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CBSE Class 12-science Answered

a person can see up to 1m the nature n power of which will enable him tosee things at 3m
Asked by manjulamurali1979 | 19 May, 2020, 09:01: PM
Expert Answer
The person can see objects only upto 1 m and not beyond 1 m. This means the person is suffering from myopia. 
Thus, to make objects distinctly visible for his eyes beyond 1 m concave lens must be used. 
 
Thus, the nature of lens is concave lens. 
 
Far point of person, v = - 1 m 
Actual far point, u = - 3m 
 
focal length, f = ?
B y space l e n s space f o r m u l a comma space
1 over f space equals space 1 over v minus 1 over u
equals fraction numerator 1 over denominator negative 1 end fraction minus fraction numerator 1 over denominator left parenthesis negative 3 right parenthesis end fraction
equals fraction numerator 1 over denominator negative 1 end fraction plus space 1 third
equals fraction numerator 3 minus 1 over denominator negative 3 end fraction equals negative space 2 over 3

F o c a l space l e n g t h space equals space minus 2 over 3 space equals space minus 0.66 space m space

T h u s comma space
P o w e r space o f space l e n s space i s comma space

P equals space fraction numerator 1 over denominator f space left parenthesis i n space m e t r e s right parenthesis end fraction space equals space fraction numerator 1 over denominator negative open parentheses begin display style bevelled 2 over 3 end style close parentheses end fraction space equals space minus 3 over 2 space equals space minus space 1.5 space D space 
 
Thus, concave lens of power -1.5 D can enable the person to see the objects distinctly upto 3 metres. 
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