Two sources are called coherent if they produce waves

Options

A. 1.of equal wavelength
B. 2.of equal velocity
C. 3.having same shape of wavefront
D. 4.having a constant phase difference.

Answer

**Step-by-Step Solution:**

1. **Understanding Coherent Sources**:
- Coherent sources are defined as two or more sources that emit waves with a constant phase difference. This means that the waves produced by these sources maintain a consistent relationship in their phases over time.

2. **Key Characteristics of Coherent Sources**:
- **Equal Wavelength**: The waves produced by coherent sources must have the same wavelength.
- **Equal Velocity**: The speed of the waves must be the same.
- **Same Shape of Wavefront**: The wavefronts produced should be identical in shape.
- **Constant Phase Difference**: The phase difference between the waves must be constant, which can be zero or any other fixed value.

3. **Evaluating Options**:
- When presented with multiple-choice options, we need to identify which option correctly describes coherent sources based on the characteristics listed above.
- Option 1 states something that does not align with the definition, making it incorrect.
- Option 2 also does not match the coherent source definition, hence incorrect.
- Option 3 fails to meet the criteria as well, thus incorrect.
- Option 4 correctly states that coherent sources have either a zero phase difference or a constant phase difference, making it the correct choice.

4. **Conclusion**:
- After evaluating all options, we conclude that the correct definition of coherent sources aligns with option 4, which states that they have a constant phase difference.

**Final Answer**: Option 4 is correct.

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