Coherence is a property of waves describing how well they line up with each other in time and space. Two sources are coherent if their waves line up perfectly. For this to happen, the waves must have the same wavelength (and hence frequency). Since waves are often emitted in bursts that are finite in length, the wave packets must also line up to be perfectly coherent.

The two wave packets above are partial coherent since the waves line up, but the packets are slightly out of alignment. Interference would be incomplete between them.

Bridge Light
Chain Interference Analogy
Hologram - Skull
Interference Patterns
Laser Demonstration
Michelson Interferometer
Multiple Raccoon Eyes
Points of Light

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