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Cambridge IGCSE Physics · 0625

Chapter 3: Waves — Part 4

Topic 3.2.3 · Thin lenses

Converging and diverging lenses

A thin converging lens brings a parallel beam to a point. A thin diverging lens spreads a parallel beam out.

The principal axis is the line through the centre of the lens. The principal focus (F) is where parallel rays on the axis are made to converge (or appear to diverge from). Focal length f is the distance from the lens centre to F.

Thin converging lens: parallel rays meeting at the principal focus F; principal axis and focal length f marked. Thin diverging lens: parallel rays spreading out as if from F.
Diagram 1: Thin converging lens: parallel rays meeting at the principal focus F; principal axis and focal length f marked. Thin diverging lens: parallel rays spreading out as if from F.

Real and virtual images

Describe images as enlarged / same size / diminished, upright / inverted, and real / virtual.

A real image is formed where rays actually meet and can be shown on a screen. A virtual image is formed where diverging rays are extrapolated backwards; it cannot form a visible projection on a screen.

Draw ray diagrams for a real image by a converging lens (object outside F) and for a virtual image (object inside F). A single converging lens used with the object inside F is a magnifying glass.

Ray diagram for a real inverted image by a converging lens (object beyond F): ray through centre undeviated, ray parallel to axis through F, image on the far side.
Diagram 2: Ray diagram for a real inverted image by a converging lens (object beyond F): ray through centre undeviated, ray parallel to axis through F, image on the far side.
Ray diagram for a virtual magnified image (object inside F) used as a magnifying glass, plus simple ray sketches of lenses correcting short-sight and long-sight.
Diagram 3: Ray diagram for a virtual magnified image (object inside F) used as a magnifying glass, plus simple ray sketches of lenses correcting short-sight and long-sight.

Correcting eyesight

Short-sight (myopia): distant objects focus in front of the retina. Correct with a diverging lens.

Long-sight (hyperopia): near objects focus behind the retina. Correct with a converging lens.

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