A lens is any transparent material (e.g. glass) of an appropriate shape that can take parallel rays of incident light and either converge the rays to a point or diverge the rays from a point.
Lenses change the direction of light rays by refraction.
Lenses are used in eyeglasses, cameras, microscopes, and telescopes. You also have lenses in your eyes!
Converging Lenses (positive lenses)
Converging lenses converge parallel rays of light and are thicker in the middle than at the edges.
Converging or concave lens focus light rays to a single point.
Diverging Lenses (negative lenses)
Diverging lenses diverge parallel rays of light and are thicker at the edges than in the middle.
Diverging or convex lenses spread out the light rays so that it looks like they all come from the same point.
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