Draw ray diagrams showing the image formation by a concave lens when an object is placed:
$(a)$ at the focus of the lens
$(b)$ between focus and twice the focal length of the lens
$(c)$ beyond twice the focal length of the lens

(N/A) For a concave lens,the image formed is always virtual,erect,and diminished,regardless of the object's position.
$(a)$ When the object is at the focus $(F_1)$,the rays appear to diverge from a point between the optical center $(O)$ and the focus $(F_1)$. The image is virtual,erect,and highly diminished.
$(b)$ When the object is between the focus $(F_1)$ and twice the focal length $(2F_1)$,the image is formed between the optical center $(O)$ and the focus $(F_1)$. The image is virtual,erect,and diminished.
$(c)$ When the object is beyond twice the focal length $(2F_1)$,the image is formed between the optical center $(O)$ and the focus $(F_1)$. The image is virtual,erect,and diminished.