What is diffraction in the context of wave physics?

Diffraction refers to the bending and spreading of waves when they encounter an obstacle or pass through a narrow opening. This phenomenon is significant in wave physics as it demonstrates the wave nature of light and other types of waves. In the context of the Cambridge International A Levels curriculum, understanding diffraction is crucial for explaining how waves interact with their environment.

How does diffraction relate to the principle of superposition?

Diffraction is closely related to the principle of superposition, which states that when two or more waves overlap, the resulting wave is the sum of the individual waves. During diffraction, waves spread out and overlap, leading to interference patterns that can be explained using superposition. This is particularly important in understanding phenomena such as the diffraction patterns observed in light and sound waves.

What are some practical examples of diffraction?

Practical examples of diffraction include the spreading of sound waves around corners, the appearance of patterns when light passes through a small aperture, and the diffraction of X-rays in crystallography. These examples illustrate how diffraction is a fundamental concept in both everyday experiences and advanced scientific research, making it a key topic in the Cambridge International A Levels Physics syllabus.

Why is the study of diffraction important in the Cambridge International A Levels Physics curriculum?

The study of diffraction is important in the Cambridge International A Levels Physics curriculum because it helps students understand the wave nature of light and other waves. It provides insights into wave behavior, interference patterns, and the limitations of wave propagation. This knowledge is essential for further studies in optics, acoustics, and various technological applications.

How does the size of the aperture affect the diffraction pattern?

The size of the aperture significantly affects the diffraction pattern. When waves pass through a smaller aperture, they spread out more, resulting in a wider diffraction pattern. Conversely, a larger aperture causes less spreading. This relationship is crucial for understanding how different wavelengths and aperture sizes influence the diffraction effects observed in experiments, a concept emphasized in the Cambridge International A Levels Physics curriculum.

Why is "Saying diffraction only happens with small gaps" a common mistake in Diffraction?

Why it happens: Effects most visible for narrow gaps. How to avoid it: Diffraction ALWAYS happens. Visible when gap is comparable to wavelength. Source: 9702 Examiner Reports 2022-2024

Cambridge International A Levels Physics (9702)

Diffraction

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Short Notes - Diffraction

Detailed Study Notes

Detailed notes on Superposition for Cambridge International A Levels Physics, covering key concepts, explanations, examples, and exam-focused revision points.

Diffraction Study Notes — Cambridge International A Level Physics 9702 Superposition (2025-2027 syllabus)

Wave spreading around obstacles or through gaps. Maximum when $\lambda \sim$ gap width.

At a glance

Diffraction = wave bending round obstacle/gap.
Max when $\lambda \sim$ gap.
Wider gap → less.
Longer $\lambda$ → more.
Sound diffracts more than light around everyday gaps.

What you’ll learn

Mapped to the Cambridge International A Level 9702 syllabus (2025-2027).

8.2.1 — Describe diffraction.
8.2.2 — Describe single-slit pattern.
8.2.3 — Explain why sound diffracts more than light.

Diffraction basics

Wave spreads.

Definition. When a wave passes through a gap or around an obstacle, it SPREADS OUT.

Amount of diffraction depends on:

Gap width $a$ : wider → less diffraction.
Wavelength $\lambda$ : longer → more diffraction.

Maximum effect. When $\lambda \sim a$ .

Example. Sound ( $\lambda \sim$ 1 m) diffracts around doors well — you hear someone behind a wall. Light ( $\lambda \sim 500$ nm) barely diffracts at everyday scales — you don't see them.

Cambridge tip. State both conditions for maximum diffraction.

Figure 1 — How gap width changes the amount of diffraction

What matters is the ratio of wavelength to gap width, not either on its own — which is why the same gap diffracts sound strongly and light hardly at all.

Figure 2 — Why sound diffracts round a doorway but light does not

A doorway is roughly one wavelength wide for sound but about two million wavelengths wide for light, so only sound spreads appreciably.

Wave bends around obstacles.
Max when $\lambda \sim$ gap.
Sound > light at everyday scales.

See the full worked example for diffraction →

Single-slit pattern

Bright central max + dimmer side maxima.

Monochromatic light through narrow slit produces:

Central bright maximum (wide and intense).
Side maxima narrower and progressively dimmer.
Dark fringes (minima) at angles where $a\sin\theta = n\lambda$ , $n = 1, 2, 3, \ldots$

Wider slit → narrower central max + more concentrated pattern.

Cambridge tip. Central max is roughly TWICE the width of side maxima.

Figure 3 — Single-slit diffraction intensity pattern

Diffraction through a single slit gives a wide central maximum about twice the width of the side maxima, with dark fringes where a sinθ = nλ.

