Question

The sun is 150,000,000 km from earth; its diameter is 1,400,000 km. A student uses a 5.6-cm-diameter lens with f = 10 cm to cast an image of the sun on a piece of paper.
Where should the paper be placed relative to the lens to get a sharp image?

Answer 1

This question is based on basic understanding of lens working and all the numerical values given are irrelevant here.  

Now steps to answer this question are-

A) What type of lens should be used?

 Since it is given that you want an image on a piece of paper, then that lens must be CONVEX because only CONVEX LENS PRODUCE REAL IMAGES.

B) How far from optical centre will we get clearer and sharper image?

The distance to obtain clear and sharp image should be equal to the focal length because focal point is the best place where we can get sharpest image possible of a light source present at infinity (sun is considered to be at infinity).

So the answer is 10cm, that is the focal length.