Properties of electromagnetic waves 1 refraction & RP9

SPEC 4.6.2.2 • PROPERTIES OF EM WAVES — REFRACTION

Refraction & RP9

The mechanism, the practical (both halves), and the Level-3 answer pattern that only 16% of students reached last summer.

Why This Lesson Matters

Refraction is one of the most reliable 6-mark questions on Paper 2. RP9 was the headline practical of Jun 2024 — and the examiner reporting makes brutal reading.

16%

Reached Level 3 on RP9 method (6-mark)

8%

All 3 marks on HT wavefronts (3-mark)

~10

Marks on the line per series

What You'll Walk Away With

The mechanism (why light bends), the full RP9 method for both refraction AND reflection investigations, and the Level-3 answer pattern that separates the 16% from the rest.

What Is Refraction?

Three definitions you must recall word-for-word. The mark scheme wording is uncompromising here — paraphrases lose marks.

Refraction

The change in direction of a wave at a boundary between two media, caused by a change in the wave's speed.

The Normal

A line drawn perpendicular (90°) to the boundary at the point where the ray meets it. All angles in refraction are measured from the normal — never from the surface.

Angles of Incidence & Refraction

Angle of incidence (i): angle between the incident ray and the normal.

Angle of refraction (r): angle between the refracted ray (inside the new medium) and the normal.

Four Things Happen at a Boundary

When a wave meets a boundary between two media, the spec lists four possible behaviours. Refraction is one of them — but the full 4.6.2.2 mark scheme expects you to know all four.

1 · REFLECT

Wave bounces back off the boundary into the original medium. Angle of incidence = angle of reflection.

2 · REFRACT

Wave passes through the boundary and changes direction because its speed changes. The focus of this lesson.

3 · TRANSMIT

Wave's energy passes through into the new medium. Refraction is one type of transmission — the wave can also pass straight through (when i = 0°).

4 · ABSORB

Wave's energy is taken up by the particles in the new medium (often heating them). The wave doesn't continue.

HT — Wavelength Dependence

Whether a wave reflects, refracts, transmits or is absorbed at a material depends on its wavelength as well as the substance. That's why glass is transparent to visible light but absorbs UV, and why X-rays pass through soft tissue but are absorbed by bone.

The Mechanism & The Direction Rule

Light bends because its velocity (speed) changes when it crosses into a different medium. The direction of bending depends on which way the speed changes.

The Direction Rule

Slowing down (entering a denser medium, e.g. air → glass) → bends toward the normal.

Speeding up (entering a less dense medium, e.g. glass → air) → bends away from the normal.

What Changes — and What Doesn't

The frequency-vs-wavelength trap is one of the most-asked single-mark questions on this topic. Memorise this table.

PropertyWhat Happens When the Wave Refracts

Speed (velocity)Changes — this is the cause of refraction. Slower in denser media.

Wavelength (λ)Changes — gets shorter when the wave slows down (because v = fλ and f is fixed).

Frequency (f)STAYS THE SAME — determined by the source, not the medium.

DirectionChanges — by the direction rule above (unless the ray hits the boundary along the normal, i.e. i = 0°, in which case it passes straight through).

Why Frequency Can't Change

Frequency = how many wave peaks pass a point per second. Wave peaks can't be created or destroyed at a boundary — so the rate at which they arrive must equal the rate at which they leave. f stays constant; λ adjusts to match the new speed.

HT Wavefronts — Why the Wave Bends

The HT explanation. Picture a wave as a row of wavefronts (the crests) marching forwards. The mark-scheme key phrase: "parts of the wavefront reach the boundary at different times."

Examiner Intelligence — HT Wavefront Question

On a recent Paper 2 HT 3-mark wavefront question, only 8% of students scored all three marks. The most commonly missed point: failing to state that different parts of the wavefront reach the boundary at different times.

The 3-Mark Mark Scheme (HT)

M1: Parts of the wavefront reach the boundary at different times.

