I understand completely how to go about doing this problem... I know that you would have to plot the points and draw a line of best fit and that the x-intercepts and y-intercepts would equal 1/f.... but what would the actual focal length be? and how do you calculate the gradient??? This is actually a review sheet for my test and the teacher didn't give us the answer to this last one because it was the last thing we covered so he didn't get the chance to. I would really like to know the exact answer so I can review it multiple times as the test is very similar to these problems. It says: A converging lens is set up on an optical bench and the distances to the object O and image I are measured. Use a graph to determine the focal length of the lens. O,cm I,cm 20 80 25 44.1 46 24.5 58 22.1 70 20.7 Thanks so much in advance!!!!