State one condition for obtaining a sustained interference of light.

Explanation

To obtain a sustained interference of light, one key condition is:

**Condition**: The two sources of light must be coherent.

### Explanation:
1. **Coherent Sources**: Coherent sources are those that emit light waves of the same frequency and maintain a constant phase difference between them. This means that the waves produced by these sources will interfere with each other in a predictable manner.

2. **Same Frequency**: The light emitted from both sources should have the same frequency. If the frequencies differ, the interference pattern will change over time, making it unsustainable.
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