Ultraviolet

This discovery revealed that the spectrum of light extends beyond the visible violet end, where darkening paper soaked in silver nitrate suggested a new type of radiation termed 'ultraviolet' because it is beyond the visible spectrum. The discovery showcased the existence of light that is not visible to the naked eye but has measurable effects.
 Johann Wilhelm Ritter Germany 1801 Ultraviolet

This discovery revealed that the spectrum of light extends beyond the visible violet end, where darkening paper soaked in silver nitrate suggested a new type of radiation termed 'ultraviolet' because it is beyond the visible spectrum. The discovery showcased the existence of light that is not visible to the naked eye but has measurable effects.