Infrared radiation refers broadly to that part of the electromagnetic spectrum between the visible and microwave regions, extending approximately over the wavelength range 0.5–100 µm, where 1 µm = 10−6 m. The first scientific report on the heating effect of infrared radiation is believed to belong to Herschel (1800). Knowing that a lens will focus the sun and burn paper, Herschel (the same German-born, English-by-choice astronomer who discovered the planet Uranus in 1781) placed a thermometer in the various colors of the solar spectrum resolved by passing daylight through a prism. He noticed an unusual large heating effect in the region immediately beyond the red end of the visible spectrum (Herschel 1800). This invisible part of the spectrum was naturally called infrared and it was defined by subsequent measurements between wavelengths of 0.76 and 350 µm.
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