# 33.2: Introduction


Light travels in a straight line at a constant speed, unless it interacts with a material.  When light travels from one medium to another, one material to another, the light’s velocity changes where the two mediums meet.  The apparent location of an object may not be the actual position of the object if the light has travelled through more than one medium, for example, if the light passes through both air and water as it travels to you.  If light passes from one medium to another at an angle, then the path of the light will change.