Opponent-Process Theory - The theory that opposing retinal processes (red-green, yellow-blue, white-black) enable color vision. For example, some cells are stimulated by green and inhibited by red; others are stimulated by red and inhibited by green.
One of my favorite things that was shown to me during my elementary age years was an optical illusion. It was the very famous Rubin Vase (pictured below) which you can see in either two ways - as a vase or as two faces that are facing each other.
Mouth of Flower - Octavio Ocampo This is good art because it is an optical illusion. It could either be just some flowers with a butterfly, or you can also see a woman's face. It is very creative in a subtle way.
Silhouette by Alexander Khokhlov Artist and photographer Alexander Khokhlov took this brilliant photo entitled “Silhouette”, as a part of the project “Weird Beauty”. Model is Alexandra Romanova and make-up artist is Valeriya Kutsan.