"Light, the visible portion of electromagnetic radiation emitted by the sun (among other stars). Following on from this, every radiation whose wavelength or frequency falls within the electromagnetic spectrum." - Brockhaus Encyclopedia
In works of art, light should chiefly illuminate the viewers and serve aesthetic purposes. Neon light, reflectors, light bulbs, and mirrors are employed by artists to let works and rooms shine. In painting, too, light has always been used in the form of fire or candles by artists such as Rembrandt to create moods.
However, one only began to speak of light art (Lichtkunst) in the 19th century, as there were no artificial light sources before.