A converging lens and a screen appear in almost every 0625 Paper 5 optics question. Switch on the lamp, slide the lens to each object distance and slide the screen until the image is sharp, judging it yourself in the screen view. Read both positions, work out f for every row and describe the image. Every attempt uses a different lens. Allow about 20 minutes.
Past-paper referenceOriginal experiment, wording and numbers, written in the style of Cambridge IGCSE Physics 0625 Paper 5 (Practical Test), Question 3. It is not copied from a past paper.
Practise next: 0625/53 May/June 2024, Question 3 (the focal length of a converging lens from sharp images) · official past papers
Practise next: 0625/52 October/November 2024, Question 3 (the image formed by a converging lens at five object distances) · official past papers
This type of experiment has come up 60 times (32 in Paper 5, the Practical Test, and 28 in Paper 6, the Alternative to Practical) in the past papers we checked. Show the list
Checked: Cambridge IGCSE Physics 0625 Papers 5 and 6, 2014 to 2025. Only the papers in our collection are listed, so the real number may be higher. Question papers are available from Cambridge.
This workout needs JavaScript.
How this question is marked
Readings and table (a)
Headings with units, v at the sharpest image for every u to 0.1 cm, f = uv / (u + v) for each row, and the mean focal length.
The image (b)
Inverted and magnified at u = 20.0 cm, with m = v / u from your own reading.
Technique and evaluation (c) to (e)
Finding the sharpest image by moving through it, keeping everything on one axis, and how the image changes as u decreases.
Habits that save marks
Bracket the sharp image: move past it both ways and choose the middle.
Measure to the centre of the lens, for u and for v.
Keep everything in line: object, lens centre and screen at the same height.
The marking follows the conventions in published Cambridge mark schemes. The experiments, readings and wording on this page are our own.