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I am sure that almost everyone at least once in their lifetime witnessed some street lights being on even in the morning. I wonder why they keep them on in the morning.

Possible Explanations

  • Electricians want to see which street lights are broken in the morning so this way they could identify those malfunctioning easily.

  • Since the only obvious changing variable is the daylight, I would suspect maybe there's some relation between the lifetime of a circuit and temperature. Resistivity and length of the resistor increase with the increase in temperature. But I'm not sure how this might affect the lifetime. This reasoning was due to a direct observation in obvious change in the temperature of the environment.

  • For some reason which is probably quite unlikely but the system malfunctions and some of the street lights keep radiating.

Is there any physics-related reason why they keep them on (e.g. not to reduce the lifetime of the resistors and etc.)?

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Assuming it's not deliberate, two things come to mind:

  1. light-sensitive switches will have significant hysteresis to avoid flickering on and off in dim light. So they will require brighter light to switch on in the morning than to switch off in the evvening.
  2. human vision perceives a given light level as being brighter during a transition from dark to light than light to dark (visual accommodation). So, coming from dark, it will seem quite bright in the morning when the streetlights are still on.
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