To find index error `e` distance between object needle and pole of the concave mirror is `20 cm`. The separation between the indices of object needle and mirror was observed to be `20.2 cm` in some observation, the observed image distance is `20.2 cm` and the object distance is `30.2 cm` find
(a) the index error e.
(b) focal length of the mirror `f`.

Answer

A, B, C

Explanation

(a) index error e =observed distance - actual distance.
= separation between indices- distance between object needle and pole of the mirror
`20.0- 20.0= 0.2 cm`
(b) `|u|= 30.2 -0.2=30 cm`
`:. u =-30 cm`
`|v| = 20.2 -0.2=20 cm`
`:. v = - 20 cm`
Using the mirror formula,
`1/f=1/v+1/u=1/(-20)+1/(-30)`
or `f =-12 cm`.