Anurag Sinha World
Mystery of the spinning dancer
The spinning dancer is an astonishing optical illusion. The dancer appears to spin clockwise for some and anti-clockwise for others. Refer to http://www.news.com.au/heraldsun/story/0,21985,22556281-661,00.html for viewing the illusion.
The reason for the illusion is very simple; the dancer is rendered in a single color - black. As a result, the mind has to make certain assumptions for body features.
Consider the following frame from the animation.
Which leg is resting on the ground? Is it the right leg, or the left?
Now have a look at the following two frames. By appropriately drawing some lines to distinguish features, the same figure can be made to appear with left leg on the ground or right. Hence, the illusion.
The brain fills in the missing lines and thus the figure appears to rotate either clockwise or anti-clockwise
Now a trick for those who are not able to spin the dancer the other way round. Cover the torso and head of the dancer with your left palm and focus just on the legs. If you see the dancer always rotating the clockwise, then focus on the right leg. The right leg would emerge out at 3 pm positon, move towards your face, and fade back at 9 pm position. When the right leg moves back at 9 pm position, make your mind believe that the leg is actually emerging from screen and moving towards your face. As soon as you are able to visualize it, remove the palm, and voila, you will see the dancer rotating in the anti-clockwise direction.
(For people who always see the dancer rotating anti-clockwise, replace clockwise with anti-clockwise, left with right and 9pm with 3pm in the above paragraph)