Sorry, wrong answer.
Snell's law tells us that the light always bends toward the thick part of the lens.
If rays parallel to the axis and close to it bend together (converge) in passing through the lens, they intersect at a real focal point on the axis, giving a converging lens and a positive focal length.
If rays parallel to the axis and close to it bend apart (diverge) in passing through the lens, they can be extended back to intersect at a virtual focal point on the axis, giving a diverging lens and a negative focal length.
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