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Question
A convex lens forms a full image of the object on a screen. If half of the lens is covered with an opaque object then
A convex lens forms a full image of the object on a screen. If half of the lens is covered with an opaque object then
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Solution
The correct option is D full image of decreased intensity is seen
When you cover half the lens you get a fainter image. Effectively you are cutting out half the light rays, but they still produce an entire image. In the diagram below you can see that rays from the top and bottom of the tree still pass through the top half of the lens to form. However half of the rays from all parts of the image are blocked, so that only half reach the image point, giving only half the intensity of the complete lens.
When you cover half the lens you get a fainter image. Effectively you are cutting out half the light rays, but they still produce an entire image. In the diagram below you can see that rays from the top and bottom of the tree still pass through the top half of the lens to form. However half of the rays from all parts of the image are blocked, so that only half reach the image point, giving only half the intensity of the complete lens.
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