In producing a pure spectrum, the incident light is passed through a narrow slit placed in the focal plane of an chromatic lens because a narrow slit

Options

A. 1.a produces less diffraction
B. 2.increases intensity
C. 3.allows onlyone colour at a time
D. 4.allows more parallel beam when it passes thorugh the lens

Answer

To solve the question regarding why a narrow slit is used in producing a pure spectrum when light is passed through a chromatic lens, we can break down the explanation into clear steps:

### Step-by-Step Solution:

1. **Understanding the Role of the Slit**:
- A narrow slit is essential in optical experiments involving light dispersion. It serves as a point source of light, which helps in producing a clear and distinct spectrum.

2. **Effect of Slit Width**:
- If the slit is wide, it allows a larger area of light to pass through. This results in multiple rays of light coming from different parts of the slit. Each part of the slit can emit light of different wavelengths (colors), leading to overlapping of these rays.

3. **Formation of Impure Spectrum**:
- When light from a wide slit overlaps, it combines at the screen, causing the formation of an impure spectrum. This is because different wavelengths interfere with each other, creating a mixture of colors rather than a clear separation.

4. **Narrow Slit Characteristics**:
- A narrow slit, on the other hand, restricts the light to a very small area. This means that the light passing through is more uniform and primarily consists of parallel rays.

5. **Resulting Pure Spectrum**:
- When these parallel rays pass through a chromatic lens, they are refracted uniformly. This uniform refraction allows for the separation of different wavelengths without overlap, resulting in a pure spectrum on the screen.

6. **Conclusion**:
- Therefore, the reason a narrow slit is used in producing a pure spectrum is that it allows only parallel beams of light to pass through, which helps in achieving a clear separation of colors when the light is refracted by the lens.

### Final Answer:
The narrow slit is used because it allows only parallel beams of light to pass through, which results in a pure spectrum when the light is refracted by the chromatic lens.

---