since θ_i is a constant in the Littman-Metcalf setup. The light that is exactly perpendicular to the reflecting mirror in the Littman-Metcalf setup will be the light that goes back into the diode, providing the optical feedback. Thus, as the angle of this reflecting mirror is changed, light diffracted at a different angle is sent back into the diode. The change in θ_m for the light fed back into the diode should then be equal to the change in the angle of the reflecting mirror. A precision mirror mount will have a known angular change per rotation of a set screw. Since we can then determine ∆θ_m, and ∆ λ can be found using the spectrometer, the change in wavelength can be measured for a change in θ_m.