Covering half of a convex lens with black paper reduces the size and brightness of the image formed.
When the upper half of a convex lens is covered with black paper, the size of the image formed will be one-half of the size of the image that would be produced without the paper. This is because every ray that comes from the object and hits the lens contributes to the image, even if only half of the lens is available. The brightness of the image will reduce due to the obstruction caused by the black paper.
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