Question

prove by a diagram that virtual image cannot be obtained on a screen ?

Answer 1

Top: The formation of a virtual image using a diverging lens. Bottom: The formation of a virtual image using a convex mirror. In both diagrams, f  is the focal point, O  is the object and I  is the image, shown in grey. Solid blue lines indicate light rays. It can be seen that the light rays appear to emanate from the virtual image but do not actually exist at the position of the virtual image. Thus an image cannot be seen by placing a screen at the position of the virtual image.In optics, a virtual image is an image formed when the outgoing rays from a point on an object always diverge. The image appears to be 

Answer 2

This is the diagram. I suppose you have known the reason for this with Gowari Shankar's answer.