Infrared Radiation
Infrared radiation (IR) is electromagnetic radiation with wavelengths ranging from about 700 nanometres (nm) — just beyond red visible light — to roughly 1 millimetre (mm), where it borders microwave radiation. Because these wavelengths are invisible to the human eye, we experience infrared primarily as heat. Every object with a temperature above absolute zero (0 K) emits infrared radiation, which is why a warm mug, a human body, and even the night sky all radiate energy without glowing visibly. The discovery that heat could travel as invisible radiation was a turning point in physics. Today, infrared technology underpins thermal cameras, TV remote controls, medical imaging, weather satellites, and even the way Earth regulates its temperature through the greenhouse effect. Understanding infrared means understanding how energy moves through the universe without requiring any physical medium — pure electromagnetic radiation carrying thermal energy across empty space. Infrared sits in a specific slice of the electromagnetic spectrum, and its behaviour follows the same wave equations as visible light, radio waves, and X-rays. However, its particular energy range — photon energies roughly between 1.2 × 10⁻³ eV and 1.7 eV — matches the vibrational energy levels of many molecules, which is why IR radiation interacts so strongly with matter, causing atoms to vibrate faster and temperature to rise.
- Position on the Electromagnetic Spectrum
- Blackbody Radiation and Wien's Law
- Stefan–Boltzmann Law: Total Power Radiated
- Molecular Absorption and the Greenhouse Effect
- Sensing and Applications
Position on the Electromagnetic Spectrum
The electromagnetic spectrum is ordered by wavelength (or equivalently, frequency). Infrared occupies wavelengths from ~700 nm to ~1 mm. Physicists subdivide it into near-infrared (NIR, 700 nm–2.5 µm), mid-infrared (MIR, 2.5–25 µm), and far-infrared (FIR, 25 µm–1 mm). Near-infrared photons are energetic enough to be detected by modified camera sensors and are used in fibre-optic communications. Far-infrared overlaps with thermal emission from room-temperature objects.
Blackbody Radiation and Wien's Law
Any object emits a continuous spectrum of electromagnetic radiation purely because of its temperature — this is called blackbody (or thermal) radiation. The peak wavelength of that emission shifts with temperature according to Wien's Displacement Law: λ_peak = b / T, where b ≈ 2.898 × 10⁻³ m·K. The human body at ~310 K emits peak radiation around 9.4 µm — squarely in the mid-infrared. The Sun's surface at ~5778 K peaks near 500 nm (visible green-yellow). Hotter objects radiate at shorter wavelengths and with far greater power.
Stefan–Boltzmann Law: Total Power Radiated
The total power emitted per unit area by a perfect blackbody scales with the fourth power of absolute temperature: P/A = σT⁴. This rapid T⁴ dependence means that doubling a body's temperature increases its radiated power by a factor of 16. Real objects emit less than a perfect blackbody by a factor called emissivity (ε, dimensionless, 0 < ε ≤ 1), giving P = εσAT⁴. Human skin has ε ≈ 0.98 — almost a perfect blackbody in the infrared, which is why thermal cameras image people so clearly regardless of skin colour.
Molecular Absorption and the Greenhouse Effect
Infrared photons have energies that match the quantised vibrational modes of polyatomic molecules. When a CO₂ or H₂O molecule absorbs an IR photon, its bonds vibrate at increased amplitude — effectively gaining thermal energy. This is exactly the mechanism behind the greenhouse effect: Earth's surface emits infrared (because it is ~288 K), and greenhouse gases in the atmosphere absorb and re-emit it, trapping heat. The same principle explains why a microwave oven's cousin — infrared grills — cook food by exciting molecular vibrations directly.
Sensing and Applications
Thermal (infrared) cameras use detector arrays (such as bolometers or InGaAs photodetectors) sensitive to IR photons. They produce false-colour images where brighter colours represent warmer regions — invaluable in firefighting, medical diagnostics (detecting inflammation or poor circulation), and security. IR remote controls use near-infrared LEDs (~940 nm) pulsed in coded patterns. Infrared spectroscopy identifies chemical bonds: every molecule has a unique IR 'fingerprint' spectrum, used in forensics, pharmaceuticals, and atmospheric monitoring.
The Key Equation
- \lambda_{\text{peak}} — Peak wavelength (metres (m)): The wavelength at which the blackbody emits the most power per unit wavelength interval.
- b — Wien's displacement constant (m·K): A universal constant equal to 2.898 × 10⁻³ m·K.
- T — Absolute temperature (kelvin (K)): The temperature of the emitting body measured from absolute zero.
Worked Example
Problem. A medical thermal camera detects radiation from human skin at 37 °C (310 K). Calculate the peak wavelength of infrared emission from the skin, and identify which sub-region of infrared this falls in.
Answer. The peak wavelength is approximately 9.35 µm, in the mid-infrared region.
Origins · 1 minWilliam Herschel and how this was discovered
In 1800, Herschel placed thermometers beyond the red end of a prism-dispersed solar spectrum and discovered that the thermometer registered a temperature rise even where no visible light fell. He called this 'calorific rays' — what we now call infrared radiation. This was the first discovery of a form of electromagnetic radiation beyond the visible spectrum, opening the entire field of non-visible electromagnetic wave research. Herschel was primarily an astronomer who discovered the planet Uranus in 1781 — the first planet found with a telescope. His infrared discovery came about almost by accident while he was studying how much heat different colours of sunlight carried.
Related Concepts
- Visible Light and the Optical Spectrum — Visible light (400–700 nm) borders infrared directly; understanding both regions together clarifies the continuous nature of the electromagnetic spectrum and how our eyes evolved to detect the Sun's peak emission.
- Blackbody Radiation and Planck's Law — Infrared emission from warm objects is a direct consequence of blackbody radiation; Planck's Law describes the full spectrum from which Wien's Law and the Stefan–Boltzmann Law are derived.
- The Electromagnetic Spectrum — Infrared is one region of the broader electromagnetic spectrum; situating it among radio, microwave, UV, X-ray, and gamma-ray regions builds a complete picture of wave-based energy transfer.
- Thermal Radiation as Heat Transfer — Radiation is one of three mechanisms of heat transfer (alongside conduction and convection); infrared is the dominant radiative mechanism for objects at everyday temperatures.