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There are two kind of lenses, converging and diverging lens. For a converging lens, parallel light rays converge to a point at the other side. This point is called the focus and the distance between the lens and the focus is called the focal length. For a diverging lens, parallel light rays will diverge away from each other after passing through the lens. However, these diverging rays seems like to come a point at the side of the incoming rays. We can consider this imaginary point as the focus and the distance between this point and the lens is the focal length of the diverging lens.