The distance between a convex lens and a plane mirror is \(10\) cm. The parallel rays incident on the convex lens, after reflection from the mirror form image at the optical centre of the lens. Focal length of the lens will be:

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. \(50\)

For a convex lens, the distance of the object is taken on X-axis and the distance of the image is taken on Y-axis, the nature of the graph so obtained is :

1. Straight line                     

2. Circle

3. Parabola                          

4. Hyperbola

The diameter of a plano-convex lens is 6 cm and thickness at the centre is 3 mm. If the

speed of light in the material of the lens is $2\times {10}^8\raisebox{1ex}{$m$}\!\left/ \!\raisebox{-1ex}{$sec$}\right.$, the focal length of the lens is

1. 15 cm                         

2. 20 cm

3. 30 cm                         

4. 10 cm