What a lens is
A lens is an optical device with curved surfaces, made of glass, plastic or another transparent material. It changes the path of light passing through it by refraction.
The lens inside your eye forms images on the retina. Telescopes and binoculars use lenses to see distant things clearly, microscopes to see tiny parts too small for the naked eye, and a magnifying glass — a simple microscope — to make small things look big.
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Most lenses are made of glass, but plastic lenses are becoming more common. Any transparent material can be used — even water or another liquid.