A contact lens is made of plastic with an index of refraction of 1.58. The lens has a focal length of +28.0 cm

Question

A contact lens is made of plastic with an index of refraction of 1.58. The lens has a focal length of +28.0 cm

Answer 1

No need to restate the same thing the others have said, but in regards to using a "converter"... 1) Normally they will cause you to lose 2-3 stops on the aperture. A 18-200mm f/3.5-5.6 will act like a 36-400 f/7.1-11 2) Some converters do not allow the lens and camera to communicate. This may present a problem (especially if you depend on auto focus, or worse...you may lose aperture control unless the lens has a manual aperture)

Answer 2

(1/r1) + (1/r2) = 1/f

The rest you do yourself.

Answer 3

use the lensmaker's equation:

1/f = (n0/n1 - 1)(1/r1 + 1/r2)

f is the focal length of the lens,

n0 is the refractive index of the lens material,

n1 is the refractive index of the medium the lens is in,

R1 is the radius of the lens closest to the light source,

R2 is the radius of the lens farthest from the light source

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if it's air where the lens is in... n0 = 1

1/[f(n0/n1 - 1)] = 1/r1 + 1/r2

1/[f(n0/n1 - 1)] - 1/r2 = 1/r1

1/r1 = 1/[28 cm (1.58 - 1)] - 1/(23 cm)

1/r1 = 1/(16.24 cm) - 1/(23 cm)

1/r1 = 0,018098093810237738273720282715785 cm^-1

r1 = 55,25 cm