AC power
April 16, 2005 4:07 PM   Subscribe

If I have AC power at 50Hz, does it mean my lightbulb is flickering 100 times a second and I just can't see it? Is this why, when passing a street light in a speeding train, it can appear as a series of dots in your vision?
posted by Pretty_Generic to Science & Nature (11 answers total)
 
I believe in the case of incandescent bulbs, the frequency is so fast that the bulb can't die down quickly enough to be off. So, even if you filmed it in slow motion, I'm guessing the most you'd see is a 50Hz cycle of slightly dim - brighter.
posted by odinsdream at 4:19 PM on April 16, 2005


Yes, the filament actually is white hot, and can't flicker with the AC. You can strobe tune a turntable with those little striped cardboard discs you put on the center, but you need a fluorescent light.
posted by weapons-grade pandemonium at 4:31 PM on April 16, 2005


But it gets slightly brighter and dimmer, right?
posted by Pretty_Generic at 4:36 PM on April 16, 2005


Wave your hand in front of your monitor and count your fingers.
posted by weapons-grade pandemonium at 4:36 PM on April 16, 2005


Those streetlights are probably lit with charged gasses. Lights like that DO flicker, the frequency depends on line power and the design of the circuitry powering it. You might even have similar lights at home... like your fluorescent lights! Although most of these nowadays, unless it's a really old fixture, will be flickering in the kHz range, and won't be visible at all.

Older, cheaper designs, in your case, really would flicker at 100 Hz (double your line frequency).
posted by shepd at 4:38 PM on April 16, 2005


Brighter and dimmer, yes, as odinsdream said, but the difference may not be noticeable.
posted by weapons-grade pandemonium at 4:39 PM on April 16, 2005


Here's the spectra of some street lights, it appears Mercury and Sodium vapour are particularly popular to light them with.

For a very SIMPLE light working on the same principle, you may find older power bars have a flickering orange "ON" light. These are lit by charging helium.

A little more info.
posted by shepd at 4:41 PM on April 16, 2005


Light bulbs do flicker at 100 Hz (120 Hz in the US) but it is not on/off but brighter/dimmer because, as odinsdream and w-gp have said this is only a variation on top of a white hot filament which cannot thermally respond so quickly. Street lights and fluorescent bulbs also have flicker at harmonics of the ac frequency (100 Hz, 200 Hz, etc.) and in addition they will flicker at ~10 kHz because many are powered by dc-dc switching power supplies which achieve the high voltage necessary for gas discharge. Awful nuisance to some of us. More than you ever wanted to know.
posted by fatllama at 5:01 PM on April 16, 2005


Yeah, I've noticed driving on the highway at night, with street lights on, that car wheels appear to freeze or spin backwards with a strobe effect. Street lights definitely flicker. However I agree that incandescent are not going to flicker in any sense. The cooldown time is so long that there is probably no measurable change in brightness, even with a high speed camera.
posted by knave at 12:29 AM on April 17, 2005


from looking around on the net it seems like the explanation for flickering lights from the train is that they're still using fairly old equipment. the modern thing seems to be high frequency ballast, which will, i guess, also vary in brightness, but at a much higher rate (whch might explain strobing on car wheels, but wouldn't give flicker through a train window).

alternatively, i guess you could be seeing the light through tree branches?
posted by andrew cooke at 6:14 AM on April 17, 2005


For a very SIMPLE light working on the same principle, you may find older power bars have a flickering orange "ON" light. These are lit by charging helium.

Neon, actually. Were you thinking of helium-neon lasers?
posted by Good Brain at 2:12 PM on April 19, 2005


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