Far point of a myopic person is 40 cm. What type of lens, should he wear so as to see the distant objects…
Far point of a myopic person is 40 cm. What type of lens, should he wear so as to see the distant objects clearly? Calculate the focal length and the power of the lens he should use.
Solution
The person should wear a **concave lens**.
For a myopic eye viewing a distant object, $u = -\infty$ and the image must form at the far point, $v = -40\ \text{cm}$.
$\dfrac{1}{f} = \dfrac{1}{v} - \dfrac{1}{u} = \dfrac{1}{-40} - \dfrac{1}{-\infty} = -\dfrac{1}{40}$
$f = -40\ \text{cm}$
$P = \dfrac{100}{f\ (\text{cm})} = \dfrac{100}{-40} = -2.5\ \text{D}$
A concave lens of focal length $40\ \text{cm}$ and power $-2.5\ \text{D}$ is needed.