What Is Radiant Energy?
Radiant energy is the form of electromagnetic radiation that can travel through empty space and interact with surfaces. Sunlight is radiant energy, as are infrared waves emitted by the walls of your room or your body. When these waves come into contact with a surface they may be absorbed, transmitted or reflected depending on the nature of that surface and the wavelength of the radiation. Radiant energy can also be transformed into thermal or electrical energy or other forms of energy. For example, sunlight can be converted to thermal energy through solar heating systems or into electricity by photovoltaic cells on a roof. Likewise, hot water or air can be used for radiant heating or as heat transfer fluid for hydronic radiant systems.
Radiation comes in a range of colors and wavelengths, from visible light to radio waves. Visible light is electromagnetic radiation that can be seen, infrared radiation is invisible but feels warm and has the property of conduction, and X-rays are high-energy electromagnetic radiation that can penetrate solid objects, such as bones, to reveal internal structures. The different types of radiant energy have many practical applications, including household uses such as radiant lighting or radiant heating and medical imaging techniques like X-rays.
The word radiant derives from the Latin radians, meaning "rays." It is a term used to describe something that radiates (or sends out rays) in any way, whether bright, cheerful, or attractive. For example, someone who is radiant is showing a great deal of happiness or joy and looks very pleased with herself. This quality is often emphasized in adjectives such as glowing, brilliant, luminous, or lustrous.
In physics, radiant energy is the electromagnetic radiation emitted by all material bodies at a temperature above absolute zero. All material bodies want to achieve thermal equilibrium with their surroundings, and that requires energy in the form of electromagnetic radiation to be absorbed by surrounding matter and transferred into thermal energy or other forms of energy. The amount of radiation emitted by an object can be calculated from its temperature using the Planck constant and the Stefan–Boltzmann law.
Because radiant energy can move through empty space, it is often represented geometrically by rays that represent electromagnetic fields. These rays are often drawn as straight lines through an opaque or semitransparent material, but these lines are mathematical abstractions and cannot reflect the exact shape of a real source. A more accurate measure of radiant energy is irradiance, which includes both the intensity and the spectral distribution of a source's electromagnetic radiation. This can be derived from a radiant flux or from the product of an elemental area (dso) and a cosine of the solid angle,