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Which type of mirror has:(a) positive focal length?(b) negative focal length?
(a) A convex mirror has a positive focal length because the focus of a convex mirror is on the right side and behind the mirror.
(b) A concave mirror has a negative focal length because the focus of a concave mirror is on the left side and in front of the mirror.
Explanation
The focal length of a concave mirror is negative, and that of a convex mirror is positive.
This is because according to the New Cartesian Sign Convention, the distances on the left side of the mirror from the pole (in front of the mirror) are given a negative sign, and in the concave mirror the focus lies in front of the mirror. On the other hand, distances on the right side of the mirror from the pole (behind the mirror) are given a positive sign, and in the convex mirror, the focus lies behind the mirror.
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