Pins far apart, angles from the normal. Trace a ray through a glass block.
Ray tracing with optics pins comes up again and again in Paper 6. You draw round a glass block, draw the normal and an incident ray, mark it with two pins, then look through the block and line up two more pins. Watch the trace built step by step with a graph of the results, then build it yourself and check it against the mark points.
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 February/March 2025, (refraction through a rectangular block, pin separation and techniques) · official past papers
Practise next: 0625/61 October/November 2025, (refraction ray trace at i = 30° and choosing more angles) · official past papers
Practise next: 0625/61 May/June 2025, (reflection in a plane mirror traced with pins) · official past papers
Examiner reports used: 0625 June 2023, November 2024, March 2025, June 2025 and November 2025.
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What a full-mark diagram shows
The normal
Drawn at 90° to the face of the block (not to the ray), and continued through the block.
Pin separation
P1 and P2, and P3 and P4, at least 5 cm apart. Pins that are too close are a common lost mark.
Angles from the normal
Protractor centre on N, zero line along the normal. Never measure from the surface or the mirror.
An accurate trace
Thin lines with a sharp pencil, thin pins, pins upright, and view the bases of the pins.
Habits the examiner reports keep asking for
Name the technique itself: “view the bases of the pins” or “draw thin lines”, not “avoid parallax”.
Answer about the method in the question. No dark room or ray box in a pin experiment; no pins in a ray-box experiment.
Compare values with numbers: say whether they are within about 10% of each other, and show the working.
Choose new values of i sensibly: at least three more, all under 90°, spread over at least 30°.
Use a 30 cm ruler so each ray is one straight line, and label the points the question asks for.