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CBSE Class 10 Answered

A concave lens has focal length of 30 cm. At what distance should the object be placed from the lens so that it forms an image at 20 cm from the lens ? Also, find the magnification produced by the lens.
Asked by lbalarajulbalaraju2 | 01 Jun, 2021, 11:09: AM
Expert Answer
f = - 30 cm ...(focal length of concave lens)
 
v = - 20cm ...(image formed by concave lens)
 
Lens formula is given as,
1 over f equals 1 over v minus 1 over u
fraction numerator 1 over denominator negative 30 end fraction equals fraction numerator 1 over denominator negative 20 end fraction minus 1 over u
1 over u space equals space fraction numerator 1 over denominator negative 30 end fraction space minus space left parenthesis fraction numerator 1 over denominator negative 20 end fraction right parenthesis
1 over u space equals space fraction numerator 1 over denominator negative 30 end fraction space plus 1 over 20 space
u space equals space minus 60 space c m space 
 m = v/u 
m = -20/-60
m = 1/3  = 0.33 
 
Thus, image formed is virtual, erect and diminished. 
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