Grab some sunglasses and watch this video to experience the Pulfrich illusion

Illustration for article titled Grab some sunglasses and watch this video to experience the Pulfrich illusion

Having a good weekend? Got some sun? Great. Grab some sunglasses and stare at the computer screen. We’ll show you an example of the Pulfrich illusion.

The Pulfrich illusion, or Pulfrich effect, was first noticed by Carl Pulfrich, who described it in 1922. All you need is something that acts like a pendulum swinging in front of you. Grab a pair of sunglasses, and put one lens in front of one eye. Look at the pendulum again. It should suddenly look like it’s moving in an elliptical path, instead of just swinging from side to side. The same thing can be done with film of objects moving in a circle on a plain. If you don’t happen to have a pendulum handy, just click the video link and watch through your single lens. It should look suddenly three dimensional.

Pulfrich theorized that this effect as due to the brightness reduction in one eye, which the eye responds to with a tiny delay in its optical information. This gets your brain to interpret the entire thing as three dimensional motion.

This isn’t unprecedented. All three dimensional images are made up by the brain. Your eyes see two slightly different pictures – due to their slight distance apart from each other. The brain melds these two images together to give us three dimensions. When looking a film, or two dimensional motion, the eyes see the same image. But if one eye sends a delayed image, while the other eye sends the its own image “on time,” the two eyes once again see two slightly different images, and the brain can do what it always does and weave them into one three dimensional image again.

Top Image: SXC



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OK, of all the dubious optical illusions IO9 has paraded in front of my visual cortex these past several years, this one actually performed as advertised.