Suppose you have focused on a screen the image of a candle flame placed at the farthest end of the laboratory table using a convex lens. If your teacher suggests you to focus the parallel rays of the sun, reaching your laboratory table, on the same screen, what are you expected to do?

In the case of the candle flame, the object is placed beyond c (center of curvature), which means the image is formed or focused between c and f (focus). In the second case, the object (sun) is at infinity, so the image will be formed at the focus. This means the distance between the image and the lens has decreased in the second case. Therefore, we have to move the screen towards the lens or the lens towards the screen. The correct option is to move the lens slightly towards the screen.