Sir William Herschel, an astronomer who discovered the planet Uranus in 1781, discovered infrared in 1800. He built his own telescopes and was therefore very familiar with lenses and mirrors. Knowing that sunlight was made up of all the colors of the spectrum, and that it was also a source of heat, Herschel wanted to find out which color(s) were responsible for heating objects. He devised an experiment using a prism, paperboard, and thermometers with blackened bulbs where he measured the temperatures of the different colors. Herschel observed an increase in temperature as he moved the thermometer from violet to red in the rainbow created by sunlight passing through the prism. He found that the hottest temperature was actually beyond red light. The radiation causing this heating was not visible; Herschel termed this invisible radiation “calorific rays.” Today, we know it as infrared.
Infrared light is part of the electromagnetic spectrum, which includes all known forms of radiation, including visible light, gamma rays, ultraviolet light, infrared light, X-rays, and radio waves.
The frequency for a given type of radiation determines its placement on the electromagnetic spectrum. Because infrared light has a lower frequency than visible light and a higher frequency than radio waves, it falls between the two on the spectrum. We can’t see or hear infrared; but, if the signals are strong enough, we can feel infrared as heat. IR does not penetrate walls and so does not interfere with other devices in adjoining rooms.
Remote controls and IrDA devices use infrared light-emitting diodes (LEDs) to emit infrared radiation which is focused by a plastic lens into a narrow beam. The beam is modulated, i.e. switched on and off, to encode the data. The receiver uses a silicon photodiode to convert the infrared radiation to an electric current. It responds only to the rapidly pulsing signal created by the transmitter, and filters out slowly changing infrared radiation from ambient light. Infrared communications are useful for indoor use in areas of high population density. These devices usually conform to standards published by IrDA, the Infrared Data Association.