The Light Sensor uses one photoresistor to detect the luminance of environment. The darker of the environment, the lower voltage will be provided.
 
Working Principle
The Light sensor also call as Light Dependent Resistor(LDR), the physical property will change under different light source as the name implied. Depend on the model of different LDR, the resistance value and curves have difference, but all follow the principle:
When the Illumination is increase, the resistance of the resistor will reduce. When the Illumination is decrease, the resistance of resistor will increase.