the operation of lasers. The correct answer to the question is that it is related to lasers.

### Final Answer:
A situation of population inversion is related to **lasers** (Option 4).