SIGN CONVENTION FOR LENSES
A lens is a curved surface that causes the light rays falling on it to either converge or diverge. The sign convention is a set of rules that determine the signs of the parameters associated with a lens. Consider a lens having radii of curvatures as R1 and R2 corresponding to the surfaces S1 and S2 respectively.
The sign of R1 and R2 depends on the shape of the surfaces constituting the lens. We can say,
R1>0 if S1 is convex
R1<0 if S1 is concave
R2>0 if S2 is concave
R2<0 if S2 is convex
We can tabulate the sign convention of the radii of curvatures of the two surfaces of a lens as:
Lens sign convention table:
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