Most street lights are sodium lamps. Sodium lamps produce an orange glow which is not very attractive, but they are extremely efficient. Electricity runs through a tube filled with sodium and a bit of argon gas. When the light first turns on, the sodium is solid, but as it heats up it evaporates into the tube. When an electron hits a molecule of sodium, the sodium absorbs some of the energy. The molecule then releases the energy as a photon of orange light.
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Timers are used to control many street lamp systems. Usually, an electronic clock is built into the entire system. The clock turns the street lights on in the evening and off in the morning automatically. In some systems, the timer will vary slightly day by day, keeping the lights on for longer periods in the winter months and shorter ones in the summer.
Some street lights use a different system to turn the lights on at night and off during the day. These lamps use photodetectors, small electric components which are sensitive to light. One of most common type of photodetector is a photoresistor. Photoresistors resist the flow of electricity, but their resistance drops when light hits them. The more resistance in a circuit, the less electric current flows through. In a photoresistor powered street light, a small current runs through the resistor. When it becomes dark, the resistance increases and the current drops. This tells the circuit to switch the light on until daylight comes and the current increases again. Like timers, these systems adjust their timing during darker months of the year. Unlike timers, they can also detect dark, overcast weather and switch on the lights to help compensate.
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