5mm LDR (Pack of 10)

The 5800B Light Dependent Resistor (LDR) White Color is a photoresistor (or light-dependent resistor, LDR, or photo-conductive cell). Which is a light-controlled variable resistor. The resistance of a photoresistor decreases with increasing incident light intensity; in other words, it exhibits photoconductivity. A photoresistor is made of a high resistance semiconductor.

Photoresistors are Semiconductor devices that use light energy to control the flow of electrons, and hence the current flowing through them. When the light level decreases, the resistance of the LDR increases. As this resistance increases in relation to the other Resistor; which has a fixed resistance, it causes the voltage dropped across the LDR to also increase. When this voltage is large enough (0.7V for a typical NPN Transistor), it will cause the Transistor to turn on.

when the light falls on its surface, then the material conductivity reduces and also the electrons in the valence band of the device are excited to the conduction band.