First slide

The lens

Question

A point object in air is in front of the curved surface of a plano-convex lens. The radius of curvature of the curved surface is 30cm and the refractive index of the lens material is 1.5, then the focal length of the lens (in cm) is ________

Easy

Solution

$L e n s M a k e r s f o r m u l a, \frac{1}{f} = (μ - 1) (\frac{1}{R_{1}} - \frac{1}{R_{2}})$
$G i v e n : R_{1} = \infty, R_{2} = - 30 c m, μ = 1.5 ∴ \frac{1}{f} = (1.5 - 1) (\frac{1}{\infty} - \frac{1}{- 30}) \Rightarrow \frac{1}{f} = \frac{0.5}{30} \Rightarrow f = 60 c m$

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