First optical telephone

A device called the Photophone was invented jointly by Alexander Graham Bell and his assistant Charles Sumner Tainter on 19 February 1880, at Bell's laboratory at 1325 L Street in Washington DC, USA. Bell's Photophone worked by projecting his voice through a mouthpiece onto a mirror. His voice caused vibrations in the mirror, which used sunlight captured and projected via lenses to a receiving mirror located some distance away. These vibrations were transformed back into sound at the receiving end of the projection by a selenium cell and a simple earpiece. The variation in light intensity produced a variation in electrical current through the earpiece to produce sound. On 21 June 1880, Bell's assistant transmitted a voice message from the roof of the Franklin School to the window of Bell's laboratory, some 213 m (about 700 ft) away. Of the 18 patents granted in Bell's name alone, and the 12 he shared with collaborators, four were for the Photophone.

Today, fibre-optic cables carry telephone conversations around the world by light. Bell's forerunner of fibre-optic communications was a century ahead of its time.