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Diffraction happens when a wave hits an obstacle or gap, diffraction is greatest when the gap is about the same size as the wavelength of the wave. The waves bend round the object or spread out when they pass through the gap, this is called diffraction.

diffraction of a wave

 Single Slit Diffraction

When monochromatic laser light is shone through a narrow single slit a diffraction pattern is produced consisting of light and dark fringes. It produces a wide central bright fringe. The other bright fringes get dimmer as you move away from the centre.

Single slit intensity graph

Diffraction grating.

A diffraction grating is a piece of glass with lots of closely spaced parallel lines on it each of which allows light to pass through it, this is a transmission diffraction grating.

diffraction grating drawing

Diffraction gratings are used in spectrometers. The diffraction grating splits up the light into a spectra.

diffraction grating diagram

diffraction grating equation

d = grating spacing in metres (m)
J = angle of diffraction
n = order number
l = wavelength on the light in metres (m)

This equation can be rearranged to

diffraction grating equation

If there are N lines per metre on a diffraction grating then d can be calculated using

Diffraction gratings are used in the spectral analysis of light from stars.