Why are coherent sources essential for interference?

Coherent sources are essential for interference because they maintain a constant phase difference, enabling constructive and destructive interference.

Interference is a fundamental concept in physics, describing the process where two or more waves superpose to form a resultant wave of greater, lower, or the same amplitude. This phenomenon can only occur when the waves come from coherent sources. Coherence, in this context, refers to the property of waves that enables them to maintain a constant phase difference. This is crucial for interference to occur because it is the phase difference between waves that determines whether they will interfere constructively (adding together to form a wave of greater amplitude) or destructively (cancelling each other out to form a wave of lower amplitude).

The requirement for coherence can be understood by considering the nature of waves. Waves are characterised by their amplitude, frequency, wavelength, and phase. If two waves have the same frequency and wavelength but different phases, they can still interfere because the phase difference is constant. However, if the phase difference is not constant, the waves will not interfere consistently. Sometimes they might interfere constructively, and at other times destructively, leading to an inconsistent and unpredictable pattern of interference.

In practical terms, creating coherent sources can be challenging. For light waves, lasers are often used because they emit light that is not only monochromatic (of a single wavelength) but also coherent. For sound waves, tuning forks or speakers driven by the same signal can be used as coherent sources. In each case, the key is to ensure that the sources are emitting waves with a constant phase difference.

In conclusion, coherent sources are essential for interference because they ensure a constant phase difference between the waves they emit. This allows for consistent and predictable patterns of interference, whether constructive or destructive. Without coherence, the interference pattern would be inconsistent and unpredictable, making it impossible to study or use in practical applications.