Select "Ruler" from the tools menu and draw a ruler through the center of the lens. Keeping the source on the principal axis, place the source at four different distances and collect data on the distance of the source from the lens (object distance, do) and the distance of the corresponding real image from the lens (image distance, di). A data table is included at the end of this section. The focal length can be found from: 1/(object distance) + 1/(image distance) = 1/(focal length) The distance from the principal axis to the object is (green dot below) the object height (h0) and the distance from the axis to the image (yellow dot) is the image height (hi) as shown in the figure below, where the object (on the left) and the image (on the right) are represented by arrows. Note that the image height will be negative in this case. The magnification (m) can be found by m = -di/do or hi/ho