Radiation transfers energy through electromagnetic waves, tiny packets of energy called photons that travel outward from a source at the speed of light. Unlike conduction (which needs direct contact between materials) or convection (which needs a fluid like air or water to carry heat), radiation requires no physical medium at all. It works perfectly well across the vacuum of space, which is exactly how the sun’s energy reaches Earth across 150 million kilometers of emptiness.
What Photons Actually Do
Every object with a temperature above absolute zero emits photons. These photons are the smallest possible particles of electromagnetic energy, and they behave as both waves and particles simultaneously. As they travel, the electric and magnetic fields they carry oscillate in intensity, swinging between high and low energy levels in a regular pattern.
The energy packed into each photon determines what kind of radiation it carries. Low-energy photons produce radio waves. Slightly higher-energy photons create microwaves, then infrared light, then visible light, then ultraviolet, X-rays, and finally gamma rays at the highest energies. This entire range is the electromagnetic spectrum, and all of it transfers energy by the same basic mechanism: photons leaving one object and being absorbed by another.
Why Hotter Objects Radiate Differently
The temperature of an object controls two things about the radiation it emits: how much total energy it puts out, and what wavelengths dominate that emission. Total radiated energy scales with the fourth power of temperature. Double an object’s temperature (in absolute terms) and it radiates 16 times as much energy. This relationship is captured by the Stefan-Boltzmann law, and it explains why a glowing coal radiates far more heat than a warm sidewalk, even though both emit infrared radiation.
Temperature also shifts the peak wavelength of emission. As an object gets hotter, its radiation peaks at shorter wavelengths. A warm human body peaks in the infrared range, invisible to the eye. Heat iron to around 500°C and the peak shifts enough that some visible red light appears. The surface of the sun, at roughly 5,500°C, peaks in the visible light range, which is why sunlight looks white. This shift is how astronomers determine the surface temperatures of distant stars without ever visiting them.
How Distance Weakens the Signal
Radiation spreads outward from its source in all directions, and its intensity drops with distance according to the inverse square law. Move twice as far from a radiation source and the intensity falls to one quarter. Move three times as far, and it drops to one ninth. This happens because the same total energy spreads over an ever-larger area as you move away.
This principle shapes everything from how bright a lamp appears across a room to how much solar energy different planets receive. At the top of Earth’s atmosphere, the sun delivers about 1,362 watts per square meter. Averaged over the entire globe (accounting for night, angle, and curvature), that works out to roughly 340 watts per square meter of solar input. Mars, being farther away, receives considerably less.
What Happens When Radiation Hits a Surface
When radiation strikes an object, three things can happen: absorption, reflection, or transmission. The incoming energy splits among these three outcomes, and the proportions depend on the wavelength of the radiation and the material it encounters.
Absorption is the mechanism that actually transfers energy into the target. The photon’s energy converts into internal energy of the material, raising its temperature. Transmission means the radiation passes straight through, the way visible light passes through glass. Reflection redirects the energy back outward, either in a single direction (like a mirror, called specular reflection) or scattered in all directions (diffuse reflection, like light bouncing off a rough wall). Whether reflection is specular or diffuse depends on how the surface roughness compares to the wavelength of the incoming radiation. Smooth surfaces relative to the wavelength produce mirror-like reflection; rough surfaces scatter it.
Emissivity: Why Materials Radiate Differently
Not all surfaces emit radiation equally well at a given temperature. Emissivity measures how efficiently a surface radiates compared to a perfect theoretical emitter (called a blackbody), which has an emissivity of 1.0. Polished metals are poor emitters: polished copper has an emissivity as low as 0.02, polished aluminum around 0.04 to 0.06, and polished gold 0.02 to 0.04. That’s why reflective materials make good insulation. They neither absorb nor emit radiation efficiently.
Rougher, darker, or more oxidized surfaces emit far more effectively. Red brick comes in at 0.93, concrete at 0.85 to 0.94, smooth ice at 0.97, and human skin around 0.95 to 0.98. Water, glass, and most organic materials all have high emissivities, meaning they readily absorb and emit infrared radiation. This is why you can feel the warmth radiating off a dark asphalt road on a summer evening long after sunset: the asphalt (emissivity 0.93) is efficiently radiating the energy it absorbed during the day.
Radiation in Everyday Life
The most familiar example of radiation transfer is sunlight warming your skin. Photons leave the sun’s surface, travel through the vacuum of space for about eight minutes, pass through Earth’s atmosphere (with some absorbed or scattered along the way), and deliver their energy to whatever they strike. No physical contact, no moving air required.
Microwave ovens use radiation transfer in a more targeted way. They emit photons at a specific frequency tuned to interact with water molecules. Water molecules are polar, meaning they have a slight positive charge on one end and negative on the other. The oscillating electric field of the microwave radiation forces these molecules to rotate rapidly, trying to align with the field. Because the field reverses billions of times per second, the molecules can’t keep up. They collide with neighboring molecules in the process, and those collisions generate heat. This is why wet foods heat quickly in a microwave while dry ceramics stay cool.
Infrared heaters, heat lamps, and even campfires all transfer energy primarily through radiation. The warmth you feel on your face sitting across from a fire isn’t carried by the air between you and the flames. It arrives as infrared photons traveling at the speed of light, absorbed directly by your skin and clothing. Step behind a barrier that blocks those photons, and the warmth vanishes instantly, even though the air temperature around you hasn’t changed.
How Radiation Differs From Conduction and Convection
All three are modes of heat transfer, but they work through fundamentally different mechanisms. Conduction transfers energy through direct molecular contact: faster-vibrating molecules bump into slower ones, passing energy along through a solid or between touching surfaces. Convection moves energy by physically transporting heated fluid (air or liquid) from one place to another. Both require matter to work.
Radiation needs nothing between the source and the receiver. It travels at the speed of light, works across any distance (though it weakens with the square of that distance), and can transfer energy across the vacuum of space. This is the only way energy from the sun reaches Earth, and it’s the reason spacecraft in orbit must carefully manage their surface coatings and orientations. With no air to conduct or convect heat away, radiation is the sole mechanism for shedding excess thermal energy in space.