The document discusses the working of a light dependent resistor (LDR) and designing a circuit to demonstrate its varying resistance. It begins by introducing LDRs and explaining that their resistivity decreases when light is absorbed. It then describes building a circuit using an LDR, resistor, battery, LED, and breadboard. The circuit works by changing the LDR's resistance based on light intensity, which controls current flow through the LED. Finally, applications of LDRs include light sensors for street lamps, alarms, and automatic controls that require detecting light levels.