In Young's double-slit interference experiment a first screen with a single narrow slit is used in addition to the double-slit screen. An interference pattern is observed on the screen. What happens if the first screen is removed and light from an extended but monochromatic source, e.g.,yellow light from large sodium vapor lamp, is allowed to illuminate the double-slit screen directly?

Options

A. The interference pattern disappears
B. The intensity of light on the viewing screen is reduced
C. The width of the dark fringes reduces noticeably
D. The width of the bright fringes reduces noticeably

Answer

To solve the problem, we need to analyze the effect of removing the first screen with a single narrow slit in Young's double-slit interference experiment when illuminated by a monochromatic source.

### Step-by-Step Solution:

1. **Understanding the Initial Setup**:
- In the original setup, a narrow slit is placed before the double-slit screen. This single slit acts as a coherent source of light waves, allowing them to spread out and create a uniform wavefront before they reach the double slits.

2. **Interference Pattern with Single Slit**:
- When light passes through the single narrow slit, it diffracts and produces a coherent wavefront. This wavefront then encounters the double slits, leading to the formation of an interference pattern on the screen due to the superposition of waves from the two slits.

3. **Removing the Single Slit**:
- If we remove the first screen (the single narrow slit) and directly illuminate the double-slit screen with light from an extended monochromatic source (like a sodium vapor lamp), the situation changes significantly.

4. **Effect of Extended Source**:
- An extended source emits light from multiple points, which means the light waves arriving at the double slits are not coherent. Coherence is essential for producing a stable interference pattern because it ensures that the light waves maintain a constant phase relationship.

5. **Conclusion**:
- As a result of the lack of coherence when using an extended source, the interference pattern that was previously observed will disappear. Instead of a clear pattern of bright and dark fringes, the screen will show a more uniform illumination without distinct interference effects.

### Final Answer:
When the first screen is removed and light from an extended monochromatic source is allowed to illuminate the double-slit screen directly, the interference pattern will disappear.

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