M2: The part that enters the new medium first changes speed first.

M3: This causes the wavefront to pivot, changing the direction of travel.

RP9 — The Refraction Investigation

Required Practical 9 (Physics-only) has two halves: refraction (covered first) and reflection (slide 11). For refraction, you investigate how the angle of refraction depends on the angle of incidence at an air-to-glass boundary.

Apparatus — Name These Exactly

Ray box (producing a thin, single ray) · Glass block (rectangular) · Plain paper · Sharp pencil · Ruler · Protractor. The mark scheme expects all six.

RP9 Refraction — Variables

Each control needs a "so that…" reason — the reason is the mark.

Independent

Angle of incidence. Range 10°–60° in 10° intervals (≥5 values).

Dependent

Angle of refraction. Measured from the normal with a protractor.

Controls

Same glass block · thin ray · sharp pencil

The Control Variables — With Their "So That…"

Same glass block throughout — so that the optical density (and hence the relationship between i and r) is constant.

A thin, narrow ray of light — so that the centre of the ray can be traced accurately. (A wide ray makes the centre hard to judge — covered on the next results slide.)

Sharp pencil and ruler — so that ray-tracing lines are thin and don't artificially widen the measured angle.

RP9 Refraction — Method (6 Steps)

Memorise this six-step sequence. Top-band answers all contain these exact moves.

Step 1

Place the glass block flat on a sheet of plain paper. Draw round the block with a sharp pencil to mark its outline.

Step 2

Shine a thin ray from the ray box at one face of the block. Mark the entry point and the point where the emergent ray leaves the block with small pencil dots.

Step 3

Remove the block. Join the entry and exit dots with a ruler — this gives the path of the ray inside the block.

Step 4

Draw the normal at the entry point — a line perpendicular (90°) to the block's edge.

Step 5

Use a protractor to measure the angle of incidence and the angle of refraction — both from the normal. Record in a table.

Step 6

Repeat for at least 5 different angles of incidence over a stated range AND interval (e.g. 10°–60° in 10° steps). Plot angle of refraction (y) against angle of incidence (x).

Alternative Method (Equally Valid)

You can also pre-set the IV: draw round the block, remove it, draw the normal, then draw the incidence line at your chosen angle (measured from the normal). Replace the block over the outline and shine the ray along the pre-drawn line. This gives more precise control of the angle of incidence.

RP9 — The Reflection Investigation

RP9's second half. Often forgotten — but it's part of the same required practical and past papers tag reflection questions "RPA9" too.

Two Versions of the Reflection Investigation

(A) Verify the law of reflection (angle of incidence = angle of reflection) using a plane mirror.

(B) Compare reflection from different surfaces — plane mirror vs white card vs matt black card — to distinguish specular from diffuse reflection.

Independent

Angle of incidence (A) or surface type (B)

Dependent

Angle of reflection + quality of reflected ray (sharp vs scattered)

Controls

Same ray box, same surface position, sharp pencil

Method

Stand the surface vertically on paper. Draw a line along its base.

Draw the normal at the point where the ray will strike.

Shine the ray at the surface at a measured angle of incidence.

Mark two points along the reflected ray; remove the surface and join them.

Measure the angle of reflection from the normal with a protractor.

Repeat for a range of incidence angles (Version A) or different surfaces (Version B).

Expected Results

Plane mirror: angle of incidence ≈ angle of reflection at every angle; one sharp reflected ray = specular reflection.

White / matt card: ray is scattered in many directions; no single bright reflected ray = diffuse reflection. The law of reflection still holds at each microscopic point on the rough surface.

Expected Results & Sources of Inaccuracy

Typical refraction data for an air-to-glass investigation. Both the shape AND the inaccuracy points are directly examined.

