A lens is a clear object that manipulates light by convergence or divergence.
A convex lens is curved outward so that it is able to manipulate light in a convergent way. The light shining through a convex lens is focused at one point. An example of a convex lens is the human eye. Eyes use convex lenses to focus on objects that are different distances from the viewer.
A concave lens is curved inward so that it is able to manipulate light in a divergent way. Light passing through a concave lens is spread out over a larger area. An example of a concave lens is a flashlight. Flashlights use a small light and a concave lens to spread the light over a large area.
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