Low-voltage solar garden lights use a device called an LED or light emitting diode to create light. LEDs are long-lasting, rugged and use far less energy than traditional incandescent light bulbs, making them perfect for solar-powered, outdoor garden lights. They work by forcing electrons across a gap between two slightly different types of silicon. When the electrons cross the gap, they release some of their energy in the form of a photon of light. LEDs can be designed to create pretty much any shade of light from infrared to ultraviolet, but the LEDs in garden lights usually release white or yellow light.
Garden lights have a built-in battery charged by a photovoltaic cell, also known as a solar cell. In some models, the cell is built into the garden light. This is an easy design to work with, since you don't have to run wires to a separate solar cell, but it means that all of your garden lights have to be in sunny spots. In other models, the solar cell is separate from the light, allowing you to position the cell just right to catch the sun's rays.
All solar garden lights turn on automatically in the dark, but different ones use different methods to detect that the sun has gone down. In some solar powered lights, a circuit monitors how much power is coming from the solar photovotaic cell. When the sun starts to go down, the solar output drops and the light knows it is dark and automatically turns on. Other lights use a separate component such as a photoresistor to measure outside light. When light shines on a photoresistor, it resists the flow of electricity only a little. When the sun sets and there is no more light on the photoresistor, however, its resistance goes up. The control circuit registers the decrease in resistance and turns the lights on.