On 3rd June 1880, Alexander Graham Bell transmitted the first wireless telephone message using his newly-invented “Photophone”.
The Photophone contained a flexible mirror that vibrated in response to the speaker’s voice. When sunlight was projected on to the mirror, the vibrations were superimposed upon the reflected beam. At the receiver, crystalline selenium cells were used to detect the light beam. The resistance of these cells decreased as the intensity of the light increased, thereby allowing the speech signal to modulate an electrical current. This current could then be converted back into audible speech using an ordinary telephone receiver. In effect, the Photophone was a telephone that used light instead of electricity to carry the signal from the transmitter to the receiver.
Bell took out four separate patents relating to the Photophone, and it is reported that he considered it to be a more important invention than the telephone because it could operate without wires. However, his design was vulnerable to interference from rain or fog, and it could only communicate over a restricted distance. It was not until the development of fibre optics that communication using a beam of light became a practical possibility.