Angle of Incidence i (°)Angle of Refraction r (°)Pattern

1510r always smaller than i (slowing → toward normal)

2516r increases as i increases

3522Not directly proportional — ratio not constant

4528Increase in r slows down as i grows

5533Curve flattens at high i

6537Approaches a limiting value

Graph Shape

Plot r (y) against i (x). The line is a curve through the origin, not a straight line. The relationship is not directly proportional — this has been explicitly tested in recent papers. Always draw a curved line of best fit with a sharp pencil.

Sources of Inaccuracy — Worth 1 Whole Mark

The mark scheme awards a separate mark for naming a specific source of inaccuracy. Any of these scores:

Width of the ray — makes it difficult to judge where the centre of the ray is

Parallax when reading the protractor (eye not directly above the scale)

Pencil thickness — thick lines can artificially widen the measured angle

Partial reflection at the boundary makes the entry/exit point harder to mark precisely

"Human error" is NOT a creditworthy answer — name the specific cause.

Examiner Alert — The RP9 6-Mark Question

"Only 16% of students reached Level 3 on a 6-mark question describing the refraction practical. 38% reached Level 2. Most students failed to describe the range and interval of angles used. Top-band answers explicitly drew round the block, drew the normal at each boundary, used a stated range of incidence angles, and recorded data in a clearly labelled table." — Drawn from AQA examiner reporting on recent RP9 method questions

What Got Level 1 (1–2 marks)

"Shine the ray at the glass and measure the angles." Vague verbs, no apparatus named, no normal mentioned, single angle implied. The student knows the topic but writes nothing the mark scheme can credit.

What Got Level 3 (5–6 marks)

Six concrete moves: (1) draw round the block; (2) shine ray, mark entry & exit points; (3) draw the normal at the boundary; (4) measure both angles from the normal with a protractor; (5) repeat over a stated range AND interval of incidence angles (e.g. 10°–60° in 10° steps); (6) record results in a clearly labelled table.

Level 3 vs Level 1 — Side by Side

Same question, same student knowledge, very different mark. The difference is method-detail richness.

✗ Level 1 — 1/6

"Shine the ray at the glass block. Measure the angle of incidence and the angle of refraction with a protractor. Repeat for different angles."

What's missing: drawing round the block, marking entry/exit, the normal, "from the normal", range AND interval , a table.

✓ Level 3 — 6/6

"Place the glass block on plain paper and draw round it. Shine a ray from a ray box at one face and mark the entry point and emergent ray with pencil dots. Remove the block and join the dots to find the internal path. Draw the normal at 90° to the block edge at the entry point. Use a protractor to measure angle of incidence and angle of refraction, both from the normal. Repeat for at least 5 angles of incidence between 10° and 60° in 10° intervals and record in a clearly labelled table."

The Six Keywords That Score

"Draw round the block" · "Mark entry/exit" · "The normal" · "From the normal" · "Range AND interval" · "Table". Without these six, Level 3 is out of reach no matter how confident the writing sounds.

Three Errors That Cost Marks

Patterns that recur across multiple examiner reports on refraction questions. Fix these now.

✗ Wrong

"Light bends because it gets dimmer / less intense."Confuses absorption with refraction.

✓ Right

"Light bends because its velocity changes at the boundary."Velocity (speed) change is the cause; bending is the consequence.

✗ Wrong

"The frequency changes during refraction."Confuses wavelength with frequency.

✓ Right

"Frequency stays the same; wavelength changes."Frequency is set by the source; λ adjusts to the new speed.

✗ Wrong

"Measure the angle from the surface of the glass."Definition failure — zero credit.

✓ Right

"Measure all angles from the normal."Every angle in optics is measured from the normal.

Practice Question

A student investigates how the angle of refraction varies with the angle of incidence as light passes from air into a glass block.

Describe a method the student could use. Your answer may include a labelled diagram. State one possible source of inaccuracy in the data. 6 marks + 1

M1

Apparatus & setup: Place the glass block on plain paper. Draw round the block with a sharp pencil. Set up a ray box pointing at one face.

