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| Lenses |
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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. |
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| Basically, the lenses are classified as |
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 convex lens or converging lens |
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 concave or diverging lens |
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| A lens which is thicker in the middle and thinner at the edges is called a convex lens. |
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| 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 |
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 bi-convex or double convex |
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 convexo - plano |
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 convexo - concave |
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| A lens which is thinner at the middle and thicker at the edges is called a concave lens. |
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| Like convex lenses these lenses are also classified as |
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 bi-concave |
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 concave - plano |
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 concavo - convex |
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