The centre of curvature of a convex lens is at 40 cm and its central portion is wrapped in black paper as shown below. If an object is placed at 90 cm, find the nature of image formed.
The intensity of the image formed will decrease.
Let us first analyse the situation when the central portion of the lens is not wrapped. As the object is placed beyond 2F, so the image will be formed between F and 2F and will be real and inverted and of smaller size than the object.
Now, when the central portion of the lens is wrapped in black paper, same nature of image will be formed at the same place by the light rays passing through the top and bottom of the lens. The rays that were passing earlier through the black region are not passing now so the intensity of the image will be reduced.