Electromagnetic waves within the frequency range of 400 THz – 790 THz are detected by the human eye. Light transmission is line of sight propagation and is blocked by obstacles.
Optical signalling can be obtained by laser signals. For example, the LANs in two buildings can be connected by installing laser signalling system on the rooftops. Laser rays are unidirectional. So both the transmitter and the receiver need perfectly aligned photo-emitter and photo-detector.
A more used variant of light transmission is visual light communication (VLC). VLC refers to data communication using visible spectrum of the light. It is part of optical wireless communication, which refers to data communication using infrared, visible and ultraviolet lights. VLC uses florescent lamps or LEDs to generate optical signals. The signals are received by photodiodes.
Communication using light signals is safe for eyes and cost effective. Also, it does not require any license for usage. So, it is increasingly being used for ubiquitous computing and IoT systems.