Minimum number of prisms required to make a combination which is 'Achromatic' as well as 'Direct Vision' is:

Options

A. 2
B. 3
C. 4
D. 11

Explanation

To solve the problem of finding the minimum number of prisms required to create a combination that is both 'Achromatic' and 'Direct Vision', we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Direct Vision and Achromatic Conditions**:
- **Direct Vision** means that light passes through the system without any deviation. This implies that the central wavelength (yellow light for white light) should emerge undeviated.
- **Achromatic** means that light does not disperse into its constituent colors. This means that the combination of prisms should recombine the colors back into white light.

2. **Using Two Prisms**:
- We can use two prisms to achieve both conditions. The first prism will cause dispersion (breaking white light into its constituent colors), and the second prism will reverse this effect.
- If we take two equilateral prisms, we can orient the second prism in the opposite direction (180 degrees rotated) relative to the first prism.

3. **Effect of the First Prism**:
- When white light enters the first prism, it will split into its constituent colors (red, orange, yellow, green, blue, indigo, violet) due to refraction.

4. **Effect of the Second Prism**:
- The second prism, being oriented oppositely, will take the dispersed light from the first prism and recombine it. This means it will reverse the deviation caused by the first prism.
- Since both prisms are equilateral, the angles of deviation for each color will be equal and opposite, leading to the recombination of colors.

5. **Resulting Output**:
- The output from this combination will be white light, as the colors are recombined. The yellow light, which is the central wavelength, will emerge undeviated, satisfying the direct vision condition.

6. **Conclusion**:
- Therefore, the minimum number of prisms required to create a combination that is both achromatic and provides direct vision is **two**.

### Final Answer:
The minimum number of prisms required is **2**.