Infrared radiation comes from everything that has a temperature. Every object, from a human body to a distant star, emits infrared energy as its atoms and molecules vibrate. The hotter the object, the more infrared radiation it produces and the shorter the wavelength of that radiation. This simple principle connects an enormous range of sources, from the sun overhead to the remote control on your couch.
Why All Objects Emit Infrared
At the atomic level, infrared radiation is produced by the vibrations and rotations of molecules. Atoms within a molecule are constantly jiggling, stretching apart and snapping back together, bending at different angles. Each of these movements corresponds to a specific energy level, and when a molecule shifts between energy levels, it can release (or absorb) a photon of infrared light. The energy of that photon matches the exact energy difference between the two vibrational states.
This is why infrared is sometimes called “heat radiation.” Anything with thermal energy has molecules in motion, and those molecules radiate infrared photons as a result. A cup of coffee, a sidewalk in summer, your own skin: all of them are constantly broadcasting infrared light. You can’t see it, but an infrared camera can, which is how thermal imaging works.
There’s a useful relationship between an object’s temperature and the wavelength where it radiates most intensely. Known as Wien’s displacement law, it says that hotter objects peak at shorter wavelengths. The sun, with a surface temperature around 5,500°C, peaks in visible light. Your body, at roughly 37°C, peaks deep in the infrared, around 10 micrometers. A warm stovetop sits somewhere in between. The principle is the same in every case: thermal energy drives molecular motion, and that motion produces infrared photons.
The Sun: The Biggest Nearby Source
The sun is by far the dominant source of infrared radiation reaching Earth. While most people think of sunlight as visible light, that’s actually the smaller portion. The solar spectrum breaks down to roughly 42% to 43% visible light, 52% to 55% near-infrared light, and only 3% to 5% ultraviolet. In other words, more than half the energy the sun delivers to Earth arrives as infrared.
This near-infrared sunlight is what warms your skin on a sunny day even before you feel “hot.” It penetrates slightly deeper into surfaces than visible light does, which is part of why solar heating is so effective. Materials like asphalt, soil, and water absorb this infrared energy throughout the day and then re-emit it as longer-wavelength infrared radiation after sunset, which is why summer evenings stay warm long after the sun goes down.
The Earth Itself Radiates Infrared
Once the Earth absorbs solar energy, it doesn’t keep it forever. The planet re-emits that energy back toward space, and regardless of what wavelength arrived, the outgoing radiation is infrared. This is a fundamental point in climate science: Earth absorbs a mix of visible and near-infrared sunlight, then radiates it all back as longer-wavelength infrared, sometimes called thermal infrared or longwave radiation.
Greenhouse gases in the atmosphere, like carbon dioxide and water vapor, interact specifically with this outgoing infrared. Their molecular structures vibrate at frequencies that match infrared wavelengths, so they absorb the radiation the Earth is trying to shed. Those gas molecules then re-emit infrared in all directions, including back toward the surface. This is the greenhouse effect: the atmosphere acts as a partial blanket that traps infrared energy and keeps the planet warmer than it would otherwise be.
Stars, Dust Clouds, and the Infrared Universe
Beyond our solar system, some of the most important infrared sources are objects that are too cool or too hidden to see in visible light. Newly forming stars, called protostars, are a prime example. A protostar is still collapsing from a cloud of gas and dust, and it hasn’t yet ignited hydrogen fusion. It’s warm but not white-hot, so it radiates primarily in the infrared. The thick cocoon of dust surrounding it absorbs any visible light the protostar does produce, heats up, and re-radiates that energy as infrared. Until the young star clears away its dust shell, infrared is the only way to detect it.
As these stars mature and begin blowing away surrounding material with intense stellar winds, the warm dust that remains still glows brightly in the infrared. Entire star-forming regions, like the famous pillars in the Eagle Nebula, look dramatically different in infrared images compared to visible-light photographs. In the infrared, the dust becomes transparent and the newborn stars hidden behind it become visible. Dense, cold clumps of dust known as Bok globules, which appear as dark patches against the background sky in visible light, turn out to contain protostars when observed in infrared. One well-studied example, Barnard 5, contains at least four protostars invisible to optical telescopes.
Cool red dwarf stars, the most common type of star in the galaxy, also emit a large fraction of their energy as infrared. And galaxies undergoing intense bursts of star formation can be among the brightest objects in the infrared sky, their output dominated by heated dust rather than direct starlight.
Everyday Technology That Uses Infrared
You interact with artificial infrared sources more often than you might realize. The most familiar is probably a TV or stereo remote control. These devices contain a small LED that emits infrared light at a wavelength of about 940 nanometers, just beyond what the human eye can detect. The LED blinks in coded patterns that a sensor on the device reads as commands. You can actually see this blinking if you point a remote at your phone’s camera, which is sensitive enough to pick up near-infrared light as a faint purple or white flash.
Infrared heaters work on a larger scale. Space heaters, patio heaters, and industrial drying systems all use heated elements that radiate infrared energy directly onto surfaces rather than warming the air first. This is why a patio heater feels warm on your face even on a breezy night: the infrared travels in a straight line from the element to your skin, and the wind can’t blow it away the way it would blow away heated air. Industrial versions use specialized alloy or ceramic elements that can reach extremely high temperatures, over 1,800°C in some applications, producing intense short-wavelength infrared for tasks like drying coatings, curing materials, or forging metal.
Night-vision devices, thermal cameras, fiber-optic communication systems, and motion-activated security lights all rely on infrared as well. Thermal cameras detect the infrared your body naturally emits, which is how airport fever-screening systems and building energy audits work. Fiber-optic internet transmits data as pulses of near-infrared laser light through glass cables.
How Infrared Differs Across the Spectrum
Not all infrared is the same. The infrared portion of the electromagnetic spectrum spans a wide range, from about 700 nanometers (just past red visible light) to around 1 millimeter (where microwave radiation begins). Scientists typically divide this into three bands. Near-infrared, from about 700 nanometers to 1.4 micrometers, is closest to visible light and is what remote controls, fiber optics, and the sun’s peak infrared output fall into. Mid-infrared, roughly 1.4 to 8 micrometers, is where many chemical molecules absorb strongly, making it useful for identifying substances. Far-infrared, from about 8 micrometers to 1 millimeter, is the deep thermal radiation emitted by room-temperature objects and the Earth’s surface.
Each band comes from different sources and interacts with matter differently. Near-infrared passes through glass easily, which is why sunlight warms the inside of your car through the windshield. Far-infrared does not pass through glass, which is partly why greenhouses retain heat. The wavelength also determines how deeply infrared penetrates skin: near-infrared reaches several millimeters below the surface, while far-infrared is absorbed almost entirely by the outermost layer.
Whether it’s a protostar buried in a dust cloud, the warm ground beneath your feet, or the LED in your remote control, the underlying mechanism is the same. Molecules vibrate, energy shifts between states, and infrared photons fly outward. The temperature of the source determines the wavelength, and the wavelength determines what that infrared radiation can do when it arrives somewhere else.