An object is placed at a distance of 20cm from the optical centre of a convex lens of focal length 15cm .find the distance of the image from the lens what will be the height of the image if the object is 4cm tall

       Use the lens form:

                                1/f=1/v-1/u

                                1/30=1/v-(1/-20)

                                 1/30=1/v+1/20

                                 1/30-1/20=1/v

                                  2-3/60=1/v

                                  -1/60=1/v                    ;;;v =  -60 cm

              m = h of image/h of object  =  v/u

                                    H/2 = -60/-20

                                     H = 3*2

                                       H = 6 cm

.) the observation by semmelweis that more mothers died of childbed fever when their doctors also worked in the morgue