Diffraction grating equation

A diffraction grating is a slide containing many equally spaced slits very close together. 

When light is passed through a diffraction grating, it forms an interference pattern composed of light and dark fringes. 

The ray of light passing through the centre of a diffraction grating is called the zero order line, lines either side of the zero order are the first order lines, then the lines outside the two first order lines are the second order lines, and so on as showcased in the diagram below.

The diffraction grating equation is:

dsinθ = nλ

Where d is the distance between the slits (in the diffraction grating), θ is the angle to the normal made by the maximum (light fringe), n is the order and λ is the wavelength.