Concave mirrors produce real and virtual images. The object is at P1. Ray 1 goes straight towards the mirror and reflects through F (focal point). Ray 2 passes through the focal point and reflects straight back. The point where the two reflected rays intersect is the image. The focal point is the point where rays parallel to the optical axis of a mirror converge to a point. There is an equation to find the distance to the image or object from the mirror. This is 1/F = 1/Di + 1/Do. The ratio of 1 over the focal length is equal to the ratio of 1 over the distance of the image plus 1 over the distance of the object. There is also an equation to find the ratio of the size of the image. Hi, to the size of the object, Ho. This is the magnification (M = Hi/Ho = -Di/Do).