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 30 cm and the refractive index of the lens material is 1.5, then the focal length of the lens (in cm) is ______.
Answer (integer)
60
Solution
Lens-maker formula
<br><br>$${1 \over f} = \left( {\mu - 1} \right)\left( {{1 \over {{R_1}}} - {1 \over {{R_2}}}} \right)$$
<br><br>for plano-convex lens
<br><br>${{R_1}}$ = $\infty$ and R<sub>2</sub> = -R
<br><br>$\therefore$ f = ${R \over {\mu - 1}}$ = ${{30} \over {1.5 - 1}}$ = 60 cm
About this question
Subject: Physics · Chapter: Optics · Topic: Lenses and Optical Instruments
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