63 The laser derives from a suggestion made by Albert Einstein in 1916 according to which, under proper circumstances, atoms could release excess energy as light. Give a definition of antenna. beam: raggio harmful: dannoso path: percorso peak: picco pointer: puntatore OTHER FORMS OF TRANSMISSION Laser The word laser stands for Light Amplification by Stimulated Emission of Radiation, which is basically the description of how it works. A laser is created when electrons in the atoms of optical materials like glass, crystal, or gas absorb the energy from an electrical current or light. That extra energy excites the electrons enough to move them from a lower-energy orbit to a higher-energy orbit around the atom s nucleus, producing a very narrow beam of radiation. The emission generally covers an extremely limited range of visible, infrared, or ultraviolet wavelengths. Lasers are used in many different fields such as in healthcare from cosmetic surgery to oncology in industry, for high-precision tasks such as cutting, drilling, and welding, and in optical communication through optical fibres. In our everyday life we can find lasers in barcode scanners, pointers, printers, etc. Li-Fi 146 ELECTRONICS AND ELECTROTECHNOLOGY Li-Fi technology Li-Fi (Light Fidelity) is a bidirectional wireless system that transmits data via LED or infrared light. It was first discovered in 2011 and, unlike WiFi, which uses radio frequency, Li-Fi technology only needs a light source with a chip to transmit an Internet signal through light waves. Here are some advantages: security: the signal cannot penetrate walls, so data cannot be hacked remotely from environments which require high security such as banks, hospitals, military stations, etc.; speed: it is 100 times faster than Wi-Fi, with very high peak of data rates (10 Gbps); energy efficiency: it has low energy consumption because it uses LED lighting technology; safe for human health: it operates on optical bands which are not harmful for humans, so it offers secure wireless connectivity in workplaces. Satellite transmission Satellite transmission consists of a propagation path from a ground station to a communication satellite and back to an Earth station. Satellites orbit at about 36,000 km above the Earth and act like repeaters that amplify the signal and change the frequency. The ground station includes antennas , buildings, and electronics necessary to receive and transmit signals.