The Radius Of Curvature Of The Convex Surface Of A Thin Plano-convex Lens Is 20 Cm. Refractive Index Of The Medium Of The Lens Is 1.5. Calculate The Focal Length Of The Lens.   

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The radius of curvature of the convex surface of a thin plano-convex lens is 20 cm. Refractive index of the medium of the lens is 1.5. Calculate the focal length of the lens.   


Correct option is

40 cm


For the focal length f of a lens in air, we have


Where n is the refractive index of lens material (glass) with respect to air, and R1 and R2 are radii of curvature of the lens surfaces.

Here n = 1.5, R1 = +20 cm and R2 = ∞ (plane).


or     f = 40 cm.  



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