Most people know Alexander Graham Bell invented the telephone. But not many know about another talking device (equipment) he invented just four years later, in 1880. He called the device the photophone. "Photophone" comes form the Greek words for "light" and "sound". The photophone did not use electricity to carry sound over wires between two people.  _ . Instead, the photophone used a beam of sunlight traveling through the air. With his invention, Bell could actually transmit   human speech on a beam of light. To do so, he spoke close to a thin mirror that was reflecting sunlight. The vibrations   of his voice caused the mirror to vibrate, and the vibrating mirror caused the light to vibrate. The vibrating light hit a light-sensitive cell in a receiver placed some distance away. The cell changed the light patterns into electrical signals. Earphones changed the signals back into sounds. Bell believed that his idea of talking on a beam of light would prove to be important. He often called the photophone his greatest invention. Even though the photophone depended on a source of energy that was not constant--the sun--Bell wasn't a bit discouraged. He felt sure that people would one day talk using beams of light. Two much more recent developments made Bell's dream come true. In 1960, a scientist built the laser. A laser produces a highly concentrated beam of light. Shortly afterward, other scientists developed a new kind of optical fiber. The optical fiber is a glass thread. The new fiber could carry light beams long distances--as far as several miles. This passage is mainly about   _  .
how to use light to carry sound what the telephone is what the photophone is how to use electricity to carry sound