Huygens theory could not explain

Options

A. photoelectric effect.
B. reflection of light.
C. diffraction of light.
D. interference of light.

Explanation

**Step-by-Step Solution:**

1. **Understanding Huygens' Theory**: Huygens' 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 theory primarily explains the wave nature of light, including phenomena like reflection and refraction.

2. **Identifying the Limitations of Huygens' Theory**: While Huygens' theory successfully explains many wave phenomena, it has limitations. It does not account for certain behaviors of light that require a particle description, particularly in situations where light interacts with matter.

3. **Analyzing the Options**:
- **Photoelectric Effect**: This phenomenon occurs when light of a certain frequency strikes a metal surface and ejects electrons. It demonstrates the particle nature of light, which Huygens' theory cannot explain.
- **Reflection of Light**: Huygens' theory can explain reflection as it involves wavefronts.
- **Diffraction of Light**: Huygens' principle can also explain diffraction, as it involves the bending of waves around obstacles.
- **Interference of Light**: Huygens' theory can explain interference patterns created by overlapping wavefronts.

4. **Conclusion**: Since Huygens' theory cannot explain the photoelectric effect, which is a clear demonstration of the particle nature of light, the correct answer is the first option: **Photoelectric Effect**.