Many aas diagrams show a diffraction grating and monochromator but wuts exactly the purpose of it? (1 Viewer)

[VictorTango-9W]

I believe the diffraction grating splits up the light into separate wavelengths but the thing is the cathode lamp itself passed light of a specific wavelength. Like the lamp shines a specific wavelength of light only which matches the energy difference So yh wuts the purpose of it can't we just let the light pass through the atomiser sample and chuck the resulting light into a photometer to measure intensity?

 

[Vall]

The light has been emitted (from the cathode lamp) then passes the sample (in the burner/atomiser) then it enters the diffraction grating monochromator. The monochromator selects only one wavelength to analyse and is needed because the burner (being fire) adds more wavelengths to the ones emitted by the cathode lamp. From my teacher's ppt: