Which of the following phenomenon is not explanined by Huygen's construction of wavefront ?

Options

A. reflection
B. diffraction
C. refraction
D. origin of spectra

Explanation

To solve the question of which phenomenon is not explained by Huygen's construction of wavefronts, we can analyze each of the given options step by step.

### Step-by-Step Solution:

1. **Understanding Huygen's Principle**:
Huygen's principle states that every point on a wavefront can be considered a source of secondary wavelets. The new wavefront is formed by the envelope of these wavelets. This principle helps explain various wave phenomena.

2. **Analyzing Reflection**:
- When a wavefront strikes a reflective surface, it bounces back.
- According to Huygen's principle, the wavefront before and after reflection can be constructed using secondary wavelets from the points on the wavefront.
- **Conclusion**: Reflection is explained by Huygen's principle.

3. **Analyzing Refraction**:
- Refraction occurs when a wavefront passes from one medium to another, changing speed and bending at the interface.
- Huygen's principle can also be used to construct the new wavefront in the second medium by considering the wavelets from the wavefront in the first medium.
- **Conclusion**: Refraction is explained by Huygen's principle.

4. **Analyzing Diffraction**:
- Diffraction is the bending of waves around obstacles and the spreading of waves when they pass through narrow openings.
- Huygen's principle is applicable here as well, as it allows for the construction of wavefronts around obstacles using secondary wavelets.
- **Conclusion**: Diffraction is explained by Huygen's principle.

5. **Analyzing Origin of Spectra**:
- The origin of spectra refers to the separation of light into its constituent colors, which is a phenomenon explained by atomic physics and quantum mechanics, not wave optics.
- Huygen's principle does not apply to the explanation of spectral lines or the origin of spectra.
- **Conclusion**: The origin of spectra is not explained by Huygen's principle.

### Final Answer:
The phenomenon that is not explained by Huygen's construction of wavefronts is **the origin of a spectra**.