From the figure - 21, we can say that PI X = AB AB/AIBI = PIF/AIF   ..... (3) from the equations (1),(3) we can write AC/AIC = PIF/AIF   ..... (4) If the paraxial rays (rays which are travelling very near to principal axis) are considered, we can say that PI coincides with P Then PIF = PF AC/AIC = PF/AIF    ..... (5) We can observe from fig. 21, that AIC = PA − PC AC = PC − PAI AIF = PAI − PF by substituting these in equation (5) PA - PC /PC - PAI = PF/PAI - PF   ..... (6) we know that PA = u, PC = R = 2f, PAI= v, PF = f

This is called mirror formula. While using this formula, we have to use sign convension in every situation