Interference

Interference is a special case of superposition where the waves that combine are coherent. The waves overlap and form a repeating interference pattern of maxima and minima areas. If the waves weren’t coherent the interference pattern would change rapidly and continuously.

Coherence: Waves which are of the same frequency, wavelength, polarisation and amplitude and in a constant phase relationship. A laser is a coherent source but a light bulb is not.

Constructive Interference: The path difference between the waves is a whole number of wavelengths so the waves arrive in phase adding together to give a large wave. 2 peaks overlap

Destructive Interference: The path difference between the waves is a half number of wavelengths so the waves arrive out of phase cancelling out to give no wave at all. A peak and trough overlap