Place the Optics bench on the table. Attach the light source at one end, with the image pointing towards the bench. Note its position as A. Depending on which case you are doing, you would place the screen, lenses or mirrors, at different positions on the bench and measure their distances. CASE A: Focal Length of a Convex Lens. (Ref. Fig. 6) 1. Measure the position of the light source (object). Record this as position A. 2. Place the convex lens on the optics bench at some distance from the light source. Measure and record the position of the lens as B. The distance between the Light Source and Lens is the Object Distance 'p', i.e. p = B - A. Figure 3 Figure 4 Figure 5