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CBSE Class 10 Answered

When a person sees a distant object the focal length of eye lens is 40 cm. What will be the focal length of eye lens when he focuses on a near object?
Asked by saradamahapatra9999 | 18 Mar, 2019, 02:07: PM
answered-by-expert Expert Answer
when distant object is getting focused at retina by eye lens, we assume the distant object is at infinite distance.
Hence lens-to-image distance is very near to its focal length.
But given focal length of eye lens is 40 cm which is impossible to human eye or eye of any living being.
if focal length of eye lens  is 40 cm, the lens to retina distance also nearly 40 cm. That is not possible
 
focal length of eye is around 17 mm. Eye lens to retina distance also around 17 mm
 
Hence we will do this question by assuming focal length of eye lens is 17 mm.
 
when we see nearby distance means, we assume the recomended distance of near point to eye, i.e., 25 cm or 250 mm.
 
Hence lens to object distance is 250 mm.
 
we have lens equation :  (1/v) - (1/u) = 1/f
where v is lens to image distance = 17 mm,  u is lens to object distance = -250 mm ( sign convention applied) and f is focal length
 
hence 1/f = (1/17)+(1/250)   or  f ≈ 16 mm
Answered by Thiyagarajan K | 18 Mar, 2019, 07:29: PM

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