What's going on: Turning sound into light

Audio signals can be carried in radio waves through space and in electrical pulses through wires. Other forms of electromagnetic radiation, including visible light, can carry audio signals, too.

The battery provides a steady DC current to light the LED. Under the influence of the battery alone, the LED glows with a fixed brightness. The resistor limits the current so the LED does not burn out. When the radio is turned on, the weak but fluctuating radio signal is added to the constant signal from the battery. The LED still glows, but now it flickers in synchronization with the radio signal, as the amount of current passing through it varies.

The flickering light hitting the solar cell causes it to generate electrical signals which again vary in synchronization with the original radio signal. These signals are amplified and fed to the speaker, recreating the original sounds from the radio.

Placing an opaque barrier between the LED and the solar cell cuts off the light, and the solar cell is unable to generate any electrical signals. Blocking some, but not all, of the light decreases the amplitude of the electrical signal, which affects the volume of the radio signal.

Modern communications systems, such as long-distance phone lines and high-bandwidth communication lines for computers, commonly use modulated signals carried on a beam of light. The relatively high frequencies of visual light can carry a lot more information than lower frequency radio waves. The light that is used is normally from a laser, not an LED, and the signals are carried by a clear fiber-optic cable, rather than through air, but the principle is the same.