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The photophone used crystalline selenium cells as the receiver. This material's electrical resistance varies inversely with the illumination, i.e., its resistance is higher when it is in the dark, and lower when it is lighted. The idea of the photophone was thus to modulate a light beam: the resulting varying illumination of the receiver would induce corresponding varying resistance in the selenium cells, which could be used by a telephone to regenerate the sounds captured at the receiver. The modulation of the light beam was done by a vibrating mirror: a thin mirror would alternate between concave and convex forms, thus focussing or dispersing the light from the light source. The photophone functioned similarly to the telephone, except the photophone used light as a means of projecting the information, while the telephone relied on electricity.

In one experiment in Washington, D.C., Bell and his co-inventor Charles Sumner Tainter succeeded to communicate clearly over a distance of some 830  ftA foot is a non- SI unit of distance or length. The popular belief is that original standard was the length of a man's foot. The average foot length is about 240 mm (9. 4 in) for current Europeans. About 996 of 1000 British men have a foot that is less th. (about 250  m), using plain sunlight as the light source. The receiver was a parabolic mirror with the selenium cells in its focal point. The selenium cells had an electrical resistance varying between 300  ΩThe ohm is the SI derived unit of electrical resistance (derived from the ampere and the watt). Its symbol is the Greek capital letter omega (Ω). The ohm is named for Georg Ohm, a German physicist who discovered the relation between voltage and curr and 100  ΩThe ohm is the SI derived unit of electrical resistance (derived from the ampere and the watt). Its symbol is the Greek capital letter omega (Ω). The ohm is named for Georg Ohm, a German physicist who discovered the relation between voltage and curr.

Although the photophone was an extremely important invention, it was many years before the significance of Bell's work was fully recognized. Bell's original photophone failed to protect transmissions from outside interferenceIn communications, interference is anything which alters, modifies, or disrupts a message; as it travels along a channel, between a source and a receiver. Below is an article about the physical phenomenon of wave interference. See also the legal concept os, such as clouds, that easily disrupted transport. Until the development of modern fiber optics, technology for the secure transport of light inhibited use of Bell's invention. Bell's photophone is recognized as the progenitor of the modern fiber optics that today transport over eight percent of the world's telecommunicationTelecommunication is the extension of communication over a distance. In practice it also recognizes that something may be lost in the process; hence the term 'telecommunication' covers all forms of distance and/or conversion of the original communicationss.