Christian Christiansen (9 October 1843 in Lønborg, Denmark – 28 November 1917 Frederiksberg) was a Danish physicist. Christiansen first taught at the local polytechnical school. In 1886, he was appointed to a chair for physics at the University of Copenhagen. He mainly studied radiant heat and optical dispersion, discovering the Christiansen effect (Christiansen filter). Around 1917, he discovered the anomalous dispersion of numerous dyes, including aniline red (fuchsine), by recording absorption spectra. In 1884, he confirmed the Stefan–Boltzmann law. Christiansen was elected a member of the Royal Swedish Academy of Sciences in 1902. He was doctoral advisor to Niels Bohr. In 1912, he retired and Martin Knudsen became professor.