The calibration of the absolute wavelength is subject to
several potential sources of error. These include error in
alignment of the rotating arm, error in alignment of the grating,
and deviations between the grating line density (1000 lines/
mm) specified by the manufacturer and actual line density. To
check this, it is convenient to exploit the fact that light that
appears orange to the eye spans a very narrow wavelength
range near 589 nm and should be diffracted at θ = 36°