The splitting of single spectral lines of an emission or absorption spectrum of a substance into several components when the substance is placed in an electric field. The effect occurs when several electron orbitals in the same shell, which normally have the same energy level, have different energies due to their different orientations in the electric field. Quantum mechanical predictions of this effect are extremely accurate, a fact that provided compelling early evidence for quantum mechanics. The Stark effect is named after its discoverer, German physicist Johannes Stark (1874-1957). Compare Zeeman effect.