A very usual picture for tourists in Pisa (Italy), with a variety of options.

Despite what you may think the blue blocks appearing at the top of the cube are the same colour on both images.

Start by looking at the white dot. For a long time. Then, move backwards slowly and you will notice that the red circle will disappear progressively.

Eclipse of Planet Mars

On the drawing below, the grey colour of the squares marked as A and B is exactly the same. Still…

From Edward H. Adelson, Professor of Vision Science at MIT

It is not often that you find the most simple and lighter illusions described as actively than on the web site for the MIT (Massachussets Institute of Technology).

Lightness Perception and Lightness Illusions
interactive movies based on a paper by Edward H. Adelson

Anamorphosis is a technique to hide images in another image. It has long been used by painters. And some of them are quite well known like the The Ambassadors (Hans Holbein the Younger, 1533) below.

The Ambassadors (Hans Holbein the Younger, 1533)

In this painting, there is a highly deformed skull appearing like a whitish blade at the bottom of the painting.

Learn everything about anamorphosis from Jessica Palmer in ScienceBlogs.

Instructions:
1. Stare at the image for 10 seconds
2. Look at something — your hand, a book, your friend
3. Enjoy!

For more on illusions — and the delight of our buggy minds — see Al Seckel’s 2004 TEDTalk.

It seems difficult to believe but these drawings are nothing more than chalk on the surface of a pavement leading to perspective effects inducing dizziness and vertigo.

Rescue

Waterfall

Source: Julian Beever.

Rotate it upside down and the hussar becomes its own horse.