Line spectrum was first of all theoretically explained by

Options

A. Swan
B. Fraunhofer
C. Kirchoff
D. Bohr

Answer

To solve the question "Line spectrum was first of all theoretically explained by," we need to identify the scientist who first provided a theoretical explanation for the line spectrum in atomic physics.

### Step-by-Step Solution:

1. **Understanding Line Spectrum**:
- A line spectrum is a spectrum that contains only specific wavelengths of light, which correspond to the transitions of electrons between energy levels in an atom. Each element has a unique line spectrum.

2. **Historical Context**:
- The line spectrum was observed experimentally, but a theoretical explanation was needed to understand why these specific wavelengths were emitted.

3. **Key Theoretical Model**:
- The Bohr model of the atom, proposed by Niels Bohr in 1913, provided a theoretical framework for understanding the line spectrum. Bohr suggested that electrons orbit the nucleus in fixed paths or orbits and that they can only occupy certain energy levels.

4. **Energy Level Transitions**:
- According to the Bohr model, when an electron transitions from a higher energy level to a lower energy level, it emits a photon of light with a wavelength corresponding to the energy difference between those levels. This results in the observed line spectrum.

5. **Conclusion**:
- Therefore, the line spectrum was first theoretically explained by the Bohr model of the atom.

### Final Answer:
The line spectrum was first theoretically explained by Niels Bohr.

---