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Q(i). If f = 0.5 m for a glass lens, what is the power of the lens? (ii) The radii of curvature of the faces of a double convex lens are 10 cm and 15 cm. Its focal length is 12 cm. What is the refractive index of glass? (iii) A convex lens has 20 cm focal length in air. What is focal length in water ? (Refractive index of air-water =  4 3 , refractive index for air-glass =  3 2 ,)
 

Dear Student,
                      1)  Power =1f=1.5=2D2)  We have        1f=(n-1)1R1-1R2 for convex lens      112=(n-1)110+115   n=1.5 3) we ahve 120=(1.5-1)1R1+1R2-----11fw=(1.33-1)1R1+1R2-----2divide 1 by 2 we get fw=.5.33×20=30.33cm
Regards

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