In 1800, after passing solar light through a prism, astronomer George Herschel put a thermometer below the red band in the spectrum.

It heated up, indicating an invisible form of light or radiation.

Thus was the discovery of infrared radiation.

Later it was established that, along with visible light, infrared radiation is simply part of a vast "electromagneti" spectrum, analogous to the organ keys that Herschel had played since a child.