Take a live candle, put it in front of a convex lens mounted on an optical bench. Move the candle along the axis of the bench and take its full image on a screen. Now cover the lower half of the lens with a black paper without changing the positions of candle, lens and screen.
You will observe that the full image of the candle is still seen on the screen, but the intensity of image is reduced. The reason is that a large number of rays incident on the lens are refracted to form the image. In the case of covered lower half of lens with black paper, the rays starting from candle and incident on lens are refracted from the upper part and form the full image.