Solar radiation is the primary source of power influencing our planet. This energy spans a spectrum from high-energy ultraviolet (UV) light to visible light and lower-energy infrared (IR) radiation. When this radiant energy encounters any object or material on Earth, it must interact with the object’s matter. The fate of this energy determines everything from an object’s temperature to its appearance.
The Three Fates of Incoming Energy
When solar radiation strikes an object, the energy is partitioned into three processes that account for the entire amount of incoming energy. The first is absorption, where the energy is taken into the material itself. The second is reflection, where the energy immediately bounces off the surface and is redirected away. Finally, transmission occurs when the energy passes directly through the object and exits on the opposite side. Every object exhibits a unique combination of these three properties.
Absorption and Conversion to Heat
Absorption involves the transfer of energy from incoming photons to the atoms and molecules of the object. When a photon is absorbed, its energy excites the electrons in the material to a higher energy level. This excited state is unstable, and the energy is quickly released through molecular collisions and vibrations. The absorbed radiant power is transformed into internal energy, predominantly thermal energy, or heat.
This conversion directly increases the average kinetic energy of the object’s molecules, causing a rise in temperature. This explains why objects heat up in the sun. The ability of a material to absorb solar radiation varies significantly across the electromagnetic spectrum. For instance, some materials may absorb visible light readily but allow infrared radiation to pass through, or vice-versa.
Reflection and the Role of Surface Color
Reflection is governed by the object’s surface properties, primarily its color and texture. The measure of an object’s reflectivity is quantified by its albedo, which is the fraction of incoming energy that is reflected.
Surfaces with a high albedo appear bright because they reflect a large percentage of visible light, such as fresh snow, which can reflect up to 90% of incoming energy. Conversely, dark-colored surfaces, like asphalt or dark soil, have a low albedo and reflect less than 10% of the energy, absorbing the remainder.
Smooth, polished surfaces exhibit specular reflection, where the light bounces off in a single direction, like a mirror. In contrast, rough, textured surfaces cause diffuse reflection, scattering the light in many directions.
Transmission Through Different Materials
Transmission is the passage of solar radiation completely through a material, with the energy emerging on the other side. This occurs in transparent or translucent materials like glass, water, or certain plastics. The molecular structure of a material dictates its ability to transmit energy, allowing photons to pass through without being absorbed or reflected.
Clear soda-lime glass is highly transmissive to visible light, allowing approximately 90% to pass through. However, the material’s ability to transmit varies depending on the radiation’s wavelength. While clear glass transmits visible light well, it absorbs much of the incoming UV and infrared radiation. This selective transmission is why some materials are used to filter specific parts of the solar spectrum for applications ranging from greenhouses to optical lenses.