The sun emits radiation, which is how energy travels from the star to our planet. Solar radiation is a form of electromagnetic energy that moves outward from a source in a wave-like manner. This energy transfer mechanism drives nearly all weather, climate, and biological processes on Earth.
Understanding Energy and Radiation
Energy moves between objects and environments in three distinct ways: conduction, convection, and radiation. Conduction involves the transfer of heat through direct contact between molecules, like the warmth felt when touching a hot stove. Convection describes heat transfer through the movement of fluids, such as the circulation of warm air rising and cool air sinking in a room.
The immense distance between the sun and Earth is largely a vacuum, meaning conduction or convection cannot occur. Radiation is the only mechanism that allows energy to bridge the vast, empty expanse of space. This energy travels as electromagnetic waves, which are self-propagating waves of electric and magnetic fields.
Electromagnetic radiation travels at the speed of light and does not require any substance for its transmission. This allows the sun’s energy, generated by nuclear fusion, to reach us nearly 150 million kilometers away. The energy is released as photons, which are discrete packets of energy that make up these electromagnetic waves.
The Components of Solar Energy
Solar energy is not composed of a single type of light but is a broad spectrum of electromagnetic waves classified by their wavelength. The sun’s output is conventionally divided into three components: ultraviolet (UV), visible light, and infrared (IR) radiation. The wavelength determines the energy level, with shorter wavelengths carrying more energy per photon.
Ultraviolet radiation has the shortest wavelengths, generally ranging from 100 to 400 nanometers, and therefore carries the highest energy. Visible light occupies a narrow band from about 400 to 700 nanometers, representing the portion of the spectrum detectable by the human eye. Infrared radiation has the longest wavelengths, starting at 700 nanometers, and is primarily responsible for the heat we feel.
When solar energy reaches the Earth’s surface, the distribution is far from uniform across these three bands. Infrared radiation accounts for the largest fraction of the total energy, typically between 49 and 55 percent of the solar power reaching the surface. Visible light makes up the next largest portion, contributing approximately 42 to 47 percent of the energy. Ultraviolet radiation constitutes the smallest portion of the energy at the surface, usually only about 3 to 8 percent.
Earth’s Atmospheric Filter
Before solar radiation reaches the surface, Earth’s atmosphere acts as a complex filter, absorbing, reflecting, and scattering the incoming energy. The stratospheric ozone layer, located 15 to 35 kilometers above the surface, is a particularly effective shield. It absorbs nearly all of the highest-energy UV-C radiation and most of the UV-B radiation, preventing these damaging wavelengths from reaching life on the ground.
Incoming visible light is also affected by the atmosphere through a process called Rayleigh scattering. This process occurs when sunlight strikes air molecules, which are much smaller than the light’s wavelength. Shorter wavelengths, such as blue and violet light, scatter much more effectively than longer wavelengths, which is why the sky appears blue.
Larger particles, such as water droplets in clouds, cause non-selective scattering, which scatters all visible wavelengths equally and makes clouds appear white or gray. Greenhouse gases, including water vapor and carbon dioxide, primarily affect thermal energy after it is absorbed and re-emitted by the Earth’s surface as longwave infrared radiation. These gases absorb this outgoing heat, trapping it in the lower atmosphere and maintaining the natural greenhouse effect.