[Physics] Why doesn’t the brightness of a bulb change with time

electricitythermal-radiation

Household bulbs get alternating current, which means that the voltage of source and current in circuit keep changing with time, which implies that the power supply isn't constant. However, we don't see any changes in brightness of the bulb. Why is that ?

Best Answer

Two reasons:

  • An incandescent bulb glows not (directly) because it has electricity going through it, but because it is hot. Even when the power going through the bulb decreases, it takes some time for the filament to cool down. Even once the bulb is turned off, it takes some time (a fraction of a second) for the light to fade.
  • What variation there is in the light is too fast for our eyes to see.

You can see the AC flicker in slow motion videos if the camera has a sufficient frame rate, for instance this one.

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