Question

Large number of thin strips of black paper are made on the surface of a convex lens of focal length 20 cm to catch the image of a white horse. The image will be

• A. a zebra of black stripes
• B. a horse of black stripes
• C. a zebra of less brightness
• D. a horse of less brightness

Hint:

Covering any part of a lens never cuts off or alters the shape of the image.
It only reduces the amount of light passing through, which decreases the brightness of the complete image.

Answer 1

The Correct Option is D

Step 1: Recall how a lens actually builds an image.
A lens does not use just one path of light for a given point on the object, every unblocked part of the lens sends its own set of rays from that same point, and all of them bend to meet again at the corresponding image point.
Step 2: Think about what the black strips do.
The black paper strips only block the particular rays that were travelling through those covered patches, they do not belong to any special part of the horse's image, they are just some of the many rays converging on each image point.
Step 3: See what happens to each image point.
Since plenty of rays still get through the open parts of the lens, every point of the horse still gets its rays converging properly, so a complete, unbroken image of the horse is formed, with no stripes appearing on it.
Step 4: Account for the missing light.
What has changed is only the total amount of light energy reaching the screen, since some rays were blocked, less light arrives per point, so the picture we get is a complete horse but dimmer than before.
\[ \boxed{\text{a horse of less brightness}} \]