Infrared (IR) radiation, a segment of the electromagnetic spectrum, exists beyond what human eyes can perceive. It is often associated with heat, as many objects emit IR radiation as a function of their temperature. Materials interact with this radiation in various ways, with absorption being a fundamental process. Understanding how different substances absorb IR light is important for explaining many natural phenomena and for developing a range of technologies.
How Materials Absorb IR
Infrared radiation absorption occurs at the molecular level when the energy of IR photons matches the natural vibrational frequencies of a molecule’s bonds. Atoms within molecules are not static; they constantly vibrate, stretching and bending their chemical bonds. When a molecule encounters IR radiation with a frequency that corresponds to one of these vibrational modes, it absorbs the energy. This absorption increases the amplitude of the molecule’s vibrations, gaining energy.
For a molecule to absorb IR radiation, its vibration must cause a temporary change in its dipole moment, which describes the distribution of electrical charge within the molecule. Even if a molecule does not have a permanent dipole moment, certain vibrations can create a temporary one, allowing it to interact with the electric field of the IR radiation. This principle explains why some molecules readily absorb IR while others, like symmetrical diatomic gases such as nitrogen and oxygen, do not. The specific frequencies of IR radiation absorbed depend on factors like the mass of the atoms, the strength of the bonds, and the overall molecular structure.
Common IR Absorbers
Many common materials absorb infrared radiation, ranging from gases in the atmosphere to liquids and various solids. Among atmospheric gases, water vapor, carbon dioxide, and methane are significant IR absorbers. Water vapor is a primary absorber, responsible for a large portion of atmospheric absorption. Carbon dioxide also strongly absorbs IR, especially at a wavelength of 15 micrometers. Methane and other trace gases like nitrous oxide and chlorofluorocarbons (CFCs) also contribute to IR absorption in the atmosphere.
Water in its liquid form is another strong absorber of IR radiation. Liquid water exhibits several prominent absorption bands across the near-infrared spectrum.
Solid materials also demonstrate varying degrees of IR absorption. Dark-colored objects, such as black paint or fabrics, absorb a high percentage of IR light, making them feel warm when exposed to sunlight. Other everyday solids like glass, plastics, wood, brick, stone, asphalt, and paper can absorb IR radiation. Certain polymers are specifically engineered to have high IR absorption capacities, finding use in applications like thermoforming and welding. Even some metals like gold, manganese, and copper absorb IR well.
IR Absorption in Daily Life
The absorption of infrared radiation plays a role in numerous everyday experiences and technological applications. A common example is the warming sensation felt when sunlight hits dark surfaces. Dark objects absorb more of the sun’s visible light and also its infrared components, converting this absorbed energy into heat. This principle is also evident in how a car’s dark paint absorbs IR, contributing to the interior warming when parked in the sun.
Atmospheric gases that absorb IR radiation influence Earth’s temperature. Water vapor, carbon dioxide, and methane absorb outgoing infrared radiation emitted from Earth’s surface. These gases then re-emit the absorbed energy in all directions, including back toward the Earth, which contributes to warming the planet.
Infrared absorption is also fundamental to the operation of remote controls for electronic devices. When a button is pressed on a remote, an internal light-emitting diode (LED) sends out pulses of invisible infrared light. These light pulses carry specific binary codes that are then absorbed and decoded by a sensor in the television or other device, initiating the desired command.
Thermal imaging devices rely on the fact that objects emit and absorb IR radiation based on their temperature. These cameras detect the variations in IR radiation, allowing them to create images that represent temperature differences rather than visible light. Such technology is used in diverse fields, from night vision to building inspections and medical diagnostics.