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CBSE Class 10 Answered

A concave lens has focal length of 20 cm. At what distance from the lens a 5 cm tall object be placed so that it forms an image at 15 cm from the lens? Also calculate the size of the image formed.
Asked by rk9154247 | 01 Jun, 2021, 02:25: PM
Expert Answer

f=-20cm
v=-15 cm
1 over v minus 1 over u equals 1 over f
fraction numerator begin display style 1 end style over denominator begin display style negative 15 end style end fraction minus fraction numerator begin display style 1 end style over denominator begin display style u end style end fraction equals fraction numerator begin display style 1 end style over denominator begin display style negative 20 end style end fraction
fraction numerator begin display style 1 end style over denominator begin display style u end style end fraction equals fraction numerator begin display style 1 end style over denominator begin display style 20 end style end fraction minus fraction numerator begin display style 1 end style over denominator begin display style 15 end style end fraction
fraction numerator begin display style 1 end style over denominator begin display style u end style end fraction equals fraction numerator begin display style 3 minus 4 end style over denominator begin display style 60 end style end fraction
fraction numerator begin display style 1 end style over denominator begin display style u end style end fraction equals fraction numerator begin display style negative 1 end style over denominator begin display style 60 end style end fraction
u equals negative 60
m equals v over u equals fraction numerator H e i g h t space o f space i m a g e left parenthesis h subscript i right parenthesis over denominator H e i g h t space o f space o b j e c t left parenthesis h subscript o right parenthesis end fraction
fraction numerator negative 15 over denominator negative 60 end fraction equals h subscript i over 5
h subscript i equals 5 over 4 equals 1.25 c m

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