reflection grating

hi,

I am bothered by some question for a long time.
From basic optics, i know that the the angle of incidence equals the angle of reflection [where we define the incident angle as the angle between the beam and the normal to the surface of course].

What I can't understand is why when a polychromatic beam reflects from the surface of a reflection grating [for example, with a periodic structure] the reflection angle of the beam depends on the wavelength? Why the reflection angle should not be the same for all the wavelengths that compose the polychromatic beam and equal the incident angle [which is, in my opinion, the same for the different wavelengths that compose the beam]??

I hope my question is clear.