A convex lens is used to form an image of an object on a screen. If the upper half of the lens is blackened so that it becomes opaque, then:

A point object is placed at a distance of \(60~\text{cm}\) from a convex lens of focal length \(30~\text{cm}\). If a plane mirror were put perpendicular to the principal axis of the lens and at a distance of \(40~\text{cm}\) from it, the final image would be formed at a distance of:

A screen is placed \(90~\text{cm}\) from an object. The image of the object on the screen is formed by a convex lens at two different locations separated by \(20~\text{cm}\). The focal length of the lens is:
1. \(18.81~\text{cm}\)
2. \(20.04~\text{cm}\)
3. \(13.01~\text{cm}\)
4. \(21.39~\text{cm}\)

A card sheet divided into squares each of size \(1~\text{mm}^{2}\) is being viewed at a distance of \(9~\text{cm}\) through a magnifying glass (a converging lens of focal length \(10~\text{cm}\)) held close to the eye. What is the magnification produced by the lens? 

A convex lens A of focal length \(20~\text{cm}\) and a concave lens \(B\) of focal length \(5~\text{cm}\) are kept along the same axis with the distance \(d\) between them. If a parallel beam of light falling on \(A\) leaves \(B\) as a parallel beam, then distance \(d\) in \(\text{cm}\) will be:
1. \(25\)                         
2. \(15\) 
3. \(30\)                         
4. \(50\)