Question

Why is the radius of curvature of the surface of sun glasses kept the same ???

Answer 1

Is it to keep the focal length same???
If focal length is different for both lenses what will happen??

If a person has defective eye-sight, say myopia, then also will the radius of curvature be kept the same???

Answer 2

For normal eye , power of sun glasses should be 0.
In order to keep the power 0 , it is given as reciprocal of focal length,
hence focal length, and accordingly radius of curvature will be 1/0 or infinity.
However, if there is some defect in the eye, the power and radius of curvature of the sunglasses may not be same. more curvature would cause you to see distortedly

Answer 3

In other words, for all ordinary sun glasses, all of them have no curvature as all have a focal point at infinity.

Answer 4

(And yes, for custom sun glasses, they will have a curvature.)