Reflection & Refraction
Waves · 1 question type
Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 9% of your exam marks.
Ray diagrams, Snell's Law and critical angle calculations appear regularly.
Definitions
- Reflection is what happens when a wave hits the boundary between two media and bounces back into the original medium without crossing the boundary
- Refraction is what happens when a wave passes through a boundary between two transparent media and changes direction as it does so
- The word medium (plural media) means any material that transmits the wave, such as air, glass, water, or perspex
The normal line
- Every ray diagram for reflection or refraction is measured from an imaginary line drawn at 90° to the surface at the exact point the ray hits; this line is called the
- Angles are measured between the ray and the normal, not between the ray and the surface itself
Law of reflection
- The states:
angle of incidence (i) = angle of reflection (r)
- where:
- angle of incidence is the angle between the incoming (incident) ray and the normal
- angle of reflection is the angle between the outgoing (reflected) ray and the normal
- On a flat mirror, every ray reflects neatly, which is and produces a clear image. On a rough surface every tiny patch tilts a different way, so reflected rays scatter, which is and produces no image
Drawing a reflected ray and stating the law
What comes up: A diagram shows a ray hitting a mirror; you are asked to mark the angle of incidence, measure it, and draw the reflected ray at the correct angle.
Write (two marks): (1) State that the angle of incidence equals the angle of reflection. (2) Draw the reflected ray so that it makes that same angle with the normal — not with the mirror surface.
Watch out: The most common error is measuring the angle between the ray and the mirror surface instead of between the ray and the normal. The mark scheme treats any angle measured from the surface (roughly the complement of the correct value) as a separate wrong answer — it does not carry the mark for a correctly measured angle of incidence.
Refraction: which way the bend goes
- When a light ray crosses from one transparent medium to another, its speed changes, and that speed change forces the ray to change direction
- The bend depends on which way the speed is changing:
- Entering an optically denser medium (for example, air into glass): the ray slows and tilts towards the normal
- Leaving a denser medium for a thinner one (for example, glass back into air): the ray speeds up and tilts away from the normal
- A ray that strikes the surface along the (perpendicular, 0°) keeps going straight; only its speed changes, never its direction
Explaining why a ray changes direction at a boundary
What comes up: "Explain why the ray bends when it enters the glass block" or "Explain why the direction of the ray changes at the boundary between water and air."
Write (two marks): (1) Light travels at a different speed in the two media — it slows down when entering a denser medium (such as glass or water). (2) Because it slows, the ray bends towards the normal; when it speeds up leaving a denser medium, it bends away from the normal.
Watch out: State the speed change first, then the direction of the bend — both points are needed for full marks. A direction answer alone ("it bends towards the normal") without the reason (speed change) earns only one mark.
What changes and what does not
- During refraction, two wave properties change and one stays the same:
- Speed changes (slows in denser medium, speeds up in less dense one)
- Wavelength changes in step with the speed (using v = f × λ)
- Frequency stays the same, because the source still emits the same number of crests per second, so the same number must arrive in the new medium. A red ray of light staying red when it dives into water is the everyday evidence of this