Light Sensitive Plane

Remember that a complete circuit sends electricity from the power source (the battery) through the wires and components and back to the power source again. Some materials allow electricity to pass through them easily. These materials are known as conductors and include metals like gold, aluminum, copper, and iron. Other materials do not allow electricity to pass through easily. These are known as insulators and include materials like glass, rubber, paper, air, and plastic.

Some materials don't completely block electricity but resist it. Manufacturers can use these materials to make components that reduce the flow of electricity in a circuit. These components are known as resistors. Generally, resistors are static. They provide the same amount of resistance on a circuit all of the time. However, the photo resistor is a special type of resistor. The resistance it places on a circuit varies (or changes) based on how much light is in the surrounding environment. When a lot of light is present, resistance is small (allowing electricity to flow) so the LED lights up. When the surrounding light is limited, resistance increases so the LED gets dimmer or turns off entirely.

For more information on resistors, see this tutorial.

For more information about how resistance impacts electric voltage and current, see this tutorial.