Infrared (IR) radiation is a form of energy that exists as part of the electromagnetic spectrum, positioned between visible light and microwaves. This radiation is invisible to the human eye but is often felt as heat, leading to its common reference as heat radiation. The classification of infrared radiation as strictly “longwave” or “shortwave” is inaccurate because the term encompasses a vast range of wavelengths. This spectrum includes relatively shorter wavelengths near visible light and much longer wavelengths approaching the microwave region.
Understanding Wavelengths in the Electromagnetic Spectrum
All electromagnetic radiation, including infrared, travels through space as waves defined by their wavelength, frequency, and energy. Wavelength is the physical distance between two consecutive peaks, typically measured in micrometers (µm) for IR. Frequency measures how many waves pass a fixed point per second, while energy is the power carried by the wave’s photons.
The relationship between these three properties is inverse for wavelength and direct for frequency and energy. A shorter wavelength means the wave has a higher frequency and carries a greater amount of energy. Conversely, a longer wavelength corresponds to a lower frequency and less energy. This principle governs the entire electromagnetic spectrum, from high-energy gamma rays to low-energy radio waves.
Infrared radiation sits next to red visible light, meaning it has a longer wavelength and lower energy than visible light. However, the total IR range still covers a significant spread of physical properties.
The True Nature of Infrared Radiation
The infrared spectrum covers a range from approximately 0.7 micrometers (µm) up to about 1,000 µm (one millimeter). Due to this massive span, scientists and engineers divide the spectrum into multiple distinct categories, each with unique properties and applications. These subdivisions clarify that infrared radiation is not a single type of wave but a continuum.
Near-Infrared (NIR) and Short-Wave Infrared (SWIR)
The shortest-wavelength end of the spectrum begins with Near-Infrared (NIR), which ranges from about 0.7 µm to 1.4 µm. NIR radiation is the closest to visible light and is often used in fiber optic telecommunications and night-vision devices because it behaves more like reflected light. Immediately following is Short-Wave Infrared (SWIR), spanning roughly 1.4 µm to 3 µm. SWIR is also considered reflected infrared and is useful for imaging through atmospheric obscurants like haze and fog.
Mid-Wave Infrared (MWIR) and Long-Wave Infrared (LWIR)
Moving to the longer wavelengths, we find Mid-Wave Infrared (MWIR), typically defined as 3 µm to 8 µm. MWIR begins to transition into the thermal region, where the energy is primarily emitted as heat rather than reflected light. The longest segment is Long-Wave Infrared (LWIR), or Thermal Infrared, which extends from about 8 µm to 15 µm. LWIR is the pure heat-sensing portion of the spectrum and is used by thermal cameras to detect the heat radiated by objects at ambient temperatures.
Because the infrared spectrum includes the shorter NIR and SWIR bands and the much longer LWIR band, it is impossible to label all infrared radiation with a single “longwave” or “shortwave” designation. The physical properties of the radiation vary dramatically across this range.
Contextual Definitions of Shortwave and Longwave
The confusion regarding the “longwave” or “shortwave” label arises not from the physics of the waves themselves, but from a practical simplification used in atmospheric science and climatology. In this context, the terms describe the source of the radiation and its role in the Earth’s energy budget, rather than a precise physical boundary.
Shortwave Radiation, in meteorology, is defined as the incoming energy from the Sun. The Sun has an extremely high surface temperature of approximately 6,000 Kelvin, causing it to emit radiation with peak intensity in short wavelengths, specifically visible and near-infrared light. This solar energy is collectively referred to as shortwave radiation.
Longwave Radiation, conversely, refers to the heat emitted by the Earth’s surface and atmosphere back toward space. Since the Earth is far cooler than the Sun, it radiates thermal energy at much longer wavelengths, primarily in the Long-Wave Infrared (LWIR) range (typically between 4 µm and 30 µm). This terrestrial emission is termed longwave radiation.
This distinction is useful for modeling the Earth’s climate because it separates the high-energy solar input from the lower-energy thermal output. While a portion of the infrared spectrum (NIR/SWIR) is included in the sun’s shortwave emission, and another portion (LWIR) is the Earth’s longwave emission, the entire infrared spectrum is not uniformly one or the other. The classification depends entirely on whether the discussion is about the physical properties of the wave or its source in a climate context.