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Thermodynamics

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The Sign Convection

The distance of the object or the image from the refracting surface is a vector quantity and these distances must be represented with proper signs. The convention of signs used is in accordance with the convention of coordinate geometry as shown in figure.

(i) The figures are drawn with the incident light travelling from left to right.

(ii) The centre of the refracting surface is at the origin O and its axes is along XX’.

(iii) Distances measured to the left of O are taken as negative.

(iv) Distances measured to the right of O are taken as positive.

(v) Distances measured upward and normal to the X-axis are taken as positive, while downward distances are taken as negative.

In figure, if AB represents an object, its distance OA is negative. If PQ represents the image, its distance OP is positive. The size of the object AB is positive while the size of the image PQ is negative.