Question

Far point of a myopic eye is 3m. Find the focal length of the corrective lens ?

Answer 1

$\color{darkblue}\underline{\underline{\sf Given-}}$

• Far point of myopic eye is (v) = 3m

$\color{darkblue}\underline{\underline{\sf To\:Find-}}$

• Focal Lenght of the corrective lens (f)

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$\color{red}\underline{\underline{\sf NOTE-}}$

• If the reference of object is not given take it as $\color{blue}{\sf 25cm\:or\:0.25m}$

• Distance of far point (v) = negative (-)

• Distance of Object (u) = negative (-)

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$\color{skyblue}\odot\boxed{\dfrac{1}{f}=\dfrac{1}{v}-\dfrac{1}{u}}$

$\implies{\sf \dfrac{1}{f}=-\dfrac{1}{3}-\left(-\dfrac{1}{0.25}\right)}$

$\implies{\sf \dfrac{1}{f}=-\dfrac{1}{3}+\dfrac{1}{0.25}}$

$\implies{\sf \dfrac{1}{f}=\dfrac{0.25+(-3)}{-3×0.25} }$

$\implies{\sf \dfrac{1}{f}=\dfrac{-2.75}{-0.75}}$

$\color{orange}\implies{\sf \dfrac{1}{f}=P=+3.66D }$

We know that -

$\color{violet}\odot{\sf P=\dfrac{1}{f}}$

$\implies{\sf f=\dfrac{1}{P}}$

$\color{red}\implies{\sf f=0.27m\:or\:f=27cm}$

$\color{darkblue}\underline{\underline{\sf Answer-}}$

Focal Lenght will be $\color{red}{\sf 0.27m\:or\:27cm}$