
The Rydberg formula underlies all atomic spectroscopy. With its help, elements are identified in stars and the interstellar medium: the position of hydrogen lines in a galaxy's spectrum indicates its redshift and distance. In laboratory plasma, hydrogen lines are used to determine temperature and density. Even ordinary fluorescent lamps and neon signs glow due to similar electron transitions, only in more complex atoms. For hydrogen-like ions (He⁺, Li²⁺), the formula is slightly modified by replacing R_H with R_M = R_∞ / (1 + m_e/M), but the structure remains the same.