Coherence is where the phase difference between two waves is constant. For this to occur they must share the same frequency.

Monochromatic is where only one wavelength of the light is present.

Laser light is a source of coherent monochromatic light.

Young’s double slit experiment

Laser light which passes through a silt is diffracted by the process. If there are two slits present then the light will be diffracted at both of them.

When a screen is positioned on the other side of the slits from the light then the interference pattern of the laser can be observed. A number of bright and dark fringes are produced. The spacing of these fringes can be calculate using the following equation:

w = ?D / s

In which:

w = fringe spacing in metres (m)
? = wavelength of the light (m)
D = distance between the double slits and the screen (m)
s = slit separation (m)