1. What coherence means
Coherence tells me how well two light waves “match” each other in terms of phase and frequency. For interference patterns to stay stable, the two light waves must maintain a constant phase difference and have the same frequency.
When these conditions are met, the sources are called coherent. Otherwise, they are incoherent.
2. Coherent sources
Coherent sources emit waves that stay in step with each other. Their peaks and troughs maintain a fixed relationship.
2.1. Key conditions
- Same frequency (same colour of light)
- Constant phase difference
When these are met, the interference pattern is sharp and steady.
2.2. How coherent sources are created
Two independent bulbs cannot create coherent light. Instead, coherent sources are created by splitting light from a single source. Examples:
- The two slits in Young’s experiment
- A beam splitter creating two identical beams
- Lasers (naturally coherent)
3. Incoherent sources
Incoherent sources emit waves independently. Their phases change randomly with time, so the waves do not maintain a stable relationship.
3.1. Why they cannot produce steady interference
The random phase variation makes the bright and dark regions shift unpredictably. As a result, any interference pattern averages out and disappears.
That’s why two ordinary bulbs or LEDs do not produce an observable interference pattern.
3.2. Examples of incoherent sources
- Two separate bulbs
- Two candles
- Sunlight coming through different openings
4. Why coherence is essential for interference
Interference requires stable overlapping of two waves. This is only possible when:
- The waves have exactly the same frequency.
- Their phase difference remains constant.
If either condition fails, the bright and dark fringes keep shifting and eventually blur out.
5. Visualising coherence
I sometimes picture coherence using the idea of clocks:
- Coherent sources → clocks ticking at the same rate and in sync.
- Incoherent sources → clocks ticking randomly.
When the ticks match, the wave peaks and troughs align perfectly, creating interference.
6. Where coherence appears in practical setups
Coherence is essential in devices and experiments that rely on interference:
- Holography
- Laser interferometers
- YDSE
- Optical communication using lasers
Without coherence, the interference effects needed for these technologies would not work.