Who this is for: Cambridge IGCSE Physics (0625) students who want light — reflection, refraction, total internal reflection and lenses — to become reliable marks instead of ray diagrams they draw from memory without rules.
What query it owns: how to understand and revise light in Cambridge IGCSE Physics.
Why this is safe: this page owns the light revision-guide angle, while Tutopiya’s Light subtopic page owns the learning resource and the free Light quiz owns the practice.

Light is one of the highest-mark topics in the Cambridge IGCSE Physics (0625) Waves unit. Examiners regularly test ray diagrams for plane mirrors, glass blocks, total internal reflection in optical fibres, and converging lenses forming real and virtual images. This guide covers the syllabus content, the ray rules you must know, and the question types that appear every year.

Key takeaways

• Law of reflection: angle of incidence = angle of reflection; measured from the normal.
• Refraction: light slows down entering a denser medium → bends towards the normal.
• Total internal reflection (TIR) occurs when light travels from dense to less dense medium above the critical angle.
• Converging lens: parallel rays converge at the principal focus F; use three standard rays for image location.
• Dispersion: white light splits into a spectrum because different colours travel at slightly different speeds in glass.

What is light in Cambridge IGCSE Physics?

Light is a transverse electromagnetic wave that travels at approximately 3 × 10⁸ m/s in a vacuum. In Cambridge IGCSE Physics (0625) you model light using rays — straight lines showing the direction of travel. You must draw and interpret reflection at plane mirrors, refraction at boundaries, total internal reflection, and images formed by thin converging lenses.

You can read the full explanation, worked examples and notes on Tutopiya’s Light subtopic page before you attempt questions.

The core ideas you must master

Idea	What it means	How the exam uses it
Normal	Line perpendicular to surface at point of incidence	”Mark the angle of incidence.”
Reflection	i = r; image same size, virtual, laterally inverted	”Complete the ray diagram for a plane mirror.”
Refraction	Speed change → direction change	”Explain why the ray bends towards the normal.”
Critical angle	Angle in dense medium when refracted ray is along boundary	”Calculate the critical angle.”
Converging lens	Thicker in middle; focuses parallel rays	”Draw a ray diagram to locate the image.”

How to draw lens ray diagrams — step by step

1. Draw the principal axis and mark the optical centre O and focal points F (and F′).
2. Ray parallel to axis → refracts through F on the far side.
3. Ray through O → continues straight (undeviated).
4. Ray through F → emerges parallel to the axis.
5. Where refracted rays meet (or their extensions) locates the image — state if real or virtual, upright or inverted, magnified or diminished.

Once you have worked through a few, test yourself with the free Light quiz.

Reflection vs refraction vs TIR: which rule applies?

Situation	What happens	Key condition
Plane mirror	Reflection; i = r	Smooth reflecting surface
Glass block	Refraction at entry and exit	Light enters denser then less dense medium
Total internal reflection	Full reflection, no refraction	Dense → less dense; angle > critical angle
Optical fibre	Repeated TIR	Core denser than cladding

Light in past-paper wording: command words that matter

Command word / phrase	What the question wants	Typical light stem
Draw / Sketch	Accurate ray diagram with labels	”Draw a ray diagram to show reflection.”
Describe	Image properties	”Describe the image formed by the mirror.”
Explain	Cause and effect	”Explain why the ray bends towards the normal.”
Calculate	Use n = 1/sin c or scale diagrams	”Calculate the critical angle for glass of refractive index 1.5.”
State	Short factual answer	”State two uses of total internal reflection.”

Worked exam-style stems (how to answer the wording)

1. “A ray of light strikes a plane mirror at an angle of incidence of 35°. State the angle of reflection.” Angle of reflection = 35° (law of reflection: i = r). Mark-scheme reward: equal angles, both measured from normal.
2. “Explain why an optical fibre can carry light around a bend.” Light enters the core at an angle greater than the critical angle → undergoes total internal reflection repeatedly at the core-cladding boundary → light stays inside the fibre. Reward: TIR named + angle condition + repeated reflection.
3. “An object is placed beyond 2F from a converging lens. Describe the image formed.” Real, inverted, diminished, on the opposite side of the lens. Reward: all four properties stated.

When you can recognise the wording instantly, work the full set on the Waves topical past paper questions and the Light quiz to lock the ray rules in.

How light connects to the rest of the syllabus

Light is part of the electromagnetic spectrum covered in Electromagnetic Spectrum. Ray skills build on General Properties of Waves. The Cambridge IGCSE Physics resource hub links every Waves subtopic.

Common mistakes students make

• Measuring angles from the surface instead of the normal.
• Bending refracted rays the wrong way at a boundary (towards normal entering denser medium).
• Drawing lens rays that do not pass through F or O correctly.
• Confusing real images (rays actually meet) with virtual images (extensions meet).
• Forgetting that TIR only occurs from dense to less dense medium.

When you need more support

If ray diagram questions keep costing marks — especially converging lens image location — work through the Waves topical past paper questions and the Light quiz, then get focused help from a Cambridge IGCSE Physics tutor.

Frequently asked questions

Is light hard in Cambridge IGCSE Physics? The theory is logical, but marks are lost on ray diagrams — especially lens images and TIR conditions.

What is the critical angle? The angle of incidence in the denser medium when the refracted ray travels along the boundary (angle of refraction = 90°).

How do I remember lens ray rules? Parallel in → through F; through O → straight; through F → parallel out. Practise all three on every diagram.

What is dispersion? Splitting of white light into a spectrum because different wavelengths refract by different amounts in glass.

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