In `u-v` method, we require the distance between object or image from the pole (vertex) of the mirror (actual distance). But practically we measure the distance between the indices `A` and `B`. (Observed distance). Which need not exactly co-inside with object and pole, there can be a slight mismatch called index error, which will be constant for every observation. Index error=Observed distance-Actual distance(Just like zero error in screw guage, it is the excess reading). To determine index error, mirror and object needle and placed at arbitary position. For measuring actual distance, a knitting needle is just fitted between the pole of mirror and object needle `O`. The length of knitting needle will give the actual object distance while the seperation between indices `A` and `B` at that instant is the observed distance. So index error is- `e=`Observed distance-Actual distance `=` Separation between indices `A` and `B`- Length of knitting neddle once we get `e`, in every observation, we get Actual distance=Observed distance (separation between the indices)- Excess reading(e) There is an another term, Index corraction which is invert of index error. Index correction=-index error In some observation, the observed object distance (Separation between indices of object needle and mirror) is `30.2cm`, and the observed image distance is `19.9cm`. Using index correction from previous two equations, estimate the focal distance of the concave mirror!A. `36cm`B. `20cm`C. `12cm`D. `8cm`