Photons emitted from the surface of the Sun need to travel across the vacuum of space to reach our eyes. The transfer of energy from the sun across nearly empty space (vacuum) is accomplished primarily by radiation. Radiation is the transfer of energy by electromagnetic wave motion. Once the sun’s energy reaches earth, it is intercepted first by the atmosphere. A small part of the sun’s energy is directly absorbed, particularly by certain gases such as ozone and water vapor. Some of the sun’s energy is reflected back to space by clouds and the earth’s surface.