Experiment to Determine the Wavelength of Monochromatic Light

The wavelength of a spectral line can be measured very accurately using a diffraction grating together with a spectrometer. After the spectrometer's preliminary adjustments are made, the collimator's slit is illuminated by the monochromatic light whose wavelength is to be found, and the telescope is first aligned with the collimator to view the direct image of the slit. The grating is then mounted on the prism table with its plane perpendicular to the incident beam from the collimator. The telescope is turned to one side until the first-order diffracted image of the slit coincides with the vertical cross-wire, and this position is read; the telescope is then turned to the corresponding first-order image on the other side, and that position is read too. The difference between the two readings is $2\theta$; half of it gives $\theta$, the first-order diffraction angle, …