Figure 4 — Where the single-slit minima come from

Rays from the top and bottom of the slit travel different distances to the same point. When that difference is a whole number of wavelengths the contributions cancel across the slit.

Figure 5 — Effect of slit width on the diffraction pattern

A narrower slit diffracts more, so its pattern is more spread out — the opposite of what intuition about "letting more light through" suggests.

Central bright max wide.
Side maxima dimmer.
Minima at $a\sin\theta = n\lambda$ .

See the full worked example for diffraction →

Quick recap

Diffraction = wave spreading.
Max when $\lambda \sim$ gap.
Sound bends more than light.

Memorise this

Verbatim phrases and definitions Cambridge mark schemes credit.

Diffraction definition.
Conditions for maximum effect.
Single slit pattern.

How it’s examined

Diffraction appears every Paper 1. Most-tested: qualitative description (5-6 marks), single-slit pattern (5-7 marks).

Sources: Cambridge International A Level Physics 9702 syllabus (2025-2027); 9702 Examiner Reports 2022-2024; 9702/22 May/Jun 2024 question paper and mark scheme. Last reviewed 2026-05-11.

Take this whole topic with you

Step-by-step worked examples — Diffraction

Step-by-step solutions to past-paper-style questions on diffraction, written exactly the way a tutor would explain them at the board.

1Diffraction at gap (5 marks)
Extended• Adapted from 9702/22 May/Jun 2024• diffraction
Question
Water ripples meet a gap. Describe how diffraction varies as (a) gap is widened, (b) wavelength is increased. (5 marks)
Step-by-step solution
1. Step 1
  (a) Wider gap → LESS diffraction. Less curving of wavefronts.
2. Step 2
  (b) Longer wavelength → MORE diffraction. Effect strongest when $\lambda \sim$ gap width.
Answer
Wider gap → less. Longer $\lambda$ → more. Maximum when $\lambda \sim$ gap.
2Single-slit pattern (6 marks)
Extended• single slit
Question
Describe the appearance of single-slit diffraction with monochromatic light. (6 marks)
Step-by-step solution
1. Step 1
  Central bright maximum, wide and intense.
2. Step 2
  Side maxima narrower and dimmer, decreasing in intensity outward.
3. Step 3
  Minima at angles where $a\sin\theta = n\lambda$ , $n = 1, 2, 3, \ldots$
Answer
Central bright wide max, alternating min/max getting dimmer. Minima at $a\sin\theta = n\lambda$ .
3Diffraction of sound (5 marks)
Extended• sound diffraction
Question
Why can you hear someone speaking around a corner but not see them? (5 marks)
Step-by-step solution
1. Step 1
  Sound has wavelength $\sim$ 1 m, comparable to door/doorway.
2. Step 2
  Diffraction occurs significantly → sound bends round corners.
3. Step 3
  Light has wavelength $\sim 500$ nm, much smaller than openings.
4. Step 4
  Light diffraction negligible for everyday objects → travels in straight lines.
Answer
Sound $\lambda \sim$ gap → diffracts. Light $\lambda \ll$ gap → barely diffracts.

Key Definitions and Keywords — Diffraction

Definitions to memorise and the exact keywords mark schemes credit for diffraction answers — sharpened from recent examiner reports for the 2026 Cambridge International A Level 9702 sitting.

Diffraction
Examiner keyword
Spreading of a wave as it passes through a gap or round an obstacle.

Common Mistakes and Misconceptions — Diffraction

The traps other students keep falling into on diffraction questions — taken from recent Cambridge International A Level 9702 examiner reports and mark schemes — and how to avoid them.

✕Saying diffraction only happens with small gaps
9702 Examiner Reports 2022-2024
Why it happens
Effects most visible for narrow gaps.
How to avoid it
Diffraction ALWAYS happens. Visible when gap is comparable to wavelength.

Diffraction — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Cambridge International A Levels Physics (9702)

Diffraction

0% Complete

3 tasks remaining

Short Notes - Diffraction

Detailed Study Notes

Detailed notes on Superposition for Cambridge International A Levels Physics, covering key concepts, explanations, examples, and exam-focused revision points.

Diffraction Study Notes — Cambridge International A Level Physics 9702 Superposition (2025-2027 syllabus)

Wave spreading around obstacles or through gaps. Maximum when $\lambda \sim$ gap width.

At a glance

Diffraction = wave bending round obstacle/gap.
Max when $\lambda \sim$ gap.
Wider gap → less.
Longer $\lambda$ → more.
Sound diffracts more than light around everyday gaps.

What you’ll learn

Mapped to the Cambridge International A Level 9702 syllabus (2025-2027).

