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PhysicsInternational GCSEEdexcel & Cambridge (CIE)Years 10–11

IGCSE Physics Reflection & Refraction Notes

Reflection bounces light off a surface; refraction bends it as it changes speed. Learn the rules and ray diagrams to earn these marks.

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Key concepts

Law of reflection
The angle of incidence equals the angle of reflection, both measured from the normal.
Refraction
Light changes speed and direction when it crosses between media of different densities; it bends towards the normal entering a denser medium.
Total internal reflection
When the angle of incidence exceeds the critical angle, all light reflects back inside the denser medium.
  • Angles are always measured from the normal, not the surface.
  • Light slows down and bends towards the normal entering glass or water.
  • Optical fibres use total internal reflection to carry signals.

Key formulas

Law of reflection

angle of incidence = angle of reflection

Refractive index

n = sin i ÷ sin r

for light entering the medium from air.

Required practicals

  • Investigate reflection of light using a plane mirror and a ray box.
  • Investigate refraction of light through a glass block.

Worked example

Light hits a mirror at 30° to the normal. What is the angle of reflection?

  1. 1Recall the law of reflection.
  2. 2Angle of incidence = angle of reflection.
  3. 3The incidence angle is 30°.
  4. 4So the reflection angle is 30°.

Answer: 30° from the normal

Common mistakes

✗ Measuring angles from the surface instead of the normal.

✓ Always measure the angle from the normal (the perpendicular).

✗ Saying light bends away from the normal entering glass.

✓ Entering a denser medium, light bends towards the normal.

✗ Confusing total internal reflection with normal reflection.

✓ TIR only happens above the critical angle inside the denser medium.

Quick check

Try these, then reveal the answers.

From where are angles of incidence and reflection measured?Reveal answer

The normal.

Which way does light bend entering a denser medium?Reveal answer

Towards the normal.

What condition causes total internal reflection?Reveal answer

Angle of incidence greater than the critical angle.

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Exam tips

  • Draw the normal as a dashed line before marking any angles.
  • Label incident, reflected and refracted rays clearly.
  • State that angles are measured from the normal in written answers.

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Key terms explained

Short, exam-ready definitions with a formula, a worked example and an examiner tip.

What is refraction?Full glossary

Frequently asked questions

Why does light refract?+

It changes speed as it crosses between media of different optical densities, which changes its direction.

How do optical fibres work?+

They use total internal reflection to keep light bouncing along the fibre without escaping.

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