Year 11 Physics Module 3 ⏱ ~40 min 5 MC · 3 Short Answer Lesson 5 of 18

Reflection and Refraction of Waves

Australia's NBN fibre-to-premises network delivers data at $0.67c$ (200,000 km/s) through Corning SMF-28 optical fibre: core refractive index $n_{core} = 1.4677$, cladding $n_{clad} = 1.4640$, critical angle $\theta_c = 86.9°$. Every data pulse stays trapped inside the glass core because each time it hits the core-cladding boundary it refracts, and if the angle exceeds 86.9°, all light reflects back internally. Reflection and refraction at a boundary are two aspects of the same phenomenon, and both are essential to fibre-optic communication.

Today's hook: Australia's NBN fibre-to-premises network uses Corning SMF-28 glass fibre with core $n = 1.4677$ and cladding $n = 1.4640$ to carry data at $0.67c$. At every point where a light pulse hits the core-cladding boundary, two things happen simultaneously: some light reflects (stays in the core) and some refracts (would escape, but only if the angle is below 86.9°). Understanding both reflection ($\theta_i = \theta_r$) and refraction (speed and direction change at the boundary) is what explains how the entire NBN backbone works.
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Warm up first

Warm up first

Three quick questions from earlier lessons. Pulling old material back to mind before you learn something new makes the new material stick better, so this is not busywork.

Before you read, predict

When a wave travels from shallow water into deep water, the wave speed increases. Which way will the wavefront bend, towards the normal or away from it? Predict and explain.

Warm-up, angles in reflection are measured from:

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Learning Intentions

Learning Intentions
goals

Know

  • Law of reflection: $\theta_i = \theta_r$
  • Refraction changes wave speed and direction at a boundary
  • Frequency is unchanged during refraction

Understand

  • Why waves reflect off boundaries
  • Why waves bend when they enter a new medium at an angle
  • The relationship between speed, wavelength and direction change

Can Do

  • Draw reflection and refraction diagrams with correct angle notation
  • Predict refraction direction from speed information
  • Apply reflection and refraction to real contexts
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Key Terms

Key Terms
vocab
ReflectionA wave bounces off a boundary; the angle of incidence equals the angle of reflection (measured from the normal).
RefractionA wave changes speed (and direction) when passing from one medium to another; frequency is unchanged.
NormalA line perpendicular to the boundary at the point of incidence; angles are measured from this line.
Angle of incidence ($\theta_i$)Angle between the incoming ray and the normal.
Cross-lesson links: Reflection (this lesson) is extended to curved mirrors in L14. Refraction is quantified by Snell's law in L15, which builds directly on the qualitative rules introduced here. The NBN fibre ($n_{core}=1.4677$, $n_{clad}=1.4640$, $\theta_c=86.9°$, data at $0.67c$) connects reflection, refraction, and TIR into a single engineering system, all three phenomena in one cable.
Misconceptions to fix
✗ Wrong: Angles in reflection are measured from the surface.
✓ Right: Always measure $\theta_i$ and $\theta_r$ from the normal, not the surface.
✗ Wrong: Refraction changes the frequency of a wave.
✓ Right: Frequency is determined by the source and is unchanged. Speed and wavelength change; frequency stays constant.
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Law of Reflection

Core Content

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Law of Reflection
+5 XP

Stand in a bathroom and clap sharply once, you hear the echo a fraction of a second later as sound bounces off the hard tiled walls. Aim a torch at a bathroom mirror at an angle, the reflected beam leaves at exactly the same angle on the other side of the perpendicular. Both the sound echo and the light beam are obeying the same law: the angle the wave arrives equals the angle it leaves, measured from the perpendicular to the surface (the normal).

When a wave hits a boundary it bounces off. The angle of incidence equals the angle of reflection. Both angles are measured from the normal to the boundary at the point of contact.

Law of reflection

$\theta_i = \theta_r$

This holds for all wave types, water waves in a ripple tank, sound echoes, light in a mirror, and radar. The frequency, wavelength and speed of the reflected wave are identical to the incident wave.

The law of reflection states $\theta_i = \theta_r$ (both measured from the normal to the surface). Frequency, wavelength and speed are unchanged on reflection.

Pause, copy the highlighted law into your book before moving on.

A wave hits a boundary at 30° to the normal. The reflected ray makes an angle of __ with the normal:

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Refraction, Speed Change at a Boundary

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Refraction, Speed Change at a Boundary
+5 XP

We just saw that reflection preserves all wave properties and bounces the wave at equal angles. That raises a question: what happens if the wave enters the new medium rather than bouncing off? This card answers it → refraction changes speed and bends direction, but frequency stays constant.

