Hanleva

Hanleva, a polarised-light dancer, holds a mercury-vapour lantern whose glow loses its polarisation when a weak magnetic field is switched on.

The Physics

Shine polarised light at the mercury resonance line (253.7 nm) into mercury vapour and the atoms re-emit it as polarised resonance fluorescence. Add a weak magnetic field and the polarisation drops. Wilhelm Hanle reported this in “Über magnetische Beeinflussung der Polarisation der Resonanzfluoreszenz” (Z. Phys. 30, 93, 1924). The excited atoms behave like little oscillating dipoles that precess around the field at the Larmor frequency. If they turn through a large angle before they emit, the emitted light comes out with a scrambled direction of polarisation.

ΔB ≈ ħ / (g μ_B τ)

• ΔB: field scale over which the polarisation falls (tesla, T)
• ħ: reduced Planck constant
• g: Landé g-factor of the excited state
• μ_B: Bohr magneton
• τ: lifetime of the excited state (s)

The depolarisation happens when the Larmor precession rate becomes comparable to the decay rate, 1/τ. Measuring the width of this Hanle curve therefore measures the excited-state lifetime, which is how the effect is still used today.

Honest caveat: The line on the sticker is a simplification. The field reduces and rotates the polarisation rather than switching it off, and how much is left depends on the geometry. This is not the Zeeman effect splitting the line: the fields are too weak to resolve any splitting.

On the sticker

Hanleva lifts a glowing mercury lantern that lights her face violet. Printed text: HANLEVA with the line B KILLS POLARISATION. Pixel-art kiss-cut vinyl sticker, white, in 2″, 3″, 4″ and 6″ sizes.