Introducing the Photophone. Considered by the great inventor himself to be his “greatest achievement,” the photophone is a telecommunications device that uses light instead of electricity to transmit sound. Not only is this the first known instance of optical-audio communication, it is also the first wireless communication sent, beating the invention of radio by almost 20 years!
So how did this miracle invention work? Actually, the process is essentially the same one used in a regular telephone. In Mr. Bell’s own words “We have found that the simplest form of apparatus for producing the effect consists of a plane mirror of flexible material, such as silver mica or microscope-glass, against the back of which the speaker's voice is directed . . . . Under the action of the voice the mirror becomes alternately convex and concave and thus alternately scatters and condenses the light.” So instead of using modulated (varied) electrical impulses to convert sound to signal and back again, the photophone uses modulated light. This is accomplished by vibrating a reflective surface (such as a mirror) with sound while pointed at a receiving mirror that converts the pulses of light back into electrical impulses which are then played back on a speaker. The public demonstration of this incredible technology took place on February 19th, 1880 at Mr. Bell’s laboratory in Washington DC where the signal was transmitted over 700 feet.
So why are we still using wires for telecommunications instead of truly wireless devices? Probably for the same reason that we still don’t have practical wireless electricity, but that’s a topic for another article. While the invention was certainly successful, it did have serious practical limitations at the time. Chief among these was the reliance on sunlight to make a transmission and the fact that changes in the light introduced interference to the signal, as Mr. Bell noted when writing to his father about the invention. “I have heard articulate speech produced by sunlight! I have heard a ray of the sun laugh and cough and sing! . . . I have been able to hear a shadow, and I have even perceived by ear the passage of a cloud across the sun's disk.” While listening to the sound of clouds could be intriguing, it could also be annoying. Although Bell and his assistant, Charles Tainter, continued to develop over 50 different variations and refinements of the photophone, it was until the invention of fiber-optic cabling in the 1970s that their discovery could be put to practical use. By sending the light pulses along a glass fiber, interference is virtually eliminated, allowing for crystal-clear, long-distance communication. The fact that the world couldn’t see the potential of this discovery didn’t seem to dampen their spirits any, as Mr. Bell was so excited about it that he wanted to name his second daughter “Photophone,” but his wife gently discouraged that idea.
The photophone may not be a ubiquitous device today but its operating principle is used by nearly every person on the planet. Primary Internet traffic, or “backbone” is transmitted via photonic (light) impulses all over the world. Those cellphone towers you see dotting the landscape? Those are connected to the rest of the world via fiber optic cable. We owe a great deal of thanks Mr. Bell and Mr. Tainter!
Looking for a cool science experiment? You can build your own photophone with very simple materials. A modern version requires two speakers, a small mirror that can fit on one of the speakers, a small solar panel, a 0.1uF capacitor and an amplifier. Attach the mirror to one of the speakers, connect it to an audio source (like an MP3 player) and place it in the sunlight. Setup you solar panel several yards (or more) away and connect the capacitor to it. Then complete the connection from the capacitor to the amplifier (a simple headphone amplifier will do) and then plug in your second speaker to the amplifier. Now, point your speaker/mirror at the solar panel to shine light on it. Start the music playing and head over to your other speaker to listen on your very own photophone! Here’s a little video demonstration of one in action.
Do you have a question or topic you’d like us to address? Email and let us know!