From The Gemology Project
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As explained in nature of light, light travels in transversal waves.

Fig.1: Transversal wave

The peaks of the waves are called crests, the valleys are named throughs.

When white light hits the surface of a transparent material, like a gemstone, some part of the light gets reflected while another part gets refracted inside the gemstone. The refracted ray will eventually reach another boundary and the same process repeats itself.

The part of the light that got refracted in the first run will be partically reflected and leave the gemstone.
Thee two parts (both the origional reflected and refracted/reflected waves) will combine again after leaving the stone, trying to join together).

This can best be explained by a thin film (such as oil) laying on the street (Fig.2).

Fig.2: Thin film

The incident light (I) reaches the oil and part gets reflected (R1) while another part gets refracted inside the oil. The refracted part then reaches the boundary of oil/street and gets reflected out of the oil (R2).
Both R1 and R2 travel parallel to eachother and try to combine.

When R1 and R2 travel with the crests and throughs next to one and other, the two rays will constructively interfere with one and other, creating a more intense colour.

When one ray lags behind the other at a wavelength that is a half, they will destuctively interfere and cancel eachother out. So no color will be seen.

Fig.3: Waves in phase

Here the two waves travel with their crests and throughs at the same wavelength and they will enforce one and other (Fig.3).
As seen in the picture, the result will be a doubling in amplitude, creating a more intense wave. This is what causes the play of colors due to interference.

Note that the wavelength doesn't change.

Fig.4: Waves out of phase

In Fig.4 you see that one ray is lagging behind the other at half a wavelength. The crests of the one are at the same position as the throughs of the other. This is what we call retardation.
Retardation is the lagging of one ray behind the other.

In this case the retardation is half a wavelength, so they cancel eachother out. The result is a wave that has no amplitude.

When two rays lag behind eachother at a full wavelength (or 2 or 3 etc), we say that they are in phase and will constructively interfere, so play of color will be observed.
If the retardation is a halve a wavelength (or 1.5 etc), the waves will cancel eachother out by destructive interference and we say the two rays are out of phase.