The joule is the SI unit of heat energy. The quantity of heat required to raise the temperature of one gram of a substance one degree Celsius is called the specific heat capacity or more commonly, the specific heat . If the change occurs when energy is added the change is said to be endothermic , i.e. ice melting. If energy is given off as the process occurs the change is said to be endothermic , i.e. water freezing. Light is another form of energy.
Light travels in waves.
Each wavelength is associated with a particular energy. The longer the wavelength the lower the energy. The light we see around us is a very small portion of the electromagnetic spectrum . See figure 2.5 page 43. In other words, the amount of energy at the end of a reaction or process must be equal to the total amount of energy at the beginning.