Interference

Animation: interference of waves from 2 point sources. Crests blue, troughs red/yellow.

For two coherent sources, the spatial separation between sources is half the wavelength times the number of nodal lines. The principle of superposition of waves states that the resultant displacement at a point is equal to the vector sum of the displacements of different waves at that point. If a crest of a wave meets a crest of another wave at the same point then the crests interfere constructively and the resultant crest wave amplitude is increased; similarly troughs make a trough of increased amplitude. If a crest of a wave meets a trough of another wave then they interfere destructively, and the overall amplitude is decreased.
This form of interference can occur whenever a wave can propagate from a source to a destination by two or more paths of different lengths. Two or more sources can only be used to produce interference when there is a fixed phase relation between them, but in this case the interference generated is the same as with a single source; see Huygens' principle.
 Light from any source can be used to obtain interference patterns, for example, Newton's rings can be produced with sunlight and the colours which can be seen when sunlight is reflected in a soap-bubble are white light fringes.