Questions

The same size images are formed by a convex lens when the object is placed at 20cm or at 10cm from the lens. The focal length of convex lens is……………

detailed solution

Correct option is O

1/v – 1/u = 1/f …(1)

m = v/u ….(2)

from (1) and (2) we get ,

m = f/(f + u) ;

Given conditions

m_{1} = -m_{2} ;

f/(f – 10) = -f/(f – 20) ;

f – 20 = –f + 10 ;

2f = 30 ;

f = 15 cm

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detailed solution

Correct answer is 15

1/v – 1/u = 1/f …(1)

m = v/u ….(2)

from (1) and (2) we get ,

m = f/(f + u) ;

Given conditions

m_{1} = -m_{2} ;

f/(f – 10) = -f/(f – 20) ;

f – 20 = –f + 10 ;

2f = 30 ;

f = 15 cm

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