Starka

Starka, an armoured knight, holds two charged plates apart, and a red spectral line between them splits into a fan of lines.

The Physics

Put atoms in a strong electric field and their spectral lines split and shift. Johannes Stark observed this in hydrogen in 1913 and published it in full in Annalen der Physik (“Beobachtungen über den Effekt des elektrischen Feldes auf Spektrallinien”, Ann. Phys. 348, 965, 1914). Antonino Lo Surdo saw it independently the same year. Hydrogen is special: states with the same n but different shapes have the same energy, so the field mixes them and the shift is proportional to the field itself, giving a symmetric pattern of components.

ΔE = (3/2) n (n₁ − n₂) e a₀ F

• ΔE: energy shift of the level (J)
• n: principal quantum number
• n₁, n₂: parabolic quantum numbers
• e: elementary charge (C)
• a₀: Bohr radius (m)
• F: electric field strength (V/m)

Stark received the 1919 Nobel Prize in Physics, partly for this discovery. The linear splitting was one of the early successes of the quantum theory of the atom.

Honest caveat: The linear shift is for hydrogen, to first order. Most other atoms have no such degeneracy and shift quadratically, ΔE ∝ F², and even hydrogen picks up quadratic terms in strong fields.

On the sticker

Starka grips two round plates as a red beam between them fans out into several lines. Printed text: STARKA with the line ΔE ∝ F (LINEAR, H). Pixel-art kiss-cut vinyl sticker, white, in 2″, 3″, 4″ and 6″ sizes.