A diffraction grating is a plate with closely spaced parallel slits ruled on it.A parallel beam of monochromatic light directed normally.Light passing through each slit is diffracted.Diffracted waves from each slit reinforce in certain directions and cancel out in all other directions.The zero Order Beam is the central beam.It is in the same direction as incident beam.Each spot on either side is a higher order spectrum n=1,2,3 etc.The angle of diffraction increases if light of a longer wavelength is usedor a grating with closer slits is used.The wavelength can be calculated using

λ = s sin θ /n

where s is the slit spacing (the distance between the centres of adjacent slits) and θ is the angle between the zero order maximum and the nth order maximum measured from the centre of the grating.