Two light sources are said to be coherent if they are obtained from

Options

A. Two independent point sources emitting light of the same wavelength
B. A single point source
C. A wide source
D. Two ordinary bulbs emitting light of different wavelengths

Explanation

To determine the conditions under which two light sources are said to be coherent, we can analyze the options provided in the question.

### Step-by-Step Solution:

1. **Understanding Coherence**: Coherence refers to a fixed phase relationship between two or more light waves. For two light sources to be coherent, they must maintain a constant phase difference over time.

2. **Evaluating the Options**:
- **Option 1**: Two independent point sources emitting light of the same wavelength.
- This option suggests that two separate sources can be coherent if they emit light of the same wavelength. However, they are independent, which typically means they do not have a fixed phase relationship. Therefore, this option is not correct.

- **Option 2**: A single point source.
- This option indicates that a single point source can produce two coherent sources. When light from a single source is divided (for example, through a beam splitter), the two resulting beams will have a fixed phase relationship. This option is correct.

- **Option 3**: A wide source.
- A wide source emits light from multiple points, leading to varying phases and thus does not maintain a fixed phase relationship. Therefore, this option is not correct.

- **Option 4**: Two ordinary bulbs emitting light of different wavelengths.
- Different wavelengths imply different frequencies and phases, which means they cannot maintain a fixed phase relationship. Therefore, this option is also not correct.

3. **Conclusion**: The correct answer is that two light sources are said to be coherent if they are obtained from a **single point source**.

### Final Answer:
Two light sources are said to be coherent if they are obtained from a **single point source**.

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