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 a refractive index of $$1.7$$. The focal length of the combination is:
1. $$-20$$ cm
2. $$-25$$ cm
3. $$-50$$ cm
4. $$50$$ cm

Subtopic: Â Lens Makers' Formula |
Â 61%
From NCERT
NEET - 2015
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A plano-convex lens fits exactly into a plano-concave lens. Their plane surfaces are parallel to each other. If lenses are made of different materials of refractive indices $$\mu_1$$ and $$\mu_2$$ and $$R$$ is the radius of curvature of the curved surface of the lenses, then the focal length of the combination is:
 1 $$\dfrac{R}{2(\mu_1-\mu_2)}$$ 2 $$\dfrac{R}{(\mu_1-\mu_2)}$$ 3 $$\dfrac{2R}{(\mu_2-\mu_1)}$$ 4 $$\dfrac{R}{2(\mu_1+\mu_2)}$$
Subtopic: Â Lens Makers' Formula |
Â 66%
From NCERT
AIPMT - 2013
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When a biconvex lens of glass having a refractive index of $$1.47$$ is dipped in a liquid, it acts as a plane sheet of glass. The liquid must have a refractive index:

 1 equal to that of glass. 2 less than one. 3 greater than that of glass. 4 less than that of glass.
Subtopic: Â Lens Makers' Formula |
Â 79%
From NCERT
AIPMT - 2012
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A biconvex lens $$(\mu=1.5)$$  has a radius of curvature of magnitude $$20~\text{cm}$$. Which one of the following options, best describes, the image formed by an object of height $$2$$ cm placed $$30~\text{cm}$$ from the lens?
 1 virtual, upright, height $$=0.5$$ cm 2 real, inverted, height $$=4$$ cm 3 real, inverted, height $$=1$$ cm 4 virtual, upright, height $$=1$$ cm
Subtopic: Â Lenses |Â Lens Makers' Formula |Â Refraction at Curved Surface |
Â 70%
From NCERT
AIPMT - 2011
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