Final answer:
A flat piece of glass, such as in a window, does not form an image because it has an infinite focal length.
Step-by-step explanation:
A flat piece of glass, such as in a window, can be considered as a lens with an infinite focal length. Since the focal length is infinite, the image distance (di) becomes infinite as well. This means that no image is formed by a flat piece of glass, and there is no relation between the image distance (di) and the object distance (do).