Explain Cartesian Sign Convention - QS Study
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The following sign conventions are used by Cartesian:

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(1) All distances are measured from the pole of the mirror (in the case of lens from the optic centre).

(2) The distances measured in the same direction as the incident light, are taken as positive.

(3) The distances measured in the direction opposite to the direction of incident light are taken as negative.

(4) Heights measured perpendicular to the principal axis, in the upward direction are taken as positive.

(5) Heights measured perpendicular to the principal axis, in the downward direction are taken as negative.

(6) The size of the object is always taken as positive, but image size is positive for erect image and negative for an inverted image.

(7) The magnification is positive for erect (and virtual) image, and negative for an inverted (and real) image.