Light is that part of the spectrum of electromagnetic radiation that the human eye can see.
It lies between about 400 and 700 nanometers. All the units for measuring and defining light are based on the candela, which is the unit defining the luminous intensity from a small source, in a particular direction. This unit was originally based on the light emission from a flame. The standard later came to be defined as the glow from molten platinum. The current definition is a radical departure from the previous formulations, because it defines light intensity in terms of the unit for radiated power in general, the watt, or joule per second.