Optical communication

The heliograph[?] uses a mirror to reflect sunlight to a distant observer. By moving the mirror the distant observer sees flashes of light that can be used to send a prearranged signalling code. Navy ships often use a signal lamp[?] to signal in Morse code in a similar way.

Distress flares[?] are used by mariners in emergencies, while lighthouses and navigation lights[?] are used to communicate navigation hazards.

Aircraft use the landing lights[?] at airports to land safely, especially at night. Aircraft landing on an aircraft carrier use a similar system to land correctly on the carrier deck. The light systems communicate the correct position of the aircraft relative to the best landing glideslope[?].

In modern communications, lasers are often used as a light source. Often the light is in the infra-red spectrum, rather than being visible light, because the glass fibers transmit those frequencies better. By using different light frequencies (or colours) multiple communications can be sent optically.

• see also: free-space optical communication, Laser Optical fiber, Optical telegraph, Wave division multiplexing, telegraph .