Paper 3 at a glance
- Paper 3 carries 15 marks
Experiment: Determining the refractive index of a glass block
To determine the refractive index of a glass block by measuring the angle of incidence and the angle of refraction.
- Manipulated: Angle of incidence, i
- Responding: Angle of refraction, r
- Constant: The same glass block (same medium)
- Graph: Plot sin i (y-axis) against sin r (x-axis). A straight line through the origin shows sin i is directly proportional to sin r; the gradient is the refractive index.
- Analysis: By Snell's law, n = sin i / sin r. The gradient of the sin i against sin r graph gives the refractive index n of the glass.
Experiment: Determining the refractive index of glass by real depth and apparent depth
To determine the refractive index of glass by comparing the real depth and the apparent depth of a mark seen through glass blocks.
- Manipulated: Real depth of the mark, H (using glass blocks of different thickness)
- Responding: Apparent depth of the mark, h
- Constant: The same type of glass and the same viewing method
- Graph: Plot the real depth H (y-axis) against the apparent depth h (x-axis). A straight line through the origin shows H is directly proportional to h; the gradient is the refractive index.
- Analysis: The refractive index is n = real depth / apparent depth = H / h. The gradient of the H against h graph gives the refractive index of the glass.
Experiment: Object distance and image distance for a convex lens
To investigate the relationship between the object distance u and the image distance v for a convex lens and to find its focal length.
- Manipulated: Object distance, u
- Responding: Image distance, v
- Constant: The same convex lens (same focal length)
- Graph: Plot 1/v (y-axis) against 1/u (x-axis). The straight line cuts each axis at 1/f, so the intercept gives the focal length.
- Analysis: The lens formula is 1/f = 1/u + 1/v. The intercept of the 1/v against 1/u graph on either axis equals 1/f, so f = 1 ÷ intercept.
Paper 3 skill checklist
- Table: quantities with units in the headings, consistent decimal places, derived column calculated correctly.
- Graph: axes labelled with unit, scale using more than half the grid, points plotted with a sharp pencil, best-fit line.
- Gradient: a large triangle on the line, coordinates shown, unit of the gradient stated.
- Relationship: "y increases linearly with x" or "y is directly proportional to x" only if the line passes through the origin.
- Precaution: one that improves accuracy, stated with its reason.
More on graph skills
Exam tip
Units on every calculation line; no formula is provided in the SPM Physics papers.
More Light and Optics resources
- Chapter overview: Light and Optics
- Revision Notes
- Common Mistakes
- Practice Questions
- Paper 2 Answering Guide
- Key Terms
- Calculation practice sets
Source: DSKP KSSM Physics Form 4 and 5 (Versi English), Sijil Pelajaran Malaysia: Format Pentaksiran mulai 2021, Fizik (4531) (Bahagian Pembangunan Kurikulum (BPK), KPM)
Written by the spmphysics.com.my editorial team.· Updated 5 Sept 2026