Sunglasses don’t have power because the lenses are two curved surfaces with the same radius of curvature. The convex (outside) surface produces magnification, and the concave (inner) surface (the surface near the eye) produces demagnification of the same strength and the two cancel. Since the curvatures on either side of a sunglass are of the same radius, it has no power.
Also, the lenses on sunglasses are made of materials which are less refractive than that used for regular glasses.
It’s another thing that you can add power lenses on to your sunglasses, but sunglasses don’t come with a prescribed lens.
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