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Laser light is a source of coherent monochromatic light.

Coherence – two waves are coherent if the phase difference between them is constant. For this to be the case they must have the same frequency.

Monochromatic – means having only one wavelength of light present.

Young’s double slit experiment

When laser light passes through a slit it is diffracted. If there are two slits present the light will diffract at both slits.

If a screen is placed on the other side of the slits from the laser an interference pattern is seen. It produces a series of bright and dark fringes.

young's double slit equation

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