When waves come together they can interfere constructively or destructively. To set up a stable and clear interference pattern, two conditions must be met:

1. The sources of the waves must be coherent, which means they emit identical waves with a constant phase difference.
   
   
2. The waves should be monochromatic - they should be of a single wavelength.

Let's say we have two sources sending out identical waves in phase. Whether constructive or destructive interference occurs at a point near the sources depends on the path-length difference, d, which is the distance from the point to one source minus the distance from the point to the other source.

Condition for constructive interference: d = ml, where m is any integer.

Condition for destructive interference: d = (m + 1/2) l

The first person to observe the interference of light was Thomas Young in 1801. He used sunlight passing through two closely spaced slits. It was a difficult experiment since his source was not even close to being monochromatic, but Young's double-slit experiment provided the first indisputable evidence of light acting as a wave.

Note that you can't use two light sources (two lasers, two light bulbs, two candles, etc.) because they each experience random changes in phase about once every 10^-8 s.