Conversion of Light Into Sound

The invention of the photophone is based upon a property recently found to exist in selenium. The action of light upon that substance causes certain molecular changes that affect the ear as sound: in brief, it causes the conversion of light into sound.

The most simple photophone used consists of a small mirror of silvered mica, suspended vertically. Upon the front of this mirror is thrown, by means of a lens, a concentrated beam of sunlight; and by means of a second lens the ray is reflected upon a piece of selenium at a distance of seven hundred feet. Here the curious property of selenium comes into play. Its resistance varies with the intensity of the light; and this changeable resistance produces sound, which can be conveyed along the beam of light by placing a speaker behind the mirror.

All motions may be transmitted along the beam of light, and will then affect the ear as sounds, even though the movements themselves are inaudible.

In still other forms of the photophone, even a silent motion or the burning of a candle will act as sounds. The shadow of any object in the light of the candle will produce an audible effect.

The photophone is in only its experimental stage, but there is no doubt of its being used eventually to convey information by means of a beam of light.