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                                    ray diagram, find position, nature and size of image.A needle 4 cm high is placed in front of a lens. Image of the needle is real, inverted and 6 cm high, whenthe distance between the needle and its image is 20 cm. Find the focal length of lens and also write the type | 
                            
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Answer»  h = 4 cm h' = - 6 cm m = -6/4 = - 1.5 Since image is magnified, inverted real, it is a convex lens. | u | + v = 20 cm v - u = 20 cm as u is -ve. m = v / u => v = - 1.5 u - 1.5 u - 1 u = 20 => u = - 20/2.5 = - 8 cm v = +12 cm 1/v - 1/u = 1/f => 1/12 + 1/8 => f = 24/5 = 4.8 cm  | 
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