Here’s a picture of a fan being illuminated by a fluorescent light:
I’ve known “fans turn orange and blue sometimes” since I was a kid because I liked fans even as a kid, and sometimes they did this thing. I’ve recently become interested in what’s going on. I related it to friends and they didn’t even relate to fans turning orange and blue sometimes. So I became more interested. Luckily, this is not one of those elusive things where it doesn’t show up on a camera, hence the picture above.
I should say the picture is a bit misleading. Usually, the orange-blue pattern will be moving around, sometimes fast, sometimes slow, against or with the fan, and a bit chaotic. But I have the fan hooked up to a variable supply and managed to get the fan speed such that the pattern was almost stopped or moving slowly. There’s always a bit of chaotic movement because fan speeds are affected in complicated ways by the mechanism itself and the air system it is in. So, if I put my hand above the fan, the fan speed would slow down, and that would heat up the electromagnets in the motor more, which would in turn… and at that speed the entire fan vibrates more which slows down the fan… you get the point.
Lots of lights flicker. In North America, lots of them flicker at 120 Hz. This is because in North America, electrical outlets give AC at 60 Hz, and the lights we’re talking about do not care if current is going “forward” or “backward” — they light up either way. For those not perfectly familiar with AC, electricity is a path in which electrons move. Typically you visualize the electrons moving in a circuit in one direction, and when using a battery this indeed happens. But this is not the only way to convey electrical power — you can also slosh the electrons back and forth. And this is what outlets in North America do — electrons are sloshed one way, and then sloshed back to where they were 60 times per second. The lights we’re talking about get bright whenever electrons are moving, regardless of direction. Sloshes happen 120 times per second, so lights flicker at 120 Hz. I should also say there is time between the sloshes where the electrons aren’t moving — the electrons do not start moving back as soon as they stop moving forth — and this is when the light is dark. And I should say the electrons move smoothly, like the pendulum of a grandfather clock, not jerkily like I might’ve implied.
You can’t see the flicker with your eyes for 2 reasons. First, lights are slow; some more than others. An incandescent bulb (that is, a typical light bulb) will get bright when electrons are moved through it, and gradually dim after they’ve stopped. If you’ve ever put some metal in a fire and got it red hot, and then taken it out, you’ll notice it stays red hot for a while as it cools off. The exact same thing is happening, just at a higher temperature. This doesn’t mean lights don’t flicker, it just means they aren’t flickering between completely on and completely off. Fluorescent lights (according to simple textbooks) create light through fluorescence of phosphor, which is quicker to dim, which is one reason they are notorious for flickering and incandescents aren’t.
The second reason you can’t see lights flicker is that your eyes are slow. Digital cameras, on the other hand, are fast, and with one you can see the flicker of lights. I just had to look at my LCD screen, but you might have to fiddle with some settings, or worst come to worst, take a few pictures at the exact same settings and an exposure of, say, 1/500 seconds, and you’ll notice some pictures are lighter than others. With a video or the LCD screen, you might notice very slow flickering. This is because the camera’s capture frequency and the light flicker frequency are slightly different, and is called beating, but it’s still evidence of the light flickering.
Fluorescent lights are a bit more complicated. The fan is orange and blue, but all I’ve mentioned is bright and dark flickering. Fluorescent lights are tubes containing mercury vapor and coated with phosphor. When they are turned on, an electrical arc goes through the vapor, creating light which appears bluish-white to us, and also contains UV. The UV is absorbed by the phosphor, which then gives off light which appears orange to us. So if the arc was consistent, or even flickering at 120 Hz and being looked at by slow human eyes, the fluorescent light would appear white.
Remember how I said some lights dim faster than others? Well, the mercury vapor arc dims much faster than the phosphor. So fluorescent lights also flicker in color, from more orangey to more bluey. This is called chromatic flicker.
Finally, the fan. The fan has two blades, and it is rotating at about 60 Hz. That means, as we follow one blade around one revolution, it witnesses two flickers of the fluorescent light — from blue, to orange, to blue, to orange, and back to blue. The other blade is doing the exact same thing, just opposite the first blade. Taken with a slow shutter speed (or, again, slow human eyes) you see a disc with two orange parts and two blue parts.