Question
A person with a myopic eye cannot see objects beyond 1.2 m distinctly. What should be the type of the corrective lens used to restore proper vision?
✓
Answer
A person with a myopic eye should use a concave lens of suitable focal length or power to restore proper vision.
[Here, the person with defect of myopia has the far point nearer than infinity at a distance of 1.2 m from the eye.
So, $v=-1.2 m ; u=-\infty ; f=$ ?
So, from the lens formula
$
\begin{array}{l}
\frac{1}{f}=\frac{1}{-u}-\frac{1}{v} \\
\therefore \frac{1}{f}=\frac{1}{-(-\infty)}+\frac{1}{-1.2} \\
\therefore f=-1.2 m \\
\therefore P=\frac{1}{-1.2}=-0.83 D
\end{array}
$
$\therefore$ A concave lens of focal length 1.2 m should be used to restore proper vision.]
[Here, the person with defect of myopia has the far point nearer than infinity at a distance of 1.2 m from the eye.
So, $v=-1.2 m ; u=-\infty ; f=$ ?
So, from the lens formula
$
\begin{array}{l}
\frac{1}{f}=\frac{1}{-u}-\frac{1}{v} \\
\therefore \frac{1}{f}=\frac{1}{-(-\infty)}+\frac{1}{-1.2} \\
\therefore f=-1.2 m \\
\therefore P=\frac{1}{-1.2}=-0.83 D
\end{array}
$
$\therefore$ A concave lens of focal length 1.2 m should be used to restore proper vision.]
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