Explain what happens to the charge carriers in a semiconductor light dependant resistor when it is illuminated.
Suggest why shining light onto an LED might be expected to generate a small voltage between its terminals.
A semiconducting light-dependent resistor excites more free electrons into the conduction band when light is present. In solid state physics language, the new photoelectrons are "raised above the Fermi level" of the semiconductor.
An LED is reversible device. Electrical potential between the electrodes liberates photons (light), but photons incident upon the junction can reverse the electron excitation process, driving electrons in the opposite direction and creating a voltage at the terminals.