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Find the distance CQ if incident light r...

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` .

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In triangle CNQ, cos `i= R//2CQ rArr CQ = R//2 cos i`
As `i` increases `cos i` decreases. Hence, CQ increases.
If `i` is a small angle, `cos i~~ 1` (see Fig. (b)).
`:. CQ= R//2`
So, paraxial rays meet at a distance equal to `R//2` from center of curvature, which is call focus.
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