Absorbed radiation is most commonly reemitted as infrared radiation, which is a longer-wavelength, lower-energy form of light than what was originally absorbed. This principle drives everything from the greenhouse effect to why objects glow different colors at different temperatures. The exact form of the reemitted energy depends on what absorbed it and how hot it is, but in everyday scenarios on Earth, infrared is the dominant answer.
Why Infrared Is the Default
When an object absorbs radiation, that energy gets converted into the random motion of its atoms and molecules, essentially heat. The object then radiates energy back out at wavelengths determined by its temperature. At room temperature and the temperatures typical of Earth’s surface, objects emit almost entirely in the infrared range. NOAA describes Earth’s thermal emissions as falling between 4,000 and 100,000 nanometers, all within the infrared band.
This is a direct consequence of Wien’s displacement law: the peak wavelength of emission is inversely proportional to an object’s temperature. The Sun, at roughly 5,500°C, emits mostly visible light. Earth’s surface, averaging around 15°C, emits at much longer wavelengths. As an object heats up past a few hundred degrees Celsius, it begins emitting visible light, starting with red and progressing through orange, yellow, white, and eventually blue. Think of a heated piece of metal glowing red, then white. But at ordinary temperatures, all the emission stays in the infrared, invisible to our eyes.
The Greenhouse Effect as a Real-World Example
The greenhouse effect is the most familiar example of this process. Sunlight arrives at Earth mostly as visible and ultraviolet radiation (shortwave energy). About half of it is absorbed by the surface. Because Earth is far cooler than the Sun, it reemits that energy as infrared radiation rather than visible light.
Here’s where it gets important: greenhouse gases like carbon dioxide, methane, and water vapor are largely transparent to incoming visible light but absorb infrared radiation efficiently. Once they absorb that outgoing infrared energy, they reemit it in all directions, sending some back toward the surface and trapping heat in the atmosphere. Nitrogen and oxygen, which make up the vast majority of the atmosphere, don’t absorb visible or infrared radiation and play essentially no direct role in this warming process.
Reemission as Visible Light: Fluorescence and Phosphorescence
Not all reemission comes out as infrared. Some materials absorb radiation and reemit it as visible light through a process called luminescence. This splits into two categories based on timing.
Fluorescence happens almost instantly. A material absorbs a photon (often ultraviolet) and reemits a lower-energy photon (often visible) within nanoseconds. This is why certain minerals, laundry detergents, and highlighter inks glow under a blacklight. The absorbed UV energy kicks electrons into a higher energy state, and as they drop back down, they release visible light.
Phosphorescence is the slower version. The energy gets temporarily trapped in a different electronic state before being released, which can take microseconds to hours. Glow-in-the-dark materials work this way: they absorb light, store the energy briefly, and release it slowly as a visible afterglow.
In both cases, the reemitted light has a longer wavelength than what was absorbed. This energy gap is called the Stokes shift. Some of the absorbed energy is lost to molecular vibrations and heat before the photon is reemitted, so the outgoing photon carries less energy, which means a longer wavelength. The surrounding molecules also play a role. In polar solvents like water, the molecules around the absorbing substance physically rearrange to stabilize the new electronic state, and that rearrangement uses up some of the absorbed energy.
When Energy Isn’t Reemitted as Light at All
Sometimes absorbed radiation is never reemitted as a photon. Instead, the energy dissipates entirely as heat through non-radiative processes. After a molecule absorbs light and its electrons jump to a higher energy level, they can lose that energy through vibrations and collisions with neighboring molecules rather than by emitting a new photon. The ratio of reemitted photons to absorbed photons is called the quantum yield. A quantum yield of 1.0 would mean every absorbed photon produces a reemitted photon. Most materials fall well below that. When internal conversion between energy levels is easy (many pathways for the energy to dissipate as vibration), reemission of light effectively disappears, and all the energy becomes heat.
Good Absorbers Are Good Emitters
Kirchhoff’s law of thermal radiation establishes a fundamental rule: at any given wavelength, an object’s ability to emit radiation equals its ability to absorb it. A perfect absorber at a particular wavelength is also a perfect emitter at that wavelength. A surface that reflects most incoming radiation at a certain wavelength will also be a poor emitter at that wavelength. This is why dark-colored objects that absorb sunlight also radiate heat more efficiently than shiny, reflective surfaces.
A perfect absorber and emitter across all wavelengths is called a blackbody. Real objects approximate this to varying degrees. Earth’s surface behaves close to a blackbody in the infrared range, which is why it so efficiently converts absorbed sunlight into outgoing infrared radiation. The atmosphere’s greenhouse gases, in turn, are strong absorbers and emitters at specific infrared wavelengths, which is what makes them so effective at trapping heat.
The Short Answer
For most everyday situations, absorbed radiation is reemitted as infrared (heat) radiation at longer wavelengths than the original. The specific wavelength depends on the temperature of the emitting object. Specialized materials can reemit absorbed energy as visible light through fluorescence or phosphorescence, but in those cases the emitted light still shifts toward longer wavelengths. And in many materials, a significant fraction of absorbed energy never becomes light again, instead converting entirely to heat.