Weird moving elevator buttons, why so blue?
April 24, 2011 9:00 AM   Subscribe

Why do the elevator buttons seem to be moving? My eyes deceive me!

For the past month or so, I've noticed that whenever I'm in the elevator of my building and looking at the floor that's selected, I'm able to make the button move, visually, by focusing my eyes on it and moving my head. Try the same thing with the buttons that aren't on, and nothing happens.

Is this a known optical illusion? Does it have something to do with blue light in particular, or wearing glasses?

The buttons in question look a lot like these, only they're square.
posted by greatgefilte to Science & Nature (6 answers total) 1 user marked this as a favorite
Yeah, blue light it is. The index of refraction is frequency dependent — light bends more for blue, making it a bit harder for your eyes to focus. Try blacklights sometime, they are even more difficult to focus upon.
posted by adipocere at 9:03 AM on April 24, 2011

Yeah those blue LED christmas lights drive me nuts for the same reason.
posted by you're a kitty! at 9:30 AM on April 24, 2011

Look at this prism. If you rotated the triangle counterclockwise, the blue would move faster and further than the red.
posted by StickyCarpet at 9:31 AM on April 24, 2011

Yep, it's called "chromostereoscopy" and it's been banging around in science/vision/entertainment for a few hundred years, starting with Einthoven's observations.

Light focuses at different depths because the index of refraction (the degree to which light is bent as it passes through a material) is dependent on the wavelength. The degree to which light can be bent by a given material (like, say, the lens of your eye) is important because it determines how focused a given color/wavelength can be.

Red (long wavelength) and Blue (short wavelength) are the extremes of the spectrum. If you'd see an effect with the naked eye, it would be between blue and red.

That's not the whole story. At an extreme angle, the corneal surface and lens of your eye splits up color like a prism, causing the blue spot and the red spot to fall on different parts of your TWO retinae. The first paragraph and second image on this page tell the story very well.

Get it? You need two eyes to see depth. Because the angle to the light source is different for each eye, the image of blue is shifted a little bit relative to image of red from your left eye to your right eye and so you perceive illusory depth. In other words, it's dependent on angle, too.

Here is one decent explanation, and the introduction to this paper goes a bit further.

There are ChromaDepth glasses that exploit/enhance this effect, as well. Unfortunately, like many commercial products, the language around the product is stupidly distorted and confusing. Holo-blahblah! There's nothing holographic about the imagery. They just shove a diffraction grating in front of your eye to enhance the red-blue shift.
posted by fake at 9:43 AM on April 24, 2011 [4 favorites]

Our retinas are extremely sensitive to yellow-green light (daylight), and much much less sensitive to blue light.

This in turn means that the lens of the eye is adapted to focus yellow-green light, and has to strain quite a bit to focus blue light.

If you're staring at something that has a bunch of daylight and a little blue light, your lens is going to adjust to focus the daylight-colored stuff and the blue stuff will look really blurry.

If the sun were blue instead of yellow, we would easily see blue light but have to strain to see yellow-green light.
posted by miyabo at 10:00 AM on April 24, 2011

LED indicators are often pulsed. The pulsing frequency is high enough that you can't directly detect it, but when you move slightly you see the secondary effects like the display jumping around.

When I was a kid I used to see this all the time by staring at my bedside clock and gently grinding my teeth.
posted by intermod at 9:56 PM on April 24, 2011

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