The Hanle effect is a reduction in the polarization of light when the atoms emitting the light are subject to a magnetic field in a particular direction, and when they have themselves been excited by polarized light.
It is named after Wilhelm Hanle, who first described it in Zeitschrift für Physik in 1924. Attempts to understand the phenomenon were important in the subsequent development of quantum physics. 
- Observation of the Hanle effect on the light emitted by the sun is used to indirectly measure the magnetic fields within the sun, see:
- Hanle, Wilhelm (1924-12-01). "Über magnetische Beeinflussung der Polarisation der Resonanzfluoreszenz". Zeitschrift für Physik (in German). 30 (1): 93–105. Bibcode:1924ZPhy...30...93H. doi:10.1007/bf01331827. ISSN 0044-3328.
- J Alnis; K Blushs; M Auzinsh; S Kennedy; N Shafer-Ray; E R I Abraham (2003). "The Hanle effect and level crossing spectroscopy in Rb vapour under strong laser excitation" (PDF). Journal of Physics B. 36: 1161–1173. Bibcode:2003JPhB...36.1161A. doi:10.1088/0953-4075/36/6/307.
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