State the rules used fro drawing ray diagrams for the formation of an image by a concave lens.

Explanation

A concave lens always forms a virtual, erect and diminished image of a given object.
i. Consider an object AB placed between `F_(1) and 2F_(1)` in front of a concave lens.
ii. The incident ray AP is parallel to the principal axis. After refraction it travels along path PQ. When PQ is extended backwards, it passes through focus `F_(1)`.

iii. Now, the incident ray AO passes undeviated through optical centre O. This ray meets the backward extension of ray PQ at A' i.e., A' is the image of A.
iv. As point B lies on principal axis, its image B' will lie on principal axis beneath the point A'. In this way, A'B', the virtual, erect and diminished image of object AB is formed by a concave lens.