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
To find index error for `u`, when a knitting needle of length `20.0cm` is adjusted between pole and object needle, the sepertion between the indices of object needle and mirror was observed to be `20.2cm`. Index corraction for u is

Options

A. `-0.2cm`
B. `0.2cm`
C. `-0.1cm`
D. `0.1cm`

Answer

B

Explanation

Index error(excess reading)=Observed reading-Actual reading
`=20.2-20.0=0.2cm`