Which of the following diagrams shows correctly the dispersion of white light by a prism ?

Options

A.
B.
C.
D.

Explanation

To determine which diagram correctly shows the dispersion of white light by a prism, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Dispersion**:
- Dispersion occurs when white light passes through a prism and separates into its constituent colors (spectrum) due to different refractive indices for different colors.

2. **Analyzing the Prism**:
- A prism has two surfaces: the entry surface and the exit surface. When white light enters the prism, it bends towards the normal due to refraction.

3. **Refraction at Entry**:
- As the white light enters the prism, it bends towards the base of the prism. The different colors (red, orange, yellow, green, blue, indigo, violet) will bend at different angles because each color has a different refractive index.

4. **Refraction at Exit**:
- When the light exits the prism, it bends away from the normal. Again, different colors will emerge at different angles, leading to a spread of colors.

5. **Identifying Incorrect Diagrams**:
- Diagrams that show the emergent rays as parallel (like in diagrams 1 and 2) are incorrect because the different colors will not emerge parallel due to their different angles of refraction.

6. **Finding the Correct Diagram**:
- The correct diagram should show that the emergent rays are not parallel and that there is dispersion occurring at both surfaces of the prism.

7. **Conclusion**:
- After analyzing the diagrams, we find that option 3 correctly depicts the dispersion of white light by a prism, showing that the rays are not parallel and dispersion occurs at both surfaces.

### Final Answer:
The correct diagram that shows the dispersion of white light by a prism is **Option 3**.