Though quantum theory of light can explain a number of phenomena observed with light , it is necessary to retain the wave-nature of light to explain the phenomena of :

Options

A. Photoelectric effect
B. Diffraction
C. Compton effect
D. Black body radiation

Answer

To solve the question regarding the necessity of retaining the wave nature of light to explain certain phenomena, we will analyze the provided options step by step.

### Step-by-Step Solution:

1. **Understanding the Question**:
The question asks which phenomena can only be explained by the wave nature of light, despite the quantum theory of light being able to explain many phenomena.

2. **Analyzing the Options**:
We have four options to consider. We will evaluate each one to determine if it requires the wave theory of light for explanation.

3. **Option 1: Photoelectric Effect**:
- The photoelectric effect describes the emission of electrons from a material when it is exposed to light.
- This phenomenon can be explained using the particle theory of light (photons).
- **Conclusion**: This option does not require wave theory.

4. **Option 2: Diffraction**:
- Diffraction is the bending of light waves around obstacles and the spreading of waves when they pass through small openings.
- This phenomenon is fundamentally a wave property and cannot be adequately explained by particle theory.
- **Conclusion**: This option requires wave theory.

5. **Option 3: Compton Effect**:
- The Compton effect involves the scattering of X-rays by electrons, resulting in a change in wavelength.
- This effect can be explained using the particle theory of light, as it involves photons colliding with electrons.
- **Conclusion**: This option does not require wave theory.

6. **Option 4: Blackbody Radiation**:
- Blackbody radiation can be explained using quantum theory, but it does not specifically require wave theory.
- The phenomenon can be described by the quantization of energy levels and does not depend on the wave nature of light.
- **Conclusion**: This option does not require wave theory.

7. **Final Answer**:
After analyzing all the options, we conclude that the phenomenon that requires the wave nature of light for explanation is **Option 2: Diffraction**.