Short answers, pulled from the story.
William Herschel discovered infrared radiation in 1800 when he conducted an experiment using a glass prism to refract sunlight and measured the temperature of different colors. He found that the mercury rose highest just beyond the red light where no color appeared, proving invisible heat waves existed.
French physicist Edmond Becquerel coined the term infra-red in 1867 after decades of struggle by scientists to name and categorize what Herschel had found. English translations of articles by Vignaud Dupuy de Saint-Florent brought the concept into common usage by 1874.
The band typically extends from the nominal red edge of the visible spectrum at 780 nanometers up to one millimeter. This wavelength range corresponds to frequencies between approximately 430 terahertz down to 300 gigahertz before transitioning into the microwave portion of the electromagnetic spectrum.
Sunlight arriving at Earth contains slightly more than half its energy in the form of infrared radiation with 527 watts out of over one kilowatt per square meter being infrared. Nearly all the infrared radiation in sunlight falls within the near-infrared band shorter than four micrometers.
In 1945, the Zielgerät 1229 Vampir system became the first portable infrared device introduced for military applications. By 1958, Falcon and Sidewinder missiles were developed using infrared technology to track targets through passive guidance systems that use infrared signatures emitted by jet engine exhaust plumes.