Two plano-concave lenses (1 and 2) of glass of refractive index 1.5 have radii of curvature 25 cm and 20 cm. They are placed in contact with their curved surfaces towards each other and the space between them is filled with liquid of refractive index$\frac{4}{3}$. Then the combination is [image:9_image11.emf]

Question

Dr. Abubacker · Accepted Answer

(c) The focal length
$\frac{1}{f_{1}} = (\mu - 1)\frac{1}{- R}$
$\frac{1}{f_{1}} = (1.5 - 1)\frac{1}{- 25}$
$\frac{1}{f_{1}} = \  - 50\ cm$
The focal length
$\frac{1}{f_{2}} = (1.5 - 1) 	imes \frac{1}{- 20}$
$\frac{1}{f_{2}} = 0.5 	imes \frac{1}{- 20}\ m$
$\frac{1}{f_{2}} = - 40\ cm$
The focal length of bi-convex
$\frac{1}{f_{3}} = \left( \ _{1}n_{2} - 1 ight)\left( \frac{1}{R_{1}} + \frac{1}{R_{2}} ight)$
$\frac{1}{f_{3}} = \left( \frac{4}{3} - 1 ight)\left( \frac{1}{20} + \frac{1}{25} ight)$
$\frac{1}{f_{3}} = \frac{1}{3} 	imes (\frac{5 + 4}{100})$
$\frac{1}{f_{3}} = \frac{1}{3} 	imes \frac{9}{100}$
$\frac{1}{f_{3}} = \frac{3}{100}$
$\frac{1}{F} = \ \frac{- \ 200}{3}$
$F = - 66.3\ cm$

Q. Two plano-concave lenses (1 and 2) of glass of refractive index 1.5 have radii of curvature 25 cm and 20 cm. They are placed in contact with their curved surfaces towards each other and the space between them is filled with liquid of refractive index$\frac{4}{3}$. Then the combination is

Explanation

Reviewed by Dr. Abubacker

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