The document discusses the working principles of a flame photometer, which uses the principle of atomic emission spectroscopy to analyze the concentration of sodium and potassium in solutions. It works by nebulizing a sample into a flame, where atoms are excited and emit light at characteristic wavelengths. The light is measured with a photomultiplier tube and the intensity is directly proportional to the concentration of the element. The flame photometer provides rapid, accurate analysis of sodium and potassium but has limitations with interferences from other elements in solution.