Pendulum Illusion - Pulfrich’s Pendulum

11 April 2010

Ingredients

 

  A pendulum of some kind - a weight on a piece of cotton would be ideal   A pair of dark sunglasses

 

Instructions

 

Find somewhere that is quite badly lit -probably best somewhere that is artificially lit, and feels quite dim.

Set the pendulum swinging from something solid in as straight a line as possible.

Put the sunglasses over just one eye, and look at the pendulum.

Try holding a finger just below the bottom of the pendulum's swing. Do you notice anything odd happening.

Result

This one is quite tricky to get right, and people who have problems with depth perception may not be able to see it anyway, but you should see the pendulum appear to move in and out either side of your finger, even though it is actually still going in a straight line.

Explanation

This is an optical illusion that depends on two features of your eyes. The first is how you perceive the depth (distance) of objects which are close to you. You have two eyes, in slightly different places, so they see slightly different views of the world, particularly they see objects which are close to you as in a different place.

Try holding a finger in front of your nose, and then first close once eye and then the other. The finger should appear to move across the background, because your eyes are in different places.

The second feature of your eyes that this illusion takes advantage of is one of the ways your eyes adapt to low light levels. First they dilate your pupils and become more sensitive by accumulating more light sensitive chemicals, but eventually they have to resort to taking longer to collect light, (a bit like taking a longer exposure on a camera, but with more complex image processing as it is video) this means that it takes longer for your eyes to get the images.

Hence why it is so hard to play ball games as it is getting dark. You can still see ok, but when you swing for the tennis ball you are aiming for where it used to be, not where it is and you miss.

If put dark glasses over your left eye the processing will be slowed down for it's image, but not for the right eye. This means that your brain effectively receives two images from slightly different times at once. If an object is moving it will appear in different places for your two eyes.

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