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Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those of red light (the longest waves in the visible spectrum ), so IR is invisible to the human eye.
The wavelength range from approximately 200 μm up to a few mm is often referred to as Submillimetre astronomy, reserving far infrared for wavelengths below 200 μm. Mid-infrared, from 30 THz to 120 THz (10–2.5 μm).
An infrared heater or heat lamp is a heating appliance containing a high-temperature emitter that transfers energy to a cooler object through electromagnetic radiation. Depending on the temperature of the emitter, the wavelength of the peak of the infrared radiation ranges from 750 nm to 1 mm.
Other wavelengths, especially nearby infrared (longer than 700 nm) and ultraviolet (shorter than 400 nm) are also sometimes referred to as light. As frequency increases into the visible range, photons have enough energy to change the bond structure of some individual molecules.
The infrared portion of the electromagnetic spectrum is usually divided into three regions; the near-, mid- and far-infrared, named for their relation to the visible spectrum. The higher-energy near-IR, approximately 14,000–4,000 cm −1 (0.7–2.5 μm wavelength) can excite overtone or combination modes of molecular vibrations .
Different sources may use different boundaries to define the far infrared range. For instance, astronomers often define it as wavelengths between 25 μm and 350 μm. [3] Infrared photons possess significantly lower energy than photons in the visible light spectrum, with tens to hundreds of times less energy. [4]
For a tungsten filament at a typical temperature of 3000 K, only a small fraction of the emitted radiation is visible, and the majority is infrared light. This infrared light does not help a person see, but still transfers heat to the environment, making incandescent lights relatively inefficient as a light source. [25]
For measuring room temperature emissivities, the detectors must absorb thermal radiation completely at infrared wavelengths near 10×10 −6 metre. [15] Visible light has a wavelength range of about 0.4–0.7×10 −6 metre from violet to deep red. Emissivity measurements for many surfaces are compiled in many handbooks and texts.