When a wave enters a new medium at an angle, the change in speed causes the wavefront to bend. The side of the wavefront that enters the new medium first is the first to change speed, rotating the whole wavefront.

  • Into a slower medium (e.g. deep water → shallow, air → glass): wave bends towards the normal.
  • Into a faster medium (e.g. shallow water → deep, glass → air): wave bends away from the normal.
Key rule
Frequency is set by the source and never changes during refraction. Because $v = f\lambda$, when $v$ decreases, $\lambda$ decreases proportionally; when $v$ increases, $\lambda$ increases.

Refraction: a wave changes speed (and wavelength) when entering a new medium; it bends towards the normal when slowing down and away when speeding up. Frequency is invariant during refraction, it is set by the source.

Add the highlighted refraction rule to your notes before the check below.

When light refracts from air into glass, its frequency increases.

A wave entering a slower medium bends toward the normal.

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Activity 4, Reflection Diagrams

Activities

Activity 4, Reflection Diagrams
ApplyBand 3

Draw and label a reflection diagram for a wave arriving at 40° to the surface. Mark the normal, $\theta_i$ and $\theta_r$. State the angle of reflection.

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Activity 5, Refraction Direction

Activity 5, Refraction Direction
ApplyBand 3

For each scenario, state whether the wave bends toward or away from the normal when it enters the new medium:

  1. Air → glass (light slows)
  2. Deep water → shallow water (waves slow)
  3. Glass → air (light speeds up)
  4. Shallow water → deep water (waves speed up)
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Activity 2, Frequency During Refraction

Activity 2, Frequency During Refraction
UnderstandBand 3

A wave of frequency 500 Hz travels from air (speed 340 m/s) into water (speed 1480 m/s). Find: (a) wavelength in air, (b) wavelength in water, (c) frequency in water.

Which of the following does NOT change when a wave refracts into a new medium?

A wave travels from medium A into medium B and bends away from the normal. This tells you that:

A wave hits a smooth flat boundary at 0° to the normal (straight on). After reflection, it travels:

✓

Show what you have learned

Multiple Choice, reflection and refraction
+5 XP
Short Answer, 10 marks
+5 XP

UnderstandBand 3(3 marks) 1. State the law of reflection and explain why frequency and wavelength do not change when a wave reflects from a boundary.

ApplyBand 4(3 marks) 2. A 200 Hz wave travels from medium A (speed 200 m/s) into medium B (speed 400 m/s). State the frequency, calculate the wavelength in each medium, and identify which way the wavefront bends.

AnalyseBand 5(4 marks) 3. Radio waves entering the ionosphere slow down and refract back toward Earth. Explain this process using the concept of refraction and state the practical benefit.

Show all answers

Short Answer, Model Answers

Q1 (3 marks): The angle of incidence equals the angle of reflection ($\theta_i = \theta_r$), both measured from the normal. Frequency and wavelength do not change because the wave remains in the same medium; the boundary simply reverses the direction component perpendicular to it.

Q2 (3 marks): Frequency = 200 Hz (unchanged by refraction). $\lambda_A = 200/200 = 1$ m. $\lambda_B = 400/200 = 2$ m. Speed increases so the wave bends away from the normal.

Q3 (4 marks): At the boundary between the lower atmosphere and the ionosphere, wave speed decreases. The side of the wavefront entering the slower region first is slowed, causing the wavefront to rotate towards the normal. Progressively, the wave bends back toward Earth. Practical benefit: long-range HF radio communication without satellites, reaching over-the-horizon locations.

✓

Retrieve, reflect and finish

Check what actually stuck
How did your thinking change?

Australia's NBN fibre-to-premises network makes the lesson concrete: the glass core ($n = 1.4677$) is only slightly denser than the cladding ($n = 1.4640$), yet that tiny difference is enough that light entering the core refracts at the boundary. When the angle of incidence exceeds the critical angle of 86.9°, all light reflects back internally and data travels at $0.67c$ across the 12,000 km Sydney–Los Angeles cable. Your Think First question was about refraction direction: when a wave enters a faster medium (lower $n$), it bends away from the normal, the wavefront's leading edge speeds up, rotating the whole front outward.

← Lesson 4: Superposition Next: Diffraction of Waves →

Module overview · Physics · Checkpoint 1 · Module quiz