IGCSE Physics revision · Waves
Light
The most diagram-heavy topic on the paper. The physics lives in three rules of reflection, one law of refraction, and two special rays through a lens. Draw with a ruler; freehand rays lose marks that cost nothing to keep.
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What the syllabus demands
- —Use the law of reflection (angle of incidence = angle of reflection) and describe the image in a plane mirror
- —Describe refraction at a boundary and define refractive index n = sin i ÷ sin r
- —Explain total internal reflection and the critical angle; apply to optical fibres
- —Draw ray diagrams for a thin converging lens and describe the images formed
- —Describe dispersion of white light by a prism
Definitions that earn marks
Clear definitions to practise — check your course mark scheme
- Law of reflection
- The angle of incidence equals the angle of reflection, both measured from the normal.
- Refractive index (n)
- n = sin i ÷ sin r — a measure of how much a medium slows light: n = speed of light in vacuum ÷ speed in the medium.
- Critical angle
- The angle of incidence in the denser medium for which the angle of refraction is 90°. Above it, all the light is totally internally reflected.
- Principal focus (focal point)
- The point at which rays parallel to the principal axis converge after passing through a converging lens.
The equations
More equations to practise: the IGCSE formula sheet.
Where the marks die
Common mistakes to check
- 01
Measuring angles from the surface instead of the normal. Every angle in optics is measured from the normal — the perpendicular line at the point of incidence.
- 02
Describing the plane-mirror image incompletely. Mark scheme: same size, same distance behind the mirror as the object is in front, virtual, and laterally inverted.
- 03
Total internal reflection answers missing a condition. Two are required: the light must travel from the denser medium towards the less dense, and the angle of incidence must exceed the critical angle.
- 04
Calling the image 'virtual' or 'real' by guesswork. If the rays actually meet, it is real and can be projected; if they only appear to come from a point, it is virtual — a magnifying glass makes a virtual image, a projector makes a real one.
One worked example, done properly
Question
The refractive index of glass is 1.5. Calculate the critical angle for the glass-air boundary.
Method
- 1.sin c = 1 ÷ n = 1 ÷ 1.5 = 0.667.
- 2.c = sin⁻¹(0.667).
c ≈ 42°