Changes of resistance with light intensity

When light above a certain frequency is shone onto a metal, it releases electrons, which are known as photoelectrons. This is called the photoelectric effect.

Light-dependent resistors (LDRs) are made from photoconductive materials, meaning that they release electrons in the presence of light, as described above. Therefore, as light intensity increases, electrons are released, which increases the number of charge carriers available to conduct electricity, and the resistance of the LDR decreases.