Question

A convex (converging) lens can be used to light a fire (if you are stranded on a desert island,
for example). Consider a lens with focal length 0.200 m. How far away must a piece of
paper be held to set in on fire most effectively? Give a reason for your answer.

Answer 1

Solution

A convex lens collect all the rays comming from a object which is at infinite at the focus of the lens.

So same case happen when a piece of paper is placed at the focus of lence to burn fire, sun is at infinite distance from lens.

So to set fire most effectively paper should be placed at focal length of lence which is 0.200m