With the help of ray diagrams, explain the formation of images by a concave lens for the following positions of the object:
(i) Object at infinity
(ii) Object between infinity and optical centre O of the lens.

Image formation by a concave lens:

(i) When object is at infinity: The rays coming from an object, at infinity are parallel to each other. On refraction through the lens, the rays diverge from the focus. A virtual, erect and extremely diminished image is formed at the focus F. 


Fig. Image formed by a concave lens when the object is at infinity

(ii) When object is between infinity and optical centre O of the lens: A ray AN parallel to the principal axis, after refraction, appears to come from focus F₁. Another ray coming through a point source from infinity, passes undeviated through the optical centre O.
The two rays appear to diverge from the point A'. Thus A' is the virtual image of A.
Hence A'B' is the complete virtual, erect and diminished image of the object AB.

For all positions of the object the image is always virtual, erect and diminished and is formed between focus F₁ and the optical centre O.


Fig.  Image formed by a concave lens when the object.