When a wave hits a gap or obstacle it diffracts. If the gap is approximately the same size as the wavelength of the wave then the diffraction is at its maximum.

The light will either bend around the obstacle or spread out through the gap.

Single slit diffraction

If monochromatic laser light is shone through a single, narrow slit then the diffraction pattern produced is composed of both light and dark fringes: the central fringe is large and bright while those located on either side of this fringe become dimmer the further from it they are.

Diffraction grating

A diffraction grating consists of numerous closely spaced parallel lines in a piece of glass through which light is able to pass.

Transmission diffraction gratings are used in spectrometers to analysis with light transmitted from stars. The light is split up into a spectra by the gaps.

dsin? = n?

In which:

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

It is possible to rearrange the equation:

? = dsin? / n

It is also possible to calculate the grating spacing (d) as follows:

d=1/N