Light is not considered matter; it is a type of energy. Both matter and energy can change forms. Matter describes things that have mass, while energy describes a transferable property of objects.
The conservation laws of energy state that energy can neither be created nor destroyed. Forms of energy include kinetic, potential, mechanical, gravitational and thermal, among others. Radiant energy describes electromagnetic radiation or light. Although energy may appear massless, it actually exhibits slight mass according to the mass-energy equivalence.
The joule is the international standard unit used to measure energy. Other units of measurement include calories, kilowatt-hours and British thermal units. The watt is the measurement of the number of joules per second.