Wilhelm Carl Werner Otto Fritz Franz Wien (German: [ˈvɪlhɛlm ˈviːn] ⓘ; 13 January 1864 – 30 August 1928) was a German physicist who, in 1893, used theories about heat and electromagnetism to deduce Wien's displacement law, which calculates the emission of a blackbody at any temperature from the emission at any one reference temperature.

His arguments were based on the notion of adiabatic invariance, and were instrumental for the formulation of quantum mechanics.

Wien received the Nobel Prize in Physics in 1911 for his work on heat radiation.

Wien was very active in science politics representing conservative and nationalistic positions though being not as extreme as sharing the attitude of those going to develop the "Deutsche Physik".

In 1900 (following the work of George Frederick Charles Searle), he assumed that the entire mass of matter is of electromagnetic origin and proposed the formula

It is a device consisting of perpendicular electric and magnetic fields that can be used as a velocity filter for charged particles, for example in electron microscopes and spectrometers.

It can be configured as a charged particle energy analyzer, monochromator, or mass spectrometer.

While studying streams of ionized gas, Wien, in 1898, identified a positive particle equal in mass to the hydrogen atom.

In 1911, Wien was awarded the Nobel Prize in Physics "for his discoveries regarding the laws governing the radiation of heat".