Diffraction grating lines per MM

The pattern that you get with a large number of slits (a diffraction grating) is similar to the double slit pattern in that there are bright fringes on a dark background, but there are far fewer fringes and the gaps between them are much larger.

Double slit pattern:
(closely spaced bright fringes on a dark background)

Grating pattern:
(widely spaced bright fringes on a dark background)

Common Trick:

In a question on diffraction gratings, it is common that you are told that the grating has, for example, 600 lines per millimetre.

To use this information in the diffraction grating equation, we need to convert it into a slit spacing, d. To do this:

1. Convert it to lines per metre (for instance, 600x103 lines per metre)
2. Invert it. So 600 lines mm-1 becomes a slit spacing,

Diffraction can be used to analyse the light from stars, by separating the different wavelength waves that are emitted with a diffraction grating.

Different elements in stars will emit different frequencies from the electromagnetic spectrum, so we can work which elements are present in each star.