Question
Mon March 19, 2012 By:
 

IN MIRRORS AND LENS PLEASE EXPLAIN, WHEN WE WILL TAKE FOCAL LENGTH AS NEGATIVE OR POSITIVE. PLEASE EXPLAIN BOTH IN LENS AND MIRRORS ABOUT OBJECT. IMAGE. FOCAL LENGTH. IMAGE IS REAL AND INVERTED OR IS VIRTUAL AND ERECT AND WHEN WE WILL TAKE THE SIGNS AS POSITIVE AND NEGATIVE.I AM NOT YET CLEAR IN THIS.

Expert Reply
Wed March 21, 2012

Generally, the ‘New Cartesian Convention’ is used, according to which all distances are measured from the pole of the mirror. The distances measured in the same direction as the incident light (+ x axis) are taken to be positive, and those in the opposite direction as negative.
The heights measured upwards (above the x-axis) and normal to the principal axis of the mirror are taken as positive, and the heights measured downwards as negative.

similarly in case of lens u can take the sign.
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