Lanthanum was discovered in 1839 by the Swedish chemist Carl Gustaf Mosander, who found it concealed within a sample of what had been taken for pure cerium. Its name comes from the Greek lanthanein, to lie hidden, aptly chosen for an element that had hidden inside another. Lanthanum is a soft, silvery, highly reactive metal that heads the lanthanide series, and it is used in camera lenses, hydrogen-storage alloys and catalysts. Among the more abundant of the rare earths despite its name, lanthanum goes into the mischmetal of lighter flints, into high-refractive-index optical glass, and, historically, into the nickel-metal-hydride batteries of early hybrid cars.
Lanthanum halide crystals, particularly lanthanum bromide doped with cerium, are used in some of the most sensitive modern scintillation detectors for gamma radiation, valued for their unusually sharp energy resolution, and such detectors are now deployed in radiation-portal monitors at ports and border crossings screening cargo for smuggled nuclear or radiological material, a distinctly modern security role for an element first noticed as a stubborn contaminant in someone else's cerium. Lanthanum oxide is also blended into the glass of high-refractive-index camera and telescope lenses, since it raises the glass's bending power without the excessive colour dispersion that heavier lead-based optical glasses introduce.