For a convex lens, light rays parallel and close to the principal axis are refracted inwards and converge to a focal point, after passing through the lens. For a concave lens, light rays parallel and close to the principal axis are refracted outwards and appear to diverge from the focal point, F after passing through the lens. The focal point, F is a common point on the principal axis where all rays close and parallel to the axis converge to it after passing through a convex lens, or appear to diverge from it after passing through a concave lens.
The focal length, f of a lens is the distance between the focal point and the optical centre of the lens. For a convex lens, the ticker the lens is at the centre, the shorter is the focal length of the lens. For a concave lens, the thinner the lens is at the centre, the shorter is the focal length of the lens. Objective: To determine the focal length of a convex lens. Theory From the lens formula, 1=1+1 f where m = I v is the linear magnification. Hence the graph m versus v is a straight line graph.
The equation also shows that m is directly proportional to v. M = 1 when v = 2 f Apparatus 1 . A convex lens 2. A short transparent ruler 3. A piece of card with hole 4. A screen 5. A light source 6. A meter ruler 7. A lens holder Procedure . The focal length, f of the convex lens was estimated. 2. The apparatus was set up as shown in FIGURE 1. 3. 1. CM interval was choosing on the transparent scale of the ruler as the object. Therefore the height of the object, ho = 1. 0 CM. 4.
Object was placed at a suitable distance from the lens in order to form a real, sharp image on the screen. 5. The value of the image distance, v and the height of the sharp image, hi on the screen was measured. 6. The magnification of the image, m = hi / ho was calculated. 7. The location of the object was changed, and steps (5) and (6) were repeated until 6 sets of v and m were obtained. . A graph of m against v was plotted. 9. The focal length of the lens, f was determined from the gradient of the graph. 0. From the graph, for m ”1, the focal length was determined by using equation v = 2 f. FIGURE 1 Result and Analysis Focal length, f: Y-Y 4-1. 6 x-x = 40-20 0. 12 CM From the graph, using the equation v = if, if m=l: v=15 7. 5 CM Discussion Based on the result obtained, we can say that the experiment is conducted successfully. This is because the result was the same as the hypothesis, which is saying that magnification of the image, m is directly proportional to the image stance, v.