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:  Lens Makers' Formula |
 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:  Lens Makers' Formula |
 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:  Lens Makers' Formula |
 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:  Lenses | Lens Makers' Formula | Refraction at Curved Surface |
 70%
From NCERT
AIPMT - 2011
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