Optical Art is a mathematically-oriented form of (usually) Abstract art, which uses repetition of simple forms and colors to create vibrating effects, moiré patterns, an exaggerated sense of depth, foreground-background confusion, and other visual effects. In a sense all painting is based on tricks of visual perception: using rules of perspective to give the illusion of three-dimensional space, mixing colors to give the impression of light and shadow, and so on. With Optical Art, the rules that the eye applies to makes sense of a visual image are themselves the "subject" of the artwork. In the mid-20^th century, artists such as Josef Albers, Victor Vasarely, and M.C. Escher experimented with Optical Art. Escher's work, although not abstract, also deals extensively with various forms of visual tricks and paradoxes. In the 1960's, the term "Op Art" was coined to describe the work of a growing group of abstract painters. This movement was led by Vasarely and Bridget Riley.