Contributed by Javid
St Gabriel’s Sec/SA1/2008
The graph shows how the image distance change as an object is moved from a distance gradually towards a converging lens.
(a) What does the graph tells you about the nature of the image in region C? [1]
(b) There is a position along the principal axis where the image size is equal to the object size. Make use of this fact to find the focal length of the lens. [2]
(c) If the object is placed at P, describe the change in the size of the image observed if the object is moved closer to the lens. [1]
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Answer:
(a) The image is virtual, enlarged, upright and on the same side as the object [1].
(b) When object distance u = image distance v, the object and image should be of the same size. This occurs at 20 cm, which is actually twice the focal length [1].
Hence, focal length = 10 cm [1].
(c) The image increases in size as it gets closer to the lens [1].
