Refraction. Computer artwork of a cherry in a glass of water. The cherry is demonstrating the concept of refraction. Light travels at different speeds in different optical mediums, such as water and air. When the light from the cherry exits the water its angle of incidence with the water surface is increased. Since the brain assumes that light travels in straight lines, this leads it to believe that the cherry, when in the water, is in a different place from its true position. We see the cherry refracted off two main surfaces: the curved sides of the glass and the flat surface of the water. This provides the illusion in two different ways - hence three segments of cherry are seen in three different places.