In which direction will she move the screen to obtain a sharp image of the building ? What is the approximate focal length of this lens ?
A real image can be obtained on the screen. Therefore, the lens used is convex lens as it forms real as well as virtual image. The distance of the
real image formed by a convex lens from the lens decreases as the object distance from the lens increases. Hence, the screen has to be moved
towards the lens to obtain the sharp image of the building.
Approximate focal length of the lens = 15 cm. The rays of light from the window pane are considered to come from infinity. These rays of light are
focused by the convex lens at its focus (i.e. on the screen).