Infrared light is part of the electromagnetic spectrum. It 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.