Lenses are of two types: the spherical and cylindrical
(toric or astigmatic).
1. Spherical lenses. Spherical lenses are bounded by two spherical surfaces and are mainly of two types : convex and concave.
(i) Convex lens or plus lens is a converging lens. It may be of biconvex, plano-convex or concavo-convex (meniscus) type .
Identification of a convex lens. (i) The convex lens is thick in the centre and thin at the periphery (ii) An object held close to the lens, appears magnified. (iii) When a convex lens is moved, the object seen through it moves in the opposite direction to the lens.
Uses of convex lens. It is used (i) for correction of hypermetropia, aphakia and presbyopia; (ii) in oblique illumination (loupe and lens) examination, in indirect ophthalmoscopy, as a magnifying lens and in many other equipments.
(ii) Concave lens or minus lens is a diverging lens. It is of three types: biconcave, plano-concave and convexo-concave (meniscus) . Identification of concave lens. (i) It is thin at the centre and thick at the periphery. (ii) An object seen through it appears minified. (iii) When the lens is moved, the object seen through it moves in the same direction as the lens.
Uses of concave lens. It is used (i) for correction of myopia; (ii) as Hruby lens for fundus examination with slit-lamp. Image formation by a concave lens. A concave lens always produces a virtual, erect image of an object
2. Cylindrical lens. A cylindrical lens acts only in one axis i.e., power is incorporated in one axis, the other axis having zero power. A cylindrical lens may be convex (plus) or concave (minus). A convex cylindrical lens is a segment of a cylinder of glass cut parallel to its axis (Fig. 3.13A). Whereas a lens cast in a convex cylindrical mould is called concave cylindrical lens (Fig. 3.13B). The axis of a cylindrical lens is parallel to that of the cylinder of which it is a segment. The cylindrical lens has a power only in the direction at right angle to the axis. Therefore, the parallel rays of light after passing through a cylindrical lens do not come to a point focus but form a focal line
Identification of a cylindrical lens. (i) When the cylindrical lens is rotated around its optical axis, the object seen through it becomes distorted. (ii) The cylindrical lens acts in only one axis, so when it is moved up and down or sideways, the objects will move with the lens (in concave cylinder) or opposite to the lens (in convex cylinder) only in one direction. Uses of cylindrical lenses. (i) Prescribed to correct astigmatism (ii) As a cross cylinder used to check the refraction subjectively. Images formed by cylindrical lenses. Cylindrical or astigmatic lens may be simple (curved in one meridian only, either convex or concave), compound (curved unequally in both the meridians, either convex or concave). The compound cylindrical lens is also called sphericylinder. In mixed cylinder one meridian is convex and the other is concave.
(toric or astigmatic).
1. Spherical lenses. Spherical lenses are bounded by two spherical surfaces and are mainly of two types : convex and concave.
(i) Convex lens or plus lens is a converging lens. It may be of biconvex, plano-convex or concavo-convex (meniscus) type .
Identification of a convex lens. (i) The convex lens is thick in the centre and thin at the periphery (ii) An object held close to the lens, appears magnified. (iii) When a convex lens is moved, the object seen through it moves in the opposite direction to the lens.
Uses of convex lens. It is used (i) for correction of hypermetropia, aphakia and presbyopia; (ii) in oblique illumination (loupe and lens) examination, in indirect ophthalmoscopy, as a magnifying lens and in many other equipments.
(ii) Concave lens or minus lens is a diverging lens. It is of three types: biconcave, plano-concave and convexo-concave (meniscus) . Identification of concave lens. (i) It is thin at the centre and thick at the periphery. (ii) An object seen through it appears minified. (iii) When the lens is moved, the object seen through it moves in the same direction as the lens.
Uses of concave lens. It is used (i) for correction of myopia; (ii) as Hruby lens for fundus examination with slit-lamp. Image formation by a concave lens. A concave lens always produces a virtual, erect image of an object
2. Cylindrical lens. A cylindrical lens acts only in one axis i.e., power is incorporated in one axis, the other axis having zero power. A cylindrical lens may be convex (plus) or concave (minus). A convex cylindrical lens is a segment of a cylinder of glass cut parallel to its axis (Fig. 3.13A). Whereas a lens cast in a convex cylindrical mould is called concave cylindrical lens (Fig. 3.13B). The axis of a cylindrical lens is parallel to that of the cylinder of which it is a segment. The cylindrical lens has a power only in the direction at right angle to the axis. Therefore, the parallel rays of light after passing through a cylindrical lens do not come to a point focus but form a focal line
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| Fig. 3.13.Cylindrical lenses: convex (A) and concave (B). |
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| Fig. 3.14.Refraction through a convex cylindrical lens |
Identification of a cylindrical lens. (i) When the cylindrical lens is rotated around its optical axis, the object seen through it becomes distorted. (ii) The cylindrical lens acts in only one axis, so when it is moved up and down or sideways, the objects will move with the lens (in concave cylinder) or opposite to the lens (in convex cylinder) only in one direction. Uses of cylindrical lenses. (i) Prescribed to correct astigmatism (ii) As a cross cylinder used to check the refraction subjectively. Images formed by cylindrical lenses. Cylindrical or astigmatic lens may be simple (curved in one meridian only, either convex or concave), compound (curved unequally in both the meridians, either convex or concave). The compound cylindrical lens is also called sphericylinder. In mixed cylinder one meridian is convex and the other is concave.
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