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Asked by adjacentcalliber10 | 06 May, 2019, 09:41: AM
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(A) area of image is proportional to magnification v/u , where v is image-to-lens distance and u is sun-to-lens distance.
Since u is very large, v is very near to focal length f. hence we can write magnification f/u .
For a fixed u, magnification increases by increasing f.
 
Hence it is true that area of image πr2 is directly proportional to focal length f.
 
(B) as explained in (A) , it is false
 
(C) As magnification increases with focal length f, intensity decreases with f
 
(D) even if lowere half is blocked, full image of sun will be formed due to upper half of the lens,
since object distance from lens is large, half of the lens can view the full image of sun
Answered by Thiyagarajan K | 06 May, 2019, 11:08: AM

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