The optician’s prescription for a spectacle lens is marked +0.5D. What is the:
  1. Nature of spectacle lens?
  2. Focal length of spectacle lens?
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  1. Concave lens.
  2. $\text{P}=+0.5\text{D}$
$\text{P}=\frac{1}{\text{f}}$

$\text{f}=\frac{1}{\text{P}}=\frac{1}{0.5}=2\text{m}$
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