Optical Illusions

On a past trip to Nürnberg, I visited the "Tower of the Senses" (Turm der Sinne).  It's a great mix of science and fun -- focusing on perception or reality and fooling our senses (optical illusions).  It is located near the city center, right in one of the small towers along the old city wall surrounding Nürnberg.

One of the directors of the Tower is a professor at the University of Freiburg, and specializes in this subject area (his personal web site has been referenced by the New York Times).

The visit reminded me of the optical illusions I've come across in the past, and was inspired to look them up.

Relative Color

The gray bars below are the same color:

NOTE: To test this claim (and others below), copy the image to your computer, open the image in Paint, then cut and move the bars so they are next to each other.

This one is even crazier - the  squares labeled A and B are the same color:

The effect works with white too.  There is no triangle - the "white" is the same color as the rest of the background: 

 

Relative Size

You've probably seen this before - the yellow bars are the same length:  

Likewise, the center circles are the same size:

Again, these figures are the exact same size.  Copy, cut, and paste to verify: 

Relative Symmetry

The figure in the center is a perfect circle - the lines just make it look distorted:

 
This one is wild.  Below is not a spiral, but rather circles! 

Other Effects 

This is another common picture - there are no dots at the intersections, but your brain creates them:

Look at the image below and more your head (closer/further from the screen or tilt up and down), and the circles seem to rotate: 

In the last one, the images will appear to move without any special action on your part (your small natural eye movement is enough): 

I find these phenomena so interesting.  There are a lot more available here, with good descriptions as well:

http://www.scientificpsychic.com/graphics/