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  • Two identical thin biconvex lenses of focal length and refracti

Two identical thin biconvex lenses of focal length \mathrm{15 \mathrm{~cm}} and refractive index \mathrm{1.5} are in contact  with each other. The space between the lenses is filled with a liquid of refractive index \mathrm{1.25}.The focal length of the combination is __________ cm.

Option: 1

10


Option: 2

-


Option: 3

-


Option: 4

-


Answers (1)

best_answer

\frac{1}{\mathrm{f}}=(\mu-1)\left(\frac{2}{\mathrm{R}}\right)=\frac{1}{\mathrm{R}}=\frac{1}{15 \mathrm{~cm}}
\mathrm{R= 15\, cm}

Let the focal length of the concave lens be \mathrm{{f}'}
\mathrm{\frac{1}{{f}'}= \left ( \mu -1 \right )\left ( \frac{-2}{R} \right )= \frac{-1}{30\, cm}}
\mathrm{{f}'= -30\, cm}

\frac{1}{f_{e f f}}=\frac{2}{f}+\frac{1}{f^{\prime}}=\frac{2}{15} -\frac{1}{30}=\frac{1}{10 \mathrm{~cm}}

The focal length of the combination is \mathrm{10\, cm}
 

Posted by

Suraj Bhandari

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