Johannes Stark won the 1919 Nobel Prize in Physics for his discovery of the Doppler effect in canal rays and the splitting of spectral lines in electric fields.
Canal rays are streams of positive ions that travel through perforations, or channels, in a cathode-ray tube. Stark observed that the light emitted by these rapidly moving ions showed a Doppler shift, analogous to the change in pitch heard from a moving sound source. This provided evidence about the motion and properties of charged particles.
Stark also discovered the Stark effect: spectral lines can split or shift when atoms are placed in an electric field. This phenomenon became important in spectroscopy and helped scientists investigate atomic structure and energy levels.
Stark's Nobel citation emphasized both discoveries. His name is sometimes confused with Pieter Zeeman, whose related work concerned the splitting of spectral lines in magnetic fields and earned Zeeman the 1902 Physics Nobel. The Stark effect is specifically associated with electric fields.