Find the distance CQ if incident light ray parallel to principal axis is incident at an angle i. Also, find the distance CQ if `irarr0` .

Explanation

`cos I = (R)/(2CQ) rArr CQ = (R)/(2 cos i)`
As `i` increases `cos i` decreases, it `i` is a small angle `cos I = 1`
Hence `CQ` increases, `:. CQ = R//2`

So, paraxial rays meet at a distance equal t `R//2` form centre of curvalure which is called focus.