Concept of Lenses

A lens is a transparent medium bounded by two surfaces that refracts light. Lenses are widely used in spectacles, telescopes, cameras, and magnifying devices. They help in focusing, magnifying, or dispersing light rays to form clear images.

• Convex Lens (Converging Lens) → Thicker at the center, bends light inward, forming a real or virtual image.
• Concave Lens (Diverging Lens) → Thinner at the centre, it bends light outward, always forming a virtual image.

Types of lenses

1. Convex Lens (Diagram a)

A convex lens is formed by two spherical surfaces bulging outward.

• The surfaces S₁ and S₂ represent parts of two spheres.
• C₁ and C₂ are the centres of curvature of these spheres.
• The surface labelled 1 is part of sphere S₁, and surface 2 is part of sphere S₂.
• The lens is thicker at the centre and thinner at the edges.
• It converges light rays, making it useful in magnifying glasses, cameras, and telescopes.

2. Concave Lens (Diagram b)

A concave lens is formed by two spherical surfaces curving inward.

• The surfaces S₁ and S₂ represent sections of two spheres.
• C₁ and C₂ are the centres of curvature of these spheres.
• The surface labelled 1 is part of sphere S₁, and its surface 2 is part of sphere S₂.
• The lens is thinner at the centre and thicker at the edges.
• It diverges light rays, making it useful for correcting myopia (nearsightedness) and in laser applications.

 Cross-sections of convex and concave lenses

A lens is a transparent refracting medium bounded by either two spherical surfaces, or one spherical surface and the other surface plane.

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A lens is a transparent medium bound by two surfaces.

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A lens is a transparent medium (such as glass) bounded by two curved surfaces or one curved and one plane surface.

A lens which bulges out in the middle, is a convex lens. A light beam converges on passing through such a lens, so it is also called a converging lens.

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The lens which has two spherical surfaces which are puffed up outwards is called a convex or double convex lens.

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The lenses which are thicker in the middle and thinner at the edges, are called 'convex lenses'.

A lens which is bent inwards in the middle is a concave lens. Such a lens diverges the light rays incident on it, so it is also called a diverging lens.

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This lens is thicker near the centre as compared to the edges. The lens with both surfaces spherical on the inside is called a concave or double concave lens.

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The lenses which are thinner in the middle and thicker at the edges, are called 'concave lenses'.

• Lenses are widely used in daily life, such as in spectacles, peepholes, magnifiers, and telescopes.
• Light passing through a lens undergoes refraction twice: once on entering and once on exiting the lens.
• The shape of a lens affects the direction of light; convex lenses converge light, while concave lenses diverge it.
• Most lenses have surfaces that are parts of spheres, with common types including biconvex, biconcave, plano-convex, and meniscus lenses.

The centres of spheres whose parts form surfaces of the lenses are called centres of curvatures of the lenses.

The radii (R₁ and R₂) of the spheres whose parts form surfaces of the lenses are called the radii of curvature of the lens.

The imaginary line passing through both centres of curvature is called the principal axis of the lens.

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The line joining the centres of curvature of the surfaces of the lens is called the 'principal axis' of the lens.

The point inside a lens on the principal axis, through which light rays pass without changing their path is called the optical centre of a lens.

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The point on the principal axis of a lens such that a ray of light directed towards it emerges from the lens in the same direction, without deviation.

Principal focus (F) is the point on the principal axis at which light rays parallel to the principal axis converge after passing through a convex lens.

The distance between the optical centre and principal focus of a lens is called its focal length.