A 52-year old near-sighted person wears eye-glass with a power of –5.5 dioptres for distance viewing. His doctor prescribes a correction of +1.5 dioptres in the near-vision section of his bi-focals. This is measured relative to the main part of the lens (i) What is the focal length of his distance-viewing part of the lens? (ii) What is the focal length of the near-vision section of the lens?
(i) Focal length of lens for distance viewing f1 = 
= -0.182 m
= -18.2 cm
(ii) Focal length for near-vision f2 =
= 0.667 m = 66.7 cm.
= -0.182 m
= -18.2 cm
(ii) Focal length for near-vision f2 =
= 0.667 m = 66.7 cm.
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