A lens is a transparent refracting medium, bounded by two surfaces which form a part of a sphere (spherical lens) or a cylinder (cylindrical or toric lens).
Cardinal data of a lens
1. Centre of curvature (C) of the spherical lens is the centre of the sphere of which the refracting lens surface is a part.
2. Radius of curvature of the spherical lens is the radius of the sphere of which the refracting
surface is a part.
3. The principal axis (AB) of the lens is the
line joining the centres of curvatures of its surfaces.
4. Optical centre (O) of the lens corresponds to the nodal point of a thick lens. It is a point on the principal axis in the lens, the rays passing from where do not undergo deviation. In meniscus lenses the optical centre lies outside the lens.
5. The principal focus (F) of a lens is that point on the principal axis where parallel rays of light, after passing through the lens, converge (in convex lens) or appear to diverge (in concave lens).
6. The focal length (f) of a lens is the distance between the optical centre and the principal focus.
7. Power of a lens (P) is defined as the ability of the lens to converge a beam of light falling on the lens. For a converging (convex) lens the power is taken as positive and for a diverging (concave) lens power is taken as negative. It is measured as reciprocal of the focal length in metres i.e. P = 1/f. The unit of power is dioptre (D). One dioptre is the power of a lens of focal length one metre.
Cardinal data of a lens
1. Centre of curvature (C) of the spherical lens is the centre of the sphere of which the refracting lens surface is a part.
2. Radius of curvature of the spherical lens is the radius of the sphere of which the refracting
surface is a part.
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| Cardinal points of a convex lens: optical centre (O); principal focus (F); centre of curvature (C); and principal axis (AB). |
3. The principal axis (AB) of the lens is the
line joining the centres of curvatures of its surfaces.
4. Optical centre (O) of the lens corresponds to the nodal point of a thick lens. It is a point on the principal axis in the lens, the rays passing from where do not undergo deviation. In meniscus lenses the optical centre lies outside the lens.
5. The principal focus (F) of a lens is that point on the principal axis where parallel rays of light, after passing through the lens, converge (in convex lens) or appear to diverge (in concave lens).
6. The focal length (f) of a lens is the distance between the optical centre and the principal focus.
7. Power of a lens (P) is defined as the ability of the lens to converge a beam of light falling on the lens. For a converging (convex) lens the power is taken as positive and for a diverging (concave) lens power is taken as negative. It is measured as reciprocal of the focal length in metres i.e. P = 1/f. The unit of power is dioptre (D). One dioptre is the power of a lens of focal length one metre.
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