Question

A double convex lens has two surfaces of equal radii 'R' and refractive index n= 15.Find the focal length 'f'.

Answer 1

1f=(n-1) [(1/R1) -(1/R2)]
put the values u will get ur answer

Answer 2

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Using lens makers formula,           1/f=(n-1)[(1/R1)-(1/R2)] Here n=15, R1= R and R2= -R Therefore,           1/f=(15-1)[(1/R)-(1/-R)]    =>.  1/f= 14×[2/R]    =>.  1/f= 28/R    =>.  f = R/28