The far point of a myopic person is 150 cm in front of the eye. Calculate the focal length and the power of…
The far point of a myopic person is 150 cm in front of the eye. Calculate the focal length and the power of a lens required to enable him to see distant objects clearly.
Solution
For a myopic person viewing a distant object, $u = -\infty$ and the image must form at the far point, $v = -150\ \text{cm}$.
From the lens formula,
$\dfrac{1}{f} = \dfrac{1}{v} - \dfrac{1}{u} = \dfrac{1}{-150} - \dfrac{1}{-\infty} = -\dfrac{1}{150}$
$f = -150\ \text{cm}$
$P = \dfrac{100}{f\ (\text{cm})} = \dfrac{100}{-150} = -0.66\ \text{D}$
A **concave lens** of focal length $150\ \text{cm}$ and power $-0.66\ \text{D}$ is required.