Coherent and incoherent waves

Coherent waves (outer pattern).

Ripples from a single impact on a pond are coherent - the wave trains are regular. High quality lasers have coherence lengths of many meters. Laser pointers are much less coherent. Sunlight has a coherence length of less than a mm. Light from any distant point source (slit) that passes through two closely spaced slits is coherent with respect to itself. Young demonstrated interference when he interrupted a narrow beam of sunlight that had passed through a single narrow slit.

Light from two hot filaments is incoherent: no interference takes place. The wave patterns are similar to the expanding patterns formed by throwing a handful of sand into a pond.

Note: Young's demonstration of interference is more clearly seen when repeated with a laser pointer. 

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