Finding the image formed by a converging lens comes up again and again in 0625 Paper 6. Here a lamp lights an arrow-shaped hole in a card, and a lens and a screen stand on a bench with a metre rule fixed along it. Watch the set-up built step by step, then build it yourself, find sharp images for different object distances and check your set-up against the points the examiners look for.
Past-paper referenceOriginal experiment, wording and figures, written in the style of Cambridge IGCSE Physics 0625 Paper 6 (Alternative to Practical). It is not copied from a past paper. The “what the examiners said” notes are our paraphrase of the Principal Examiner Reports for Teachers.
Practise next: 0625/62 October/November 2024, (image distance v against object distance u, magnification and focal length) · official past papers
Practise next: 0625/62 May/June 2025, (fixed object-to-screen distance, two positions of the lens) · official past papers
Practise next: 0625/63 May/June 2024, (image height, a graph and focal length from a gradient) · official past papers
Examiner reports used: 0625 November 2023 (Papers 61 and 62), March 2024 (Paper 62), June 2024 (Paper 63), November 2024 (Papers 61 and 62) and June 2025 (Paper 62).
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What a full-mark diagram shows
The set-up
Illuminated object, converging lens in a holder and screen in a line, centres at the same height on one axis, lens and screen upright, metre rule fixed to the bench.
Finding the image
Move the screen slowly back and forth until the image is as sharp as possible, in a darkened room for better contrast.
The readings
u and v measured to the centre of the lens, to the nearest 0.1 cm; at least five different values of u.
The results
Magnification m = v / u with no unit; a smooth best-fit curve of v against u; f from the graph or f = uv / (u + v).
Habits the examiner reports keep asking for
“Move the screen” is not enough: move it slowly, back and forth, through the sharpest image.
If the object-to-screen distance is fixed, it is the lens that you move to find the sharp image.
A darkened room gives better contrast, so the edges of the image are easier to see.
Measure to the centre of the lens, and record to the precision asked for: 37.5 cm, not 37.50 cm or 38 cm.
More readings means different values of u, at least five, not the same u repeated.