Question

Two thin lenses of focal length +20 cm and - 30 cm are put incontact.What is power of the combination ?

Answer 1

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

• Focal lenght of two lens ${\sf f_1=+20cm\:and\:f_2=-30cm}$

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

• Power of combination (P)

━━━━━━━━━━━━━━━━━━━━━━━━━━━

Combination of lens is given as -

$\color{violet}\bullet\underline{\boxed{\sf \dfrac{1}{F}=\dfrac{1}{f_1}+\dfrac{1}{f_2}}}$

$\implies{\sf \dfrac{1}{F}=\dfrac{1}{20}+\left(-\dfrac{1}{30}\right)}$

$\implies{\sf \dfrac{1}{F}=\dfrac{-30+20}{20×-30}}$

$\implies{\sf \dfrac{1}{F}=\dfrac{-10}{-60} }$

$\color{orange}\implies{\sf \dfrac{1}{F}=\dfrac{1}{6} }$

━━━━━━━━━━━━━━━━━━━━━━━━━━━━

$\color{violet}\bullet\underline{\boxed{\sf P=\dfrac{1}{F}}}$

$\color{red}\implies{\sf Power (P)=6D}$

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

Power of the combination is $\color{red}{\sf +6D}$.