Question:
Two identical thin plano-convex glass lenses (refractive index 1.5) each having radius of curvature of 20 cm are placed with their convex surfaces in contact at the centre. The intervening space is filled with oil of refractive index 1.7. The focal length of the combination is
Two identical thin plano-convex glass lenses (refractive index 1.5) each having radius of curvature of 20 cm are placed with their convex surfaces in contact at the centre. The intervening space is filled with oil of refractive index 1.7. The focal length of the combination is
Updated On: Jul 18, 2024
- -50 cm
- 50 cm
- -20 cm
- -25 cm
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The Correct Option is A
Solution and Explanation
From lens maker's formula $\frac{1}{{f}} = (\mu - 1)\left(\frac{1}{R_1} - \frac{1}{R_2}\right)$
we have $ \frac{1}{f_1} = (1.5 -1) \left( \frac{1}{20}\right) = \frac{1}{40}$
$\& \hspace10mm \frac{1}{f_2} = (1.7 - 1) \left(\frac{2}{20}\right) = \frac{7}{100}$
${Now} \hspace10mm \frac{1}{f} = \frac{1}{f_1} \frac{1}{f_2}\frac{1}{f_3}$
$\Rightarrow \hspace10mm \frac{1}{f} = \frac{1}{40} + \frac{1}{40} - \frac{7}{100}$
$\Rightarrow \hspace10mm f = - 50 \,cm $
we have $ \frac{1}{f_1} = (1.5 -1) \left( \frac{1}{20}\right) = \frac{1}{40}$
$\& \hspace10mm \frac{1}{f_2} = (1.7 - 1) \left(\frac{2}{20}\right) = \frac{7}{100}$
${Now} \hspace10mm \frac{1}{f} = \frac{1}{f_1} \frac{1}{f_2}\frac{1}{f_3}$
$\Rightarrow \hspace10mm \frac{1}{f} = \frac{1}{40} + \frac{1}{40} - \frac{7}{100}$
$\Rightarrow \hspace10mm f = - 50 \,cm $
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Concepts Used:
Spherical Lenses
Lenses that are made by combining two spherical transparent surfaces are called spherical lenses. In general, there are two kinds of spherical lenses. Lenses that are made by joining two spherical surfaces that bulge outward are convex lenses, whereas lenses that are made by joining two spherical surfaces that curve inward are concave lenses.
Properties of Convex lens:
- In this, the lenses are thicker in the middle and thinner at the edges.
- They have a positive focal length.
- It intersects the incident rays towards the principal axis
- These lenses are used in the camera, focus sunlight, projector microscope, simple telescope, overhead projector, magnifying glasses, etc.
Properties of Concave lens:
- These lenses are thinner in the middle and thicker at the edges.
- They have a negative focal length.
- It parts the incident rays away from the principal axis.
- These are used in the glasses, spy holes, some telescopes in the doors, etc.