Question

when would virtual image produced by convex lens? why doesn't virtual image drawn on a screen?

Answer 1

The image formed in convex lens is always real and inverted except the situation when the object will be placed between focus and optical centre of the lens. At that time, the image will be virtual, erect and highly magnified.

There are two types of images. One is called real image and second one is the virtual image.

Real image can be hold in a plane of paper or screen. The virtual image can not be hold on a screen. It is formed by the rays which are appeared to be coming or intersecting at a point which are either behind a mirror or  in front of a lens. That's why virtual images are not drawn on a screen.