Sunlight

Figure 1. Sunlight is a crucial part of life on Earth.

Sunlight, also known as solar radiation, refers to the incoming light to the Earth that originated from the Sun. This light represents a portion of the electromagnetic spectrum that includes infrared, visible, and ultraviolet light. About half of the radiation is in the visible portion of the spectrum, with most of the rest in the near-infrared section with some in the ultraviolet spectrum. Sunlight that is incident upon the Earth's surface has been filtered through the atmosphere, with some of the ultraviolet radiation being absorbed. The radiation that isn't absorbed by the atmosphere can produce a suntan or sunburn.

In addition to providing light for Earth, sunlight also acts as a source of radiant heat, warming the Earth. The contribution of sunlight to the Earth is immense, it is a major component of the greenhouse effect, is one of Earth's major energy flows, as well as being a key factor in photosynthesis. The energy stored in fossil fuels also originates from the Sun, since fossil fuels originate from dead organisms such as algae or plankton.

Although a significant amount of solar energy reaches the Earth as sunlight, some of the incident light is reflected by the atmosphere or dispersed throughout space. Through calculations determining the solar energy to the Earth, it can be concluded that the average value for the amount of energy hitting the surface of the Earth is approximately . Additionally, the light from the Sun does not instantaneously reach the Earth, instead photons originating from the Sun must travel a significant distance to the Earth. Because of this, it takes sunlight 500 seconds, or 8 minutes and 20 seconds to reach the Earth.