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| Lenses |
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| Lens is a portion of transparent refracting medium bound by two spherical surfaces or one spherical surface and the other plane surface. |
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| Lenses are divided into two classes: |
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| Convex or Convergent Lens |
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| Convex lens are thin at the edges and thick at the centre. |
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| Concave or Divergent Lens |
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| Concave lens are thick at the edge and thin at the centre. |
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| Principal Axis |
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| It is defined as a straight line passing through the centre of curvatures of two surfaces of a lens. |
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| Optical Centre (C ) |
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| It is a point lying on the principal axis of the lens so that a ray of light whose refracted path passes through this point will have its emergent path parallel to the direction of the incident ray. |
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| Principal Focus (F) |
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| When a beam of light is incident on a lens in a direction parallel to the principal axis of the lens, the rays after refraction through lens converge to or appear to diverge from a point on the principal axis. |
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| In case of a convex lens, the rays after refraction actually come to a focus and hence a real focus is obtained at F. |
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| Convex Lens |
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| In case of a concave lens, the rays after refraction appear to come from F and hence F is a virtual focus. |
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| Convcave Lens |
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| According to sign conventions F is positive for convex lens and is negative for concave lens. |
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