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This plus sign means the lenses are converging light rays to bring images in focus closer than they really do.

Lets explain that a little further. In general, consider there a three types of lenses. Concave surfaced (minus), flat surface (plano), and Convex surface (plus).

A lens with a concave surface will expand light rays as they pass through, bringing the area of focus further away. In reality, if you expand light rays, they will never meet until they go through another (convex) refractive surface. Nearsighted (myopic) eyes require minus lenses to bring the focal area of the image further back in to the eye. A truly flat surface lens will have an overall power of zero, meaning it does not refract light.

Think of a window, you can see through it, but it does not change the image.

Notice how the image gets distorted? Consider those wrinkles as little lenses at work.

the reason we need reading glasses is to re-adjust the focal point closer now that our eyes are no longer capable of doing so on their own. So, a plus power, convex lens is just what we need. These lenses will bring our range of focus closer with each diopter we add.

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From: (what power reading glasses) http://www.readinstyle.com/pluspower.aspx