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# When to take magnification, image height, object height positive and negative?

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## Sign Convention for Spherical Mirror: Cartesian Sign Convention:In the case of a spherical mirror all signs are taken from Pole of the spherical mirror, which is often called origin or origin point. This sign convention is known as New Cartesian Sign Convention. The sign is taken as + (positive) behind the spherical mirror. For example, if an image is formed behind the mirror, the distance of image is taken as + (positive) from the pole along the principal axis. The height of is taken as + (positive) above the principal axis and taken as – (negative) below the principal axis. Sign in the case of a concave mirror: Since object is always placed in front of the mirror hence the sign of object is taken as negative. Since the center of curvature and focus lie in front of the concave mirror, so signs of the radius of curvature and focal length are taken as negative in the case of the concave mirror. When the image is formed in front of the mirror, the distance of image is taken as – (negative) and when an image is formed behind the mirror, the distance of image is taken as + (positive). The height of the image is taken as positive in the case of erect image and taken as negative in the case of inverted image. Sign in the case of a convex mirror: Since object is always placed in front of the mirror hence the sign of object is taken as negative. Since the centre of curvature and focus lies behind the convex mirror, so sign of radius of curvature and focal length are taken as + (positive) in the case of convex mirror. In the case of the convex mirror, the image always formed behind the mirror, thus the distance of image is taken as positive. In the case of a convex mirror, always an erect image is formed, thus the height of the image is taken as positive. The sign convention for spherical lenses: All distances measured above the principal axis are taken as positive. Thus, the height of an object and that of an erect image are positive and all distances measured below the principal axis are taken as negative. The distances measured in the direction of incident rays are taken as positive and all the distances measured in the direction opposite to that of the incident rays are taken as negative. All distances on the principal axis are measured from the optic center.

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