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 μ 1 and μ 2 and R is the radius of curvature of the curved surface of the lenses, then the focal length of the combination is (a) (R)/((μ₁ - μ₂)) (b) (2R)/((μ₂ - μ₁)) (c) (R)/(2(μ₁ + μ₂)) (d) (R)/(2(μ₁ - μ₂))

Answer: (a). Plano-convex: P₁ = (μ₁-1)/(R); plano-concave of same R: P₂ = -(μ₂-1)/(R). P = P₁ + P₂ = (μ₁ - μ₂)/(R) So the answer is (a) (R)/(μ₁ - μ₂).

A plano convex lens fits exactly into a plano concave lens. Their… | NEET 2013

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Combination of Lenses

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

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Answer: (a)

Plano-convex: $P_1 = \frac{\mu_1-1}{R}$ ; plano-concave of same $R$ : $P_2 = -\frac{\mu_2-1}{R}$ .
$P = P_1 + P_2 = \frac{\mu_1 - \mu_2}{R}$
So the answer is (a) $\frac{R}{\mu_1 - \mu_2}$ .

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