An optical phenomenon is a phenomenon that is related to the behavior of light. A RAINBOW is an arc of colors in the sky, usually seen after rain. Parallax is an effect seen when an object appears to move against a background when viewed from different angles. A MIRAGE is an optical illusion caused by atmospheric conditions, which makes distant objects appear to be closer than they actually are. Diffraction is the bending of light around an obstacle, which causes a wave pattern.