A very small opaque disc is placed in the path oa a monochromatic light. Its geometric shadow has

Options

A. bright point at the center of shadow surrounded by alternate bright and dark rings
B. dark point at the center of shadow surrunded by alternate bright and dark rings
C. uniform darkness
D. uniform illumination

Answer

To solve the question regarding the geometric shadow of a very small opaque disc placed in the path of monochromatic light, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Setup**:
- A very small opaque disc is placed in the path of a monochromatic light source. Monochromatic light means light of a single wavelength.

2. **Identifying the Effect of the Disc**:
- The disc will block the light, creating a shadow behind it. However, due to the wave nature of light, the situation is more complex than just a simple shadow.

3. **Diffraction Phenomenon**:
- When light encounters an obstacle (the disc), it does not just stop; instead, it bends around the edges of the disc. This bending of light waves is known as diffraction.

4. **Formation of Wavelets**:
- According to Huygens' principle, every point on a wavefront can be considered a source of secondary wavelets. The edges of the disc will act as sources of these wavelets.

5. **Interference of Wavelets**:
- The wavelets emanating from the edges of the disc will overlap and interfere with each other. This interference leads to the formation of a pattern of light and dark regions.

6. **Resulting Pattern**:
- The central region of the shadow will not be completely dark due to constructive interference from the wavelets, leading to a bright central fringe. Surrounding this central bright fringe, there will be alternating dark and bright fringes (rings) due to the interference of the wavelets.

7. **Conclusion**:
- The geometric shadow of the small opaque disc will not just be a uniform dark area; instead, it will consist of a central bright fringe surrounded by alternating dark and bright rings.

### Final Answer:
The geometric shadow of a very small opaque disc placed in the path of monochromatic light will consist of a central bright fringe surrounded by alternating dark and bright rings due to diffraction and interference.

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