The Stark effect is the shifting and splitting of spectral lines by an external electric field F. The linear Stark effect occurs in hydrogen where degenerate n>1 levels mix under H' = eFz: first-order shifts are ΔE = ∓3eaₒF·n·q where q runs over the parabolic quantum numbers. The quadratic Stark effect (non-degenerate states) gives ΔE = -½αF² where α is the polarizability — atoms are attracted to field maxima. The n=2 hydrogen level splits into 4 sublevels; the n=1 ground state is only quadratically shifted since |1s⟩ has no degenerate partner.