When you look into a plane mirror, you are seeing light rays that have reflected from somewhere else. Since light rays always travel through the air in a straight line, they appear to be coming from behind the mirror. Your brain interprets these rays as an image that is located behind the mirror. There is no actual light behind the mirror, yet this is where we see the image. When an image is located in a place where there is no actual light, it is called a virtual image.