Light can also be considered a stream of particles called photons, each of which contains energy. This concept is called the quantum theory.
So there are two ways to express how much light there is. One is based on energy (in units of watts, joules, or calories, and the other is based on the number of photons.
For example, the wavelength of green light is less than 1 millionth of an inch, and the energy of one photon of green light is equivalent to 1 million billionths of a calorie! Even though photons are particles, they are particles of energy and are different from particles in a cell such as molecules.
