Spectral study of Na source with a smartphone and a diffraction grating

A home-made spectrometer, based on the use of inexpensive diffraction transmission gratings coupled with a smartphone camera, can be assembled and employed to obtain quantitative spectra of different sources with surprisingly high accuracy to the point of being able to observe the separation of the Na-doublet (the wavelength difference between the two emission D-lines of the sodium spectrum).We describe the set up and data analysis required to acquire the emission spectra of a low-pressure sodium discharge lamp and perform such measurement. The experiment addresses a relevant and engaging topic in physics, namely optical spectroscopy, and is suitable for the undergraduate laboratory and for working with high school students.