Infrared photons are absorbed by molecules, scattered by particles, or transmitted through materials, depending on what they encounter. Unlike visible light, which has enough energy to bump electrons between shells in atoms, infrared photons carry roughly 10 to 100 times less energy. That lower energy is perfectly matched to a different kind of interaction: making molecules vibrate.
Why Molecules Absorb Infrared Light
Every molecule has natural vibrational frequencies, similar to how a guitar string vibrates at specific pitches. Bonds between atoms stretch, bend, rock, and twist, and each of these motions corresponds to a precise energy level. When an infrared photon arrives with energy that exactly matches the gap between two vibrational energy levels, the molecule absorbs it and begins vibrating more intensely. The photon ceases to exist as a separate particle, and its energy is now stored in that molecular motion.
This is why infrared photons don’t do what visible or ultraviolet light does. Visible light photons carry about 1.6 to 3.3 electron volts of energy, enough to shift electrons between the outer shells of atoms. That’s what triggers chemical reactions in your retina and lets you see. Infrared photons carry around 0.1 electron volts or less. They can’t push electrons around, but they’re ideal for exciting the closely spaced vibrational states that all molecules possess. Water is an especially aggressive absorber of infrared because its molecules have many vibrational states separated by tiny energy gaps, ranging from 0.00001 to 0.01 electron volts.
How Absorbed Infrared Becomes Heat
Once a molecule absorbs an infrared photon and starts vibrating faster, it doesn’t keep that energy to itself for long. It collides with neighboring molecules, transferring kinetic energy to them. Those neighbors bump into their neighbors, and so on. This cascade of molecular collisions is what we experience as heat. The warm feeling you get standing in sunlight comes largely from infrared radiation being absorbed by molecules in your skin.
This process also drives Earth’s energy balance. The sun heats the ground, and the ground radiates infrared photons upward. Most of that outgoing infrared is absorbed by the atmosphere before it can escape to space, and that absorbed energy warms the air. The warmed air then re-radiates infrared in all directions, including back toward the surface. This is the basic mechanism behind the greenhouse effect.
What Greenhouse Gases Do to Infrared Photons
Not every gas in the atmosphere absorbs infrared. Nitrogen and oxygen, which make up 99% of the air, are nearly transparent to it. The gases that matter are the ones whose molecular structure allows them to vibrate in ways that interact with infrared wavelengths. Carbon dioxide absorbs strongly at 4.26 microns. Methane absorbs at different wavelengths. Water vapor, the most abundant natural greenhouse gas, absorbs across a broad range of infrared wavelengths.
When one of these molecules absorbs an infrared photon, it re-emits a new infrared photon in a random direction. Some of that re-emitted energy heads back down toward Earth’s surface, some goes sideways, and some continues upward. At each step, the photon may be absorbed and re-emitted again. The net effect is that outgoing infrared takes a long, indirect path through the atmosphere rather than escaping cleanly to space. More greenhouse gas molecules in the atmosphere mean more absorption and re-emission events, which traps more energy near the surface.
There is one important exception: a range of wavelengths between roughly 8 and 13 microns where the atmosphere is mostly transparent. This is called the atmospheric window, and infrared photons in this band can pass straight through to space. Engineers have designed cooling materials that emit strongly in exactly this wavelength range, allowing surfaces to shed heat directly into outer space without the atmosphere intercepting it.
Scattering: A Different Fate
Not all infrared photons get absorbed. Some bounce off particles or molecules without being taken in, a process called scattering. The efficiency of scattering depends dramatically on wavelength. It follows an inverse fourth-power law: shorter wavelengths scatter far more than longer ones. Blue light, at around 400 nanometers, scatters about 9 times more strongly than red light at 700 nanometers. Infrared, with wavelengths starting at 700 nanometers and extending to 1 millimeter, scatters even less than red.
This is why infrared cameras can see through haze, light fog, and some types of smoke that block visible light. The particles responsible for scattering visible wavelengths are too small to interact efficiently with the longer infrared waves, so the photons pass through relatively undisturbed. It’s the same reason the sky is blue (short wavelengths scatter the most) but doesn’t glow in the infrared.
What Happens in Your Body
When infrared photons hit your skin, how deeply they penetrate depends on their wavelength. Near-infrared light, around 820 nanometers, penetrates the deepest. About 78% of it gets through the outermost 0.4 millimeters of skin. By 1 millimeter deep, only 58% remains. Through a full 2 millimeters of average skin thickness, only about 10% of the original energy is still transmitting. Thicker skin, like on the scalp (about 5.5 millimeters), blocks even more.
Far-infrared, with longer wavelengths, is absorbed almost entirely in the outermost skin layers. The water in your tissue is the primary absorber. Because infrared photons lack the energy to break chemical bonds or damage DNA the way ultraviolet light can, the main biological effect is thermal. Your skin warms up. This is why infrared heaters and saunas feel hot without producing visible light, and why you can feel the heat radiating from a campfire on your face even with your eyes closed.
Every Warm Object Emits Them
Infrared photons aren’t just absorbed. Every object above absolute zero emits them. The peak wavelength of that emission shifts with temperature, following a simple inverse relationship: hotter objects emit shorter-wavelength infrared (and eventually visible light), while cooler objects emit longer wavelengths. The sun, at about 5,800 Kelvin, actually peaks near 880 to 920 nanometers, which is in the near-infrared, just beyond what your eyes can detect. Your body, at around 310 Kelvin, peaks in the far-infrared around 9 to 10 microns.
This constant emission is why thermal cameras work. They detect the infrared photons radiating from objects and people, converting invisible wavelength differences into color maps that show temperature variations. A person in a dark room is invisible to a normal camera but glows brightly in infrared because their body is continuously emitting those photons. The photons carry away energy, which is why objects cool down when placed in a colder environment. They radiate more infrared than they absorb, losing energy until they reach thermal equilibrium with their surroundings.