In the following section, we will give an introduction to the sun as a source of energy, and some of the mechanics behind sunlight. In fact, understanding the solar radiation spectrum and how this radiation makes contact with the earth’s surface is a complicated issue in and of itself. Appreciating the details of how we determine the sun's light spectrum and intensity is very important for later understanding how light interacts with solar cells. Furthermore, once the properties of the sun and sunlight are understood, it becomes much easier to understand why creating efficient solar cells is a very challenging undertaking. The organization of this section (and subsections) was inspired by PVEducation.org.