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A lens is a portion of a transparent refracting medium bounded by two surfaces which are generally spherical or cylindrical or one curved and one plane surface.
Basically, the lenses are classified as
- convex lens or converging lens
- concave or diverging lens
A lens which is thicker in the middle and thinner at the edges is called a convex lens.
In a convex lens at least one of its surfaces is bulging out at the
middle. According to their shapes the convex lenses are classified as
- bi-convex or double convex
lens
- plano - convex lens
- concavo - convex lens
A lens which is thinner at the middle and thicker at the edges is called a concave lens.
Like convex lenses these lenses are also classified as
- bi-concave
- plano - concave
- convexo - concave
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