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REAL-IS-POSITIVE CONVENTION (1) All distances are measured from the mirror as origin. (2) Distances of real objects and images are positive. (3) Distances of virtual objects and images are negative. On the real-is-positive convention a concave mirror has a real principal focus and hence a positive focal length. A convex mirror has a virtual principal focus and […]

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NEW CARTESIAN CONVENTION (1) All distances are measured from the mirror as origin. (2) Distances measured against the incident light are negative. (3) Distances measured in same direction as incident light are positive. One advantage of this system is that, if the object is placed to the left of the mirror the ordinary graphical (Cartesian) convention of […]

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Mirror formula. Sign convention It is shown in more advanced textbooks that, for both concave and convex mirrors, We have already seen that an image is formed sometimes in front of a curved and sometimes behind it. This makes it necessary to have a sign convention so that we distinguish between the two cases and obtain the correct […]

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Magnification formula Since the image formed by a spherical mirror varies in size, we refer to the linear or transverse magnification, m, which is defined as, height of image. As a typical case, consider Fig. 22.19, in which the image has been constructed using rules 1 and 4 (page 237).

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To measure the focal length of a concave mirror We have already seen (Fig. 22.7) that when an object is placed at the centre of curvature of a concave mirror a real image is formed at the same place as the object. This fact is used in the following two methods for finding the focal length of a concave […]

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Accurate construction of ray diagrams Earlier in this chapter we mentioned that only rays parallel and close to the principal axis are brought to a true point focus. If we are to have an undistorted image the same condition of closeness to the axis applies to all rays forming the image. Nevertheless, we can locate an image […]

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Images formed by a convex mirror Unlike the concave mirror, which can produce either real or virtual images according to the position of the object, the convex mirror gives virtual images only. These are always erect and smaller than the object and are formed between P and F (see Fig. 22.12). Fig. 22.13. Polishing the 17 tonne primary […]

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Reflecting telescopes The case shown in Fig. 22.9 illustrates the principle of the reflecting telescope. When the object is a very long distance away from the mirror the rays from any particular point on it are practically parallel when they reach the mirror. Consequently, an image is formed at the principal focus. The first telescope of this type was […]

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Parallel beam from curved mirrors In connection with the reversibility of light it is worth noting that a narrow parallel beam of light may be obtained from a point source of light placed at the principal focus of a concave mirror of small aperture. We can regard the image in this case as being at infinity. If a […]

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Images formed by a concave mirror Fig. 22.4-22.9 are ray diagrams showing the image formed by a concave mirror for different positions of an object placed on the principal axis. In all ‘of these diagrams the object is represented by a vertical arrow OA and the image by lB. Starting with Fig. 22.4 in which the object is […]