Hi In the Diffraction Handbook http://gratings.newport.com/library/handbook/chapter6.asp#6.2.4 it says: A grating used in the Littrow or autocollimating configuration diffracts light of wavelength λ back along the incident light direction (Figure 6-4). In a Littrow monochromator, the spectrum is scanned by rotating the grating; this reorients the grating normal, so the angles of incidence a and diffraction b change (even though a = b for all λ). I don't understand what they mean by "even though a = b for all λ". Does anybody know, who wants to clarify this for me? Best, Niles.