LDR (light-dependent resistor)
A light-dependent resistor (LDR) or photoresistor whose resistance decreases when exposed to light. LDRs are semiconductor devices (typically cadmium selenide) with resistance inversely related to light intensity. They are nonohmic and used in light-sensing applications.
Real World
Automatic street lights use an LDR in a potential divider: at dusk, ambient light intensity falls, the LDR's resistance rises, increasing the voltage across it until a switching circuit turns the lamp on.
Exam Focus
In potential divider questions, clearly state which component's voltage increases and why — trace the logic from light change → resistance change → voltage change.
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