Perhaps the best place to start in our trip through the world of illusions is one of the most well known of all and one that you are likely to have seen before many times. Known as "Rubin's Vase" this simple illusion devised by Danish psychologist Edgar Rubin can be perceived as both a vase or as the profiles of two human faces gazing at one another. This is often considered to be one of the best examples of an optical illusion.
Image Credit: John Smithson - Public Domain
The illusion works in that the contours of one image match the contours of the other in such a way as to confuse the brain in to perceiving two interpretations of what is there."One can then state as a fundamental principle: When two fields have a common border, and one is seen as figure and the other as ground, the immediate perceptual experience is characterized by a shaping effect which emerges from the common border of the fields and which operates only on one field or operates more strongly on one than on the other."
- Edgar Rubin, 1915
OK now I did not see that coming!!!!!!