Physics 0625 · syllabus 3.2
Cambridge IGCSE Physics 0625 — Light topical past papers
Reflection in plane mirrors, refraction and refractive index, critical angle and total internal reflection, converging and diverging lenses, and dispersion by a prism.
- Questions
- 935
- Years
- 2009–2026
- With mark scheme
- 99%
- Questions 2022–2026
- 383
Anyone can browse these pages. Opening the questions and their mark schemes takes a free Momentum account.
What Light covers
Light reflected from a plane mirror leaves at the same angle as it arrives: the angle of incidence equals the angle of reflection, both measured from the normal, a line at right angles to the surface. The image in a plane mirror is virtual, upright, the same size as the object and as far behind the mirror as the object is in front. When light crosses from one transparent material into another at an angle, it refracts: going from air into glass it slows down and bends towards the normal, and coming out it speeds up and bends away from the normal. Light travelling along the normal passes straight through.
When light inside glass meets the surface, the angle of refraction in the air is larger than the angle of incidence, and at one particular angle of incidence, the critical angle, it reaches 90°. Beyond the critical angle no light escapes and all of it reflects back inside: total internal reflection, used in optical fibres and in prism periscopes and reflectors. Extended candidates define refractive index as the ratio of the speed of light in air (strictly, in a vacuum) to its speed in the material, use n = sin i / sin r and n = 1/sin c, and describe how optical fibres carry signals for telecommunications.
A thin converging lens brings a parallel beam to a point, the principal focus, one focal length from the lens, while a diverging lens spreads a parallel beam out as if it came from a principal focus on the side the light comes from. Ray diagrams show the image a converging lens forms: real and inverted when the object is further away than the principal focus and, as Extended candidates also draw, virtual, upright and enlarged when the object is closer than it, as in a magnifying glass. Extended candidates also describe correcting long sight with a converging lens and short sight with a diverging lens. A glass prism disperses white light into a spectrum — red, orange, yellow, green, blue, indigo and violet — because each colour refracts by a slightly different amount; light of a single frequency is called monochromatic.
In the syllabus
The headings this topic has in the Cambridge International 2026–2028 syllabus, as the board prints them.
- 3.2 Light
What you should be able to do
- Use the law of reflection and describe the image formed by a plane mirror
- Describe how to trace rays through glass blocks, and draw the path of a ray into and out of a block
- Explain the critical angle and total internal reflection, with examples such as optical fibres
- Use n = sin i / sin r and n = 1/sin c (Extended)
- Draw ray diagrams for a converging lens forming a real image and, for Extended, a virtual image, and describe each image
- Describe dispersion by a prism and list the colours of the spectrum in order of wavelength and of frequency
Key ideas
- Reflection: angle of incidence = angle of reflection, both measured from the normal
- Refractive index n = speed of light in air ÷ speed of light in the material = sin i / sin r (Extended)
- Critical angle: sin c = 1/n; total internal reflection when light inside the denser material meets the surface at more than c (Extended)
- Converging lens: rays parallel to the principal axis meet at the principal focus, one focal length from the lens
- Visible spectrum from lowest to highest frequency (longest to shortest wavelength): red, orange, yellow, green, blue, indigo, violet
How Light has been examined
From 2022 to 2026, Light was on Paper 4 · Theory (Extended) in 14 of 14 exam series, worth about 12% of that paper’s marks.
| Paper | Questions | Share of marks | Series |
|---|---|---|---|
| Paper 1 · Multiple Choice (Core) | 136 | 11% | 14 of 14 |
| Paper 2 · Multiple Choice (Extended) | 142 | 11% | 14 of 14 |
| Paper 3 · Theory (Core) | 42 | 11% | 14 of 14 |
| Paper 4 · Theory (Extended) | 38 | 12% | 14 of 14 |
| Paper 6 · Alternative to Practical | 25 | 21% | 12 of 14 |
Momentum’s own count, over the questions in its bank for these years: a series is one exam session, every zone included, and a question filed under two chapters counts in both. Median marks per question: 1. The bank files this topic together with neighbouring ones, so these figures count every question in its chapters, including some on those topics.
Filed in Momentum under: Rays and Waves.
A worked example
A ray in air enters a glass prism with an angle of incidence of 52°, and inside the glass its angle of refraction is 31°. Find the refractive index of the glass and its critical angle.
- n = sin i / sin r = sin 52° / sin 31° = 0.7880 / 0.5150 = 1.530
- sin c = 1/n = 1 / 1.530 = 0.6536
- c = sin⁻¹ 0.6536 = 40.8°
- So light inside this glass that meets the surface at more than about 41° is totally internally reflected
Answer: Refractive index 1.53; critical angle 40.8°, about 41°
Written by Momentum for this page, with its own numbers; it is not taken from a past paper.
Mistakes that cost marks
- Measuring angles from the surface instead of the normal — every angle of incidence, reflection and refraction is measured from the normal, so a ray at 30° to the surface has an angle of incidence of 60°
- Bending the ray the wrong way at a boundary — light entering a denser material such as glass bends towards the normal, and light leaving it bends away
- Dividing the wrong way round when light leaves glass — n = sin (angle in air) ÷ sin (angle in glass), so an answer below 1 means the angles have been swapped
- Saying total internal reflection happens whenever light meets the inside of a glass surface — the light must be travelling in the denser material and meet the surface at more than the critical angle
- Calling the image in a magnifying glass real — it is virtual: the rays only appear to come from it, so it cannot be shown on a screen
Exam technique
- Draw ray diagrams with a ruler and a sharp pencil, put arrows on the rays, and use dashed lines for virtual rays and images
- For a converging lens, use two rays from the top of the object: one parallel to the principal axis, refracted through the principal focus, and one through the centre of the lens, which carries straight on
- Describe any image with three words: enlarged, same size or diminished; upright or inverted; real or virtual
- Check that your calculator is set to degrees before using sin and sin⁻¹
Practising in Momentum
- Every question in a chapter as one list, from every paper and year in the bank, newest first or shuffled.
- Each question turns over to its own part of the mark scheme, where the board published one.
- Narrow by paper, year, session, zone and marks, and star the questions worth keeping.
- Pick chapters, a number of questions and a time, and the exam builder draws a mock to sit and mark.
Past-paper questions and mark schemes are the copyright of Cambridge International. Momentum shows them to signed-in learners for revision and does not publish them on these pages.