We know that when converging beam of rays are more converging after passing through an instrument, then we say it is of positive focal power. If angle of convergence doesn't change it is of zero focal power. Similarly negative focal power diverges.
Now if we consider an plane interface between two media. Let converging light rays pass through it from denser to rarer. Then the rays become less converging. That is negative power. But according to formula of focal power of surface when light rays pass from medium of refractive index n1 to medium n2, the focal power is n2-n1/R, where R is radius of curvature of interface. Here R=infinty. Therefore power is zero. Why a different answer?
As shown in the above figure, Power is non zero. But the formula suggests that power is zero. Why?