The radii of curvature of both the sides of a convex lens are 15 cm and the refractive index of the material of the lens is 1.5. Then the focal length of the lens in air is cm.

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(2) 15. Given: a double convex lens with both radii 15 cm, made of glass of n=1.5, in air. Idea: the lens maker's formula, 1/f=(n-1)(1/(R₁)-1/(R₂)), with the sign convention that for a biconvex lens R₁=+15 cm and R₂=-15 cm. 1/f=(1.5-1)(1/(15)+1/(15))=0.5×2/(15). That is 1/f=1/(15). So f=15 cm: whenever n=1.5 and the two radii are equal, the focal length comes out equal to the radius of curvature.

The radii of curvature of both the sides of a convex lens are 15 cm and the refractive index of the material of the lens is 1.5. Then the focal length of the lens in air is ______ cm.

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