Water flowing uphill in this impossible illusion.
Optical illusion has been a favorite topic for many artists who have created some masterpieces and they are the source of research and appreciation still now. Optical illusions are of many types and they can occur naturally also. Some very good optical illusion works have been done by the famous Dutch graphic artist M.C Escher. This artist has been famous for his drawings which were paradoxical and mathematical. He never had any proper training in mathematics but some mathematical concepts attracted him such as the subjects like plane symmetry. One of his famous works was Waterfall done in 1961. The 2D image of the waterfall gets interpreted into a 3D image by our brain. The optical illusion occurs when our brain perceives a particular image of an object which is quite different from the actual dimensions of the object. If the waterfall 2D image is to be considered then actually the 2D image is perceived as the 3D image by the brain. The 3D image that the brain sees actually cannot exist in the real universe. It is also a kind of impossible object illusion. In the example of the waterfall the impossible object is not actually invented by Escher rather it is a mathematical concept called Penrose triangle. Another famous work of Escher is the Drawing Hands. Though there has been continuous work going on optical illusions still lots more have to be done.