Refraction through Lenses — Part 1: Lenses, rays and images: visual lesson, ICSE Class 10 Physics (Science Paper 1)

Lesson

Lenses all around us

Part 1 — a lens is a piece of transparent material bounded by two surfaces, at least one of them curved. Lenses in spectacles, cameras and projectors all work by refraction.

In this lesson you will

Label the principal axis, optical centre, principal foci and focal length of a lens
Explain converging and diverging action using the lens-as-prisms model
Draw ray diagrams for a convex lens for six object positions and describe each image
Explain why a concave lens always forms a virtual, erect, diminished image

Table of four everyday devices, the kind of lens each uses and its job
Device	Lens used	Job
Reading glasses of a grandparent	Convex	Bring near print into focus
Spectacles of a short-sighted student	Concave	Spread light so distant objects are clear
Mobile phone camera	Convex (lens system)	Form a real image on the sensor
Door peephole	Concave and convex	Wide view of the corridor

Concept

Convex and concave lenses

A convex (converging) lens is thicker at the middle than at the edges; it bends a parallel beam to meet at a point. A concave (diverging) lens is thinner at the middle; it spreads a parallel beam out. Common shapes: biconvex, plano-convex, concavo-convex (thicker middle); biconcave, plano-concave, convexo-concave (thinner middle). A lens can be pictured as a set of prisms: in a convex lens their bases point towards the axis, so all rays bend towards the axis.

Diagram

Terms used for a lens

Centres of curvature $C_{1}$ , $C_{2}$ : centres of the spheres of which the lens surfaces are parts.
Principal axis: the line through both centres of curvature.
Optical centre O: the point on the axis through which a ray passes undeviated.
Principal foci $F_{1}$ , $F_{2}$ : rays parallel to the axis converge to $F_{2}$ (convex) or appear to diverge from $F_{1}$ (concave); a lens has two foci, one on each side.
Focal plane: the plane through a focus, normal to the axis.
Focal length f: distance from O to a principal focus.

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