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CBSE Class 10 Answered

Why is a convex lens with smaller focal length and not a longer one used as a magnifcation glass
Asked by Trisha Gupta | 10 Oct, 2021, 15:27: PM
answered-by-expert Expert Answer
Figure given above shows the formation of magnified image , when object is placed between lens and focal point . 
 
Image-1 is formed if focal point is F1. Similarly image-2 is formed if long focal length lens with focal point  F2 is used

Magnification of image depends on the angle subtended by the light ray passing through the focal point at
other side of lens with respect to object. 
 
As seen from figure , subtended angle θ is greater if focal length is less. We see from figure if focal point is F1,
then subtended angle θ1 is greater compare to subtended angle θ2 at Focal point F2 .
 
Hence magnification of image-1 is greater than that of image-2 .
 
Since magnitude of magnification is important in optical microscope, smaller focal length is used as objective lens
Answered by Thiyagarajan K | 10 Oct, 2021, 17:01: PM

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