An LED also operates in reverse: it converts light energy of a particular wavelength into electric current. The voltage generated is proportional to the intensity of light shining on the LED. The wavelength of light needed to produce this effect is approximately the same as the wavelength of light that the LED normally produces when operating in the electricity-to-light mode, but not identical. An LED that emits red light generates electricity from orange wavelengths of light.
These LED explorations were based on the work of Forrest Mims whose website, http://forrestmims.org/ can give you a richer content on the science behind LEDs. Some of the MVH activities are adapted from his work. Most of these explorations were used in the Global Systems Science (GSS) student series created by Lawrence Hall of Science.