Our brain are set up to receive and interpret messages from the eye. Optics, a branch of physics, studies the interaction of the light and the eye. This interaction plays an important role in optical illusions. Optical illusions use light, colors and other features to trick the mind into thinking of things that are or aren't there. For example the Lilac Chaser Illusion. In this optical illusion, the viewer sees purple blurry dots arranged in a circle around a focal point. As you stare at the plus sign, it will appear as if a space is running around the circle of lilac discs. But after the viewer continues on staring, they will eventually see a green disc moving around the circle instead of the space. Then if the viewer continues on staring, they will then see the disappearance of the blurry dots and only see the green dot moving. We perceive movement and when we see something at one point and then at another, we believe that it is in motion. Also, when blurry objects are located in the periphery of our visual field, eventually they disappear when we have our eyes fixated on a certain spot.