The principal axis of the mirror is taken as the x-axis (X’X) of the coordinate system. Since, object is always placed in front of the mirror hence the sign of object is taken as negative. Draw a diagram and apply these conventions for calculating the focal length and nature of spherical mirror which forms a 1/3 times magnified virtual image of an object placed 18 cm in front of it. The conventions are as follows –. When image is formed in front of the mirror, the distance of image is taken as – (negative) and when image is formed behind the mirror, the distance of image is taken as + (positive). List the new Cartesian sign convention for reflection of light by spherical mirrors. While dealing with the reflection of light by spherical mirrors, we shall follow a set of sign conventions called the New Cartesian Sign Convention. In the case of a convex mirror, always an erect image is formed, thus the height of image is taken as positive. 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Sign convention for Spherical Mirrors. Sign is taken as – (negative) from pole of a spherical mirror towards object along the principal axis. In this convention, the pole (P) of the mirror is taken as the origin. (iv) Distances measured perpendicular to and above the principal axis (along + y-axis) are taken as positive. Sign Convention for Reflection by Spherical Mirrors The new Cartesian sign convention is used for measuring the various distances in the ray-diagrams of spherical mirrors. This sign convention is known as New Cartesian Sign Convention. Since, the centre of curvature and focus lie in front of the concave mirror, so signs of radius of curvature and focal length are taken as negative in the case of concave mirror. (ii) All distances parallel to the principal axis are measured from the pole of the mirror. (v) Distances measured perpendicular to and below the principal axis (along –y-axis) are taken as negative. Draw a diagram and apply these conventions in the determination of focal length of a spherical mirror which forms a three times magnified real image of an object placed 16 cm in front of it. (i) The object is always placed to the left of the mirror. Sign Convention for Spherical Mirror: Cartesian Sign Convention: In the case of spherical mirror all signs are taken from Pole of the spherical mirror, which is often called origin or origin point. List the new Cartesian sign convention for reflection of light by spherical mirror. (iii) All the distances measured to the right of the origin (along + x-axis) are taken as positive while those measured to the left of the origin (along – x-axis) are taken as negative. Last updated at April 23, 2020 by Teachoo. If you would like to contribute notes or other learning material, please submit them using the button below. This implies that the light from the object falls on the mirror from the left-hand side. Height of image is taken as positive in the case of erect image and taken as negative in the case of inverted image. 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 convex mirror, image always formed behind the mirror, thus the distance of image is taken as positive. List the sign conventions for reflection of light by spherical mirrors. Draw a diagram and apply these conventions for calculating the focal length and nature of a spherical mirror which forms a 1 / 3 times magnified the virtual image of an object placed 1 8 c m in front of it.