Which of the following can give sustained interference?

Options

A. Two independent laser sources
B. Two independent light bulbs
C. Two independent sound sources
D. Two independent microwave sources

Answer

To determine which of the given options can produce sustained interference, we need to understand what sustained interference is and the conditions required for it to occur.

### Step-by-Step Solution:

1. **Understanding Sustained Interference**:
- Sustained interference occurs when the interference pattern (maxima and minima) remains constant over time. This means that the sources of interference must be coherent.

2. **Identifying Coherent Sources**:
- Coherent sources are those that maintain a constant phase relationship. This is essential for sustained interference because if the phase relationship changes, the positions of maxima and minima will also change.

3. **Analyzing the Options**:
- **Option 1: Two independent laser sources**:
- Lasers are typically coherent sources. If two lasers are used, they can produce a stable interference pattern, hence they can give sustained interference.
- **Option 2: Two independent light bulbs**:
- Light bulbs are not coherent sources. They emit light of different phases and frequencies, leading to a changing interference pattern. Therefore, they do not provide sustained interference.
- **Option 3: Laser source and sound source**:
- A laser is a coherent source, and while sound waves can interfere, they need to be from a coherent source as well. If the sound source is coherent (e.g., a tuning fork), sustained interference can occur. Thus, this option can give sustained interference.
- **Option 4: Two independent microwave sources**:
- Microwaves can be coherent if they originate from the same source or are phase-locked. Therefore, two independent microwave sources can also produce sustained interference.

4. **Conclusion**:
- The options that can give sustained interference are:
- **Option 1**: Two independent laser sources
- **Option 3**: Laser source and sound source (if coherent)
- **Option 4**: Two independent microwave sources

### Final Answer:
The options that can give sustained interference are **1st, 3rd, and 4th**.

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