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Question
The following Figure shows a concave mirror MM' on which a ray of light incident from a point P gets reflected to meet the principle axis at O.
(a) Find, by construction, the position of the centre of curvature of the concave mirror.
(b) Write down the value for the radius of curvature of the mirror.
(c) Calculate the focal length of the mirror.
(d ) Which relation is used in deducing the focal length from the radius of curvature?
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Solution
(i) Centre of curvature can be determined by constructing the imaginary sphere to which lens belongs.
(ii) Value of radius of curvature can be found by measuring the radius of this imaginary sphere geometrically.
(iii) The focal length is the midpoint of the pole and centre of curvature.
(iv) focal length of mirror = centre of curvature/2.
Centre of curvature = 2 .8 cm.
Focal length of mirror = 1.4 cm.
Focal length = radius of curvature/2.
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