The concept of reduced mass (a mutual/center-of-mass frame correction) is used in spectroscopy. When two bodies orbit their common center of mass (e.g., electron and nucleus in hydrogen), the effective mass is the reduced mass \(\mu = \frac{m_1 m_2}{m_1 + m_2}\). This corrects the Rydberg formula for finite nuclear mass, explaining isotope shifts in spectral lines.