When light is absorbed, its energy is transferred to the atoms and molecules of the material it is being absorbed by. The fate of the absorbed light depends on the nature of the material it is interacting with.
How Light Absorption Works
Light is a form of electromagnetic radiation that can be described as both a wave and a particle (photon). Photons interact with matter by being absorbed, reflected, scattered or transmitted through it. Absorption occurs when the energy and momentum of a photon is transferred to an electron in an atom or molecule. This causes the electron to move to a higher energy state or orbital around the atom’s nucleus.
There are several ways absorption can occur:
- An electron can move to a higher energy orbital after absorbing a single photon. This is common in individual atoms.
- In molecules, the energy can excite vibrational modes of the molecular bonds, resulting in transformations between rotational and vibrational energy states.
- In solids, photons can excite electrons from the valence band to the conduction band, creating mobile electron-hole pairs.
- Photons can interact with multiple electrons or pieces of matter simultaneously through various cooperative mechanisms.
The characteristic energies and states available depend on the material’s electronic structure. The probabilities of different transitions occurring are quantified by the material’s absorption spectrum.
Fate of Absorbed Visible Light
When visible light is absorbed by an object, several processes could occur to the energy it transfers:
- The energy can be changed into heat through vibrational excitation and collisions. This heat warms the material up.
- The object may re-emit lower energy light through fluorescence or phosphorescence.
- In photochemical reactions, the electronic excitation can drive chemical changes in the absorbing molecules.
- In photosynthetic organisms, the energy is used to drive biochemical synthesis reactions.
- In photovoltaics, the electron excitations create mobile charge carriers that generate electric currents.
- In photodetectors and sensors, absorption leads to measurable electrical signals.
Which process occurs depends on the detailed electronic structure and properties of the material. But in most ordinary objects, visible light absorption simply turns the photon energy into heat.
Thermal Effects of Infrared Absorption
Infrared light has longer wavelengths and lower photon energies than visible light. When infrared is absorbed by materials, the energy excitations are usually vibrations and rotations in molecules, or lattice vibrations in crystalline solids.
These excitations randomize into heat through collisions and interactions between molecules. Thus infrared absorption almost always manifests as heating of the material. Some examples include:
- Infrared lamps heating surfaces through strong infrared absorption.
- Greenhouse gases like CO2 and methane absorbing infrared radiation emitted by the Earth, causing the greenhouse effect and global warming.
- Materials absorbing ambient infrared in the environment, reaching equilibrium at the surrounding temperature.
Since different chemical bonds have characteristic energies, infrared absorption spectra can be used to identify materials through their molecular vibrations.
High Energy Photons and Ionization
Photons with high enough energies (ultravoilet, x-ray, gamma ray) can eject electrons completely from atoms or molecules via the photoelectric effect and Compton scattering. This ionizes the material by creating free electron-positive ion pairs.
In partially ionized plasmas, visible and infrared photons can also free electrons that recombine later. The fate of absorbed high energy photons is usually:
- Emission of secondary electrons that propagate through and interact with the material.
- Induction of nuclear transitions and radioactive decay in some cases.
- Heating and weak light emission through relaxation and recombination processes.
- Ejection of electrons (photoelectrons) from surfaces that are detected in spectroscopy.
High energy photons thus generally damage materials, but can also induce useful effects like sterilization, polymer curing, and radiography.
Photon Fate in Light Sensitive Biology
Special biological molecules like retinal and chlorophyll are optimized to absorb visible light. Here are some examples of how absorbed photon energy is converted into biological functions:
- Rhodopsin in the eye contains retinal chromophores that trigger nerve signals when excited, enabling vision.
- Chlorophyll in plants uses photon energy to power synthesis of sugars from carbon dioxide through photosynthesis.
- Cryptochromes and photoreceptors synchronize circadian rhythms to light/dark cycles.
- Light sensitive proteins initiate processes like DNA repair, cell movement, coloring, etc.
Beyond powering essential functions, excess absorbed light can damage biological tissues through overheating and free radical formation.
Conclusion
In summary, visible and infrared photons are generally absorbed to heat materials up. Higher energy photons ionize matter and induce chemical changes. And biological molecules absorb light to drive metabolic processes, signals, and growth. The complex interactions of light with matter enable life, technology and the world we see!
| Photon Energy | Typical Fate When Absorbed |
|---|---|
| Visible Light | Heat, fluorescence, photochemistry, photosynthesis |
| Infrared | Heat through molecular vibrations |
| Ultraviolet | Ionization, electron ejection, secondary radiation |
| X-rays, Gamma Rays | Ionization, nuclear transitions, heat |
