Explanation
In case of convex lens if rays are coming from the focus, then the emergent rays after refraction are parallel to principal axis.
A concave lens always forms virtual image for real objects.
Beam first converges and them diverges.
Since aperture of lens reduces so brightness will reduce but their will be no effect on size of image.
Convex mirror and concave lens do nor form real image. For concave mirror v > u, so image will be enlarged, hence only convex lens can be used for the purpose.
Covering a portion of lens does not effect position and size of image.
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