an interference pattern which has primary intensity maxima at angles 
from the straight-through position given by
is: an optical device consisting of a flat plate with a series of equally spaced, parallel slits on its surface. The distance between the slits is usually a few wavelengths of the radiation involved, and is called the grating spacing. The plate may be transparent (a transmission grating) or reflecting (a reflection grating) and the slits may have been produced by ruling them with an appropriate machine (ruled grating), or by taking a cast of an existing ruled grating (replica grating). [P6.1]
produces: when illuminated by normally incident monochromatic light of wavelength an interference pattern which has primary intensity maxima at angles from the straight-through position given by
where 
is the order of diffraction and 
is the grating spacing. [P5.5, P6.1]
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