Question

An object 1 cm high produces a real image 2 cm high when placed at a distance of 15 cm from a convex lens. Calculate the position of image and focal length of lens

Answer 1

given object height = h = - 1 cm

         image height = h ' =  2 cm

         object distance = u = - 15 cm   (since its a real image)

          we know that , h '\ h = v\u

                                    -1 \ 2 = v \ -15

                                          v =  15\2

                                          v = + 7.5 cm

      therefore the position of the image is 7.5 cm in front of the lens.

using lens formula to find focal length,

                                  1\v - 1\u = 1\f

                                  1\7.5 -(-) 1\15 = 1\f

                                   10 +5 \ 75 = 1\f

                                         15\75 = 1\f

                                            f = + 5 cm.

hence the focal length is +  5cm .

hope this answer was helpful.....................

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