Was ist Licht?

2015 the United nations proclaimed the “International Year of Light and Light‐Based Technologies”. Light is based on photons. First, therefore, Part 1 is concerned with the current state of knowledge on photons, which are the energy quanta of the electromagnetic radiation and have both wave and particle characteristics. The path of the photons from the sun as a natural source of radiation to the Earth's surface is then treated. But also artificial radiation sources, which are important for our visual perception of photons such as the inefficient incandescent lamp and energy‐saving inorganic and organic light‐emitting diode, are discussed. Laser as an artificial radiation source with technical importance are also addressed. In Part 2 the focus will be on applications in photocatalysis, photochemistry and inorganic and organic photovoltaic solar cells.