Resistance of an LDR

[courtyboy02]

Why does the resistance of an LDR increase with a reduction in light intensity?

[ACB312]

A photoresistor is an electronic component whose resistance decreases with increasing incident light intensity. It can also be called a light-dependent resistor (LDR), or photoconductor.

A photoresistor is made of a high-resistance semiconductor. If light falling on the device is of high enough frequency, photons absorbed by the semiconductor give bound electrons enough energy to jump into the conduction band. The resulting free electron (and its hole partner) conduct electricity, thereby lowering resistance.

A photoelectric device can be either intrinsic or extrinsic. In intrinsic devices, the only available electrons are in the valence band, and hence the photon must have enough energy to excite the electron across the entire bandgap. Extrinsic devices have impurities added, which have a ground state energy closer to the conduction band - since the electrons don't have as far to jump, lower energy photons (i.e. longer wavelengths and lower frequencies) are sufficient to trigger the device.

[Hamish]

Or in simple terms, there is a barrier to conduction in the resistor. The energy from the light helps the electons to 'jump' the barrier, hence enabling condution. Just incase you know nothing of physics!