Answer
$90\ mm$
Work Step by Step
Given:
Refractive index of glass (n) = 1.5
Focal length f = 60 mm
Let,
Radius of the first surface is $R_1 = R$
Radius of the second surface is $R_2 = -2R$
We then use the following formula to find $R$:
$\frac{1}{f} = (n-1)(\frac{1}{R_1}-\frac{1}{R_2})$
$\frac{1}{f} = (n-1)(\frac{1}{R}+\frac{1}{2R})$
$\frac{1}{f} = (n-1)\frac{3}{2R}$
$R =\frac{3f(n-1)}{2}$
$R =\frac {3( 60\ mm)(1.5-1)}{2}$
$R= 45\ mm$
Larger radius $R_2 = 2R = 2\times45\ mm = 90\ mm$