M2

Ray tracing: Shine a thin ray at the block. Mark the entry point and the point where the emergent ray leaves the opposite face with pencil dots.

M3

Find internal path: Remove the block. Join the entry and exit dots with a ruler to find the path of the ray inside the glass.

M4

Construct the normal: At the entry point, draw the normal — a line perpendicular (90°) to the boundary.

M5

Measure both angles: Use a protractor to measure the angle of incidence and the angle of refraction — both from the normal.

M6

Range & record: Repeat for at least 5 angles of incidence over a stated range AND interval (e.g. 10°–60° in 10° steps). Record in a clearly labelled table.

+1

Source of inaccuracy: The width of the light ray makes it difficult to judge where the centre of the ray is, so the entry/exit points may be marked slightly off-centre.

Framework — Numerical Refraction (ESUA)

Some refraction questions are calculations using v = f λ. The same wave's speed and wavelength change at the boundary; frequency does not.

Worked example: Monochromatic light has frequency 5.0 × 10¹⁴ Hz and travels at 3.0 × 10⁸ m/s in air. Inside glass, its speed drops to 2.0 × 10⁸ m/s. Calculate the wavelength of this light inside the glass.

E — Equation

v = f λ → λ = v ÷ f

S — Substitute (use the IN-GLASS speed; f unchanged)

λ = (2.0 × 10⁸) ÷ (5.0 × 10¹⁴)

U — Unit

metres (m) — wavelength is a distance

A — Answer

λ = 4.0 × 10⁻⁷ m

Compare with λ in air (= 6.0 × 10⁻⁷ m): the wavelength shrinks in the slower medium, while frequency stays at 5.0 × 10¹⁴ Hz throughout.

Three Things to Remember

The three habits that lift refraction answers from Level 1 to Level 3.

1. Speed (Velocity) Causes Bending

Refraction is not "light hitting glass". It's a change in speed at a boundary, which forces a change in direction. Slowing → toward normal. Speeding → away from normal.

2. Frequency Never Changes — Wavelength Does

Wave peaks can't be created or destroyed at a boundary. f is fixed; λ adjusts to the new speed. "The frequency changes" is the single most-marked-wrong refraction sentence.

3. RP9 — The Six Keywords + Sources of Inaccuracy

For 6-mark RP9 method questions: "draw round the block" · "mark entry/exit" · "draw the normal" · "measure from the normal" · "range AND interval" · "table". Plus one source of inaccuracy (e.g. width of ray) for the extra mark.

Summary

The Four Boundary Behaviours (Spec 4.6.2.2)

Reflect · Refract · Transmit · Absorb — depends on the wave's wavelength and the substance (HT)

The Mechanism

Refraction = change in direction at a boundary, caused by a change in wave speed (velocity)

Slowing (air → glass) → bends toward the normal · Speeding (glass → air) → bends away

Frequency stays the same; wavelength changes (v = f λ)

HT wavefronts: parts of the wavefront reach the boundary at different times — the part that enters first changes speed first — the front pivots

RP9 — Two Halves

Refraction: IV = angle of incidence (range AND interval, e.g. 10°–60° in 10° steps) · DV = angle of refraction · CVs = same block, thin ray, sharp pencil

Reflection: Verify law of reflection (mirror) OR compare specular vs diffuse (mirror vs white/matt card)

Method (refraction, 6 steps): draw round → mark entry/exit → join through block → draw normal → measure both from normal → repeat ≥5 angles, table

Sources of inaccuracy: width of ray · parallax · pencil thickness · partial reflection at boundary

Expected Results Pattern

Angle of refraction always smaller than angle of incidence (entering glass)

Curve through origin — NOT directly proportional

Approaches a limit as i increases

The Single Biggest Habit

Every refraction answer needs the phrase "from the normal". Every RP9 method needs the six keywords + a specific source of inaccuracy. Every numerical answer keeps frequency constant and lets wavelength scale with the new speed. Six keywords. One constant. One curve. One named source of inaccuracy.