Materials absorb and emit infrared (IR) radiation based on their molecular structure, temperature, and surface properties. The process is governed mainly by how molecules vibrate and interact with electromagnetic waves in the IR range.
1. Absorption of Infrared Radiation
- Molecular Vibrations – IR radiation corresponds to the energy needed for molecules to stretch, bend, or twist their chemical bonds.
- Selective Absorption – Different materials absorb IR at specific wavelengths depending on their molecular bonds (e.g., water strongly absorbs IR around 3 μm and 6 μm).
- Surface Color and Texture – Dark, matte surfaces generally absorb more IR than shiny, reflective ones. Metals, for example, reflect most IR, while non-metals (wood, cloth) absorb more.
- Greenhouse Gases – Molecules like CO₂, CH₄, and H₂O vapor absorb IR strongly, which is why they trap heat in the atmosphere.
2. Emission of Infrared Radiation
- Thermal Emission – Any object above absolute zero (0 K) emits IR radiation due to the motion of its atoms and molecules.
- Blackbody Radiation – Ideal “blackbodies” emit IR according to their temperature (Wien’s Law shows hotter objects emit shorter wavelengths).
- Material Dependence – Emissivity (0 to 1) measures how well a material emits IR.
- High emissivity: human skin (~0.98), soil, cloth.
- Low emissivity: polished metals, mirrors.
- Temperature Relationship – The hotter an object, the more IR it emits, and at higher intensity.
3. Everyday Examples
- A hot stovetop glows in visible red but emits most of its energy in the IR range.
- Infrared cameras detect the IR emitted by warm bodies and surfaces.
- The Earth absorbs visible sunlight and re-emits much of that energy as IR, which greenhouse gases then partially trap.