Electromagnetic radiation is a form of energy derived from the sun or produced artiﬁcially. Radiation can be characterized in two ways and although this is confusing at ﬁrst glance, it does have speciﬁc usefulness. The wave description views radiation as a continuous wave propagated through space having a frequency in time and length. The wavelength is the distance between two corresponding points on successive waves. The wave theory of radiation is most useful for understanding optics such as how radiation is reﬂected or refracted. The particle description views radiation as discrete packages of energy called photons. This theory is most suitable for explaining photobiologic phenomena such as absorption of radiation by molecules.