9.1 Lens shapes and actions

A thin converging lens is thicker at the centre than at the edges. It refracts a parallel beam toward the principal axis so the rays meet. A thin diverging lens is thinner at the centre and spreads a parallel beam outward. The refracted rays from a diverging lens appear to originate from a point on the incident side.

The description “thin lens” means its thickness is small compared with other distances in the ray diagram, allowing refraction to be represented at one central line. Real lenses have curved surfaces and finite thickness, but the thin-lens model is accurate enough for the syllabus.

9.2 Principal axis and optical centre

The principal axis is the straight line through the optical centre and the centres of curvature of the lens surfaces. A ray passing through the optical centre of a thin lens is drawn undeviated in the simplified model.

Ray diagrams must use a ruler and show arrowheads. The lens should be centred on the principal axis, and focal points must be marked at equal distances on both sides.

9.3 Principal focus and focal length

For a converging lens, rays parallel to the principal axis converge at the principal focus F on the far side. This is a real focus because light actually passes through it. For a diverging lens, parallel rays spread and their backward extensions meet at a virtual focus on the incident side.

Focal length f is the distance from the optical centre to the principal focus. A more powerful converging lens bends rays more strongly and has a shorter focal length. The O Level syllabus does not require a numerical lens-power equation.

9.4 Estimating focal length

Point a converging lens at a distant object, such as a building or tree, and place a white screen behind the lens. Move the screen until a sharp inverted image forms. The lens-to-screen distance is approximately the focal length because rays from a distant object reach the lens nearly parallel.

Measure from the optical centre of the lens, darken the surroundings if possible and repeat using several distant objects. Never look directly at the Sun through a lens; a focused solar image can damage eyes or cause burns.

9.5 Comparing lens diagrams

A converging lens is often drawn as a vertical line with outward arrowheads; a diverging lens with inward arrowheads. In full shape drawings, converging lenses bulge outward and diverging lenses curve inward at the centre.

When identifying a lens from rays, use its action rather than the sketch alone: parallel rays that converge indicate a converging lens; rays that diverge as if from a focus indicate a diverging lens.

Worked examples

Practical focus

Examination guidance

Check your understanding

1. What happens to a parallel beam through a diverging lens?
2. What is focal length?
3. Why is the distant-object method approximate?