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  • In the figure shown here, what is the equivalent focal length of the combination of lenses (Assume that all layers are thin)? <img alt="" src="https://cdn.entrance360.com/media/uploads/20

In the figure shown here, what is the equivalent focal length of the combination of lenses (Assume that all layers are thin)?

Option: 1

\mathrm{40\, cm}


Option: 2

\mathrm{-40\, cm}


Option: 3

\mathrm{-100\, cm}


Option: 4

\mathrm{-50\, cm}


Answers (1)

best_answer

 

\text { Use } \frac{1}{\mathrm{f}}=[\mu-1]\left[\frac{1}{\mathrm{R}_1}-\frac{1}{\mathrm{R}_2}\right]

\begin{aligned} & \frac{1}{f_1}=[1.6-1]\left[\frac{1}{\infty}-\frac{1}{20}\right]=\frac{-3}{100} \\ & \frac{1}{f_2}=[1.5-1]\left[\frac{1}{20}+\frac{1}{20}\right]=\frac{1}{20} \\ & \frac{1}{f_3}=\frac{-3}{100} \end{aligned}

\begin{aligned} & \frac{1}{f_{e q}}=\frac{1}{f_1}+\frac{1}{f_2}+\frac{1}{f_3} \\ & \frac{1}{f_{e q}}=-\frac{3}{100}+\frac{1}{20}-\frac{3}{100}=\frac{-1}{100} \end{aligned}

f=-100 \mathrm{~cm} \Rightarrow P =-1 D

Posted by

Riya

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