Two identical circular loops \(P\) and \(Q\) each of radius \(r\) are lying in parallel planes such that they have common axis. The current through \(P\) and \(Q\) are \(I\) and \(4I\) respectively in clockwise direction as seen from \(O\). The net magnetic field at \(O\) is: 
Find magnetic field at midpoint O. Rings have radius $R$ and direction of current is in opposite sense. 

The given figure shows image formation by a lens. Analyse the figure and answer the following questions : 
What is the type of lens used for image formation in the given ray diagram ?
If the real image is formed at a distance of \( 30 \text{ cm} \) from the lens and the size of image is twice the size of the object, then where was the object placed ?
