Question

a double convex lens is made of a glass of refractive index 155, with both faces of the same radius of curvature. find the radius of curvature required, if the focal length is 20 cm.

Answer 1

1/f =(n - 1) 1/R1 - 1/R2

acc. to sign convention,

f = - 20 ,

R1= -r ,

R2= r

1/-20 = (155-1)(1/-r - 1/r)

bcos r is same for both faces

1/-20= 154 (-2/r)

= - 308/r

reciprocating,

-20= r/-308

r = 308 × 20

r = 6160cm

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