8.2.1 — Describe diffraction.
8.2.2 — Describe single-slit pattern.
8.2.3 — Explain why sound diffracts more than light.

Diffraction basics

Wave spreads.

Definition. When a wave passes through a gap or around an obstacle, it SPREADS OUT.

Amount of diffraction depends on:

Gap width $a$ : wider → less diffraction.
Wavelength $\lambda$ : longer → more diffraction.

Maximum effect. When $\lambda \sim a$ .

Example. Sound ( $\lambda \sim$ 1 m) diffracts around doors well — you hear someone behind a wall. Light ( $\lambda \sim 500$ nm) barely diffracts at everyday scales — you don't see them.

Cambridge tip. State both conditions for maximum diffraction.

Figure 1 — How gap width changes the amount of diffraction

What matters is the ratio of wavelength to gap width, not either on its own — which is why the same gap diffracts sound strongly and light hardly at all.

Figure 2 — Why sound diffracts round a doorway but light does not

A doorway is roughly one wavelength wide for sound but about two million wavelengths wide for light, so only sound spreads appreciably.

Wave bends around obstacles.
Max when $\lambda \sim$ gap.
Sound > light at everyday scales.

See the full worked example for diffraction →

Single-slit pattern

Bright central max + dimmer side maxima.

Monochromatic light through narrow slit produces:

Central bright maximum (wide and intense).
Side maxima narrower and progressively dimmer.
Dark fringes (minima) at angles where $a\sin\theta = n\lambda$ , $n = 1, 2, 3, \ldots$

Wider slit → narrower central max + more concentrated pattern.

Cambridge tip. Central max is roughly TWICE the width of side maxima.

Figure 3 — Single-slit diffraction intensity pattern

Diffraction through a single slit gives a wide central maximum about twice the width of the side maxima, with dark fringes where a sinθ = nλ.

Figure 4 — Where the single-slit minima come from

Rays from the top and bottom of the slit travel different distances to the same point. When that difference is a whole number of wavelengths the contributions cancel across the slit.

Figure 5 — Effect of slit width on the diffraction pattern

A narrower slit diffracts more, so its pattern is more spread out — the opposite of what intuition about "letting more light through" suggests.

Central bright max wide.
Side maxima dimmer.
Minima at $a\sin\theta = n\lambda$ .

See the full worked example for diffraction →

Quick recap

Diffraction = wave spreading.
Max when $\lambda \sim$ gap.
Sound bends more than light.

Memorise this

Verbatim phrases and definitions Cambridge mark schemes credit.

Diffraction definition.
Conditions for maximum effect.
Single slit pattern.

How it’s examined

Diffraction appears every Paper 1. Most-tested: qualitative description (5-6 marks), single-slit pattern (5-7 marks).

Sources: Cambridge International A Level Physics 9702 syllabus (2025-2027); 9702 Examiner Reports 2022-2024; 9702/22 May/Jun 2024 question paper and mark scheme. Last reviewed 2026-05-11.

Take this whole topic with you

Step-by-step worked examples — Diffraction

Step-by-step solutions to past-paper-style questions on diffraction, written exactly the way a tutor would explain them at the board.

1Diffraction at gap (5 marks)
Extended• Adapted from 9702/22 May/Jun 2024• diffraction
Question
Water ripples meet a gap. Describe how diffraction varies as (a) gap is widened, (b) wavelength is increased. (5 marks)
Step-by-step solution
1. Step 1
  (a) Wider gap → LESS diffraction. Less curving of wavefronts.
2. Step 2
  (b) Longer wavelength → MORE diffraction. Effect strongest when $\lambda \sim$ gap width.
Answer
Wider gap → less. Longer $\lambda$ → more. Maximum when $\lambda \sim$ gap.
2Single-slit pattern (6 marks)
Extended• single slit
Question
Describe the appearance of single-slit diffraction with monochromatic light. (6 marks)
Step-by-step solution
1. Step 1
  Central bright maximum, wide and intense.
2. Step 2
  Side maxima narrower and dimmer, decreasing in intensity outward.
3. Step 3
  Minima at angles where $a\sin\theta = n\lambda$ , $n = 1, 2, 3, \ldots$
Answer
Central bright wide max, alternating min/max getting dimmer. Minima at $a\sin\theta = n\lambda$ .
3Diffraction of sound (5 marks)
Extended• sound diffraction
Question
Why can you hear someone speaking around a corner but not see them? (5 marks)
Step-by-step solution
1. Step 1
  Sound has wavelength $\sim$ 1 m, comparable to door/doorway.
2. Step 2
  Diffraction occurs significantly → sound bends round corners.
3. Step 3
  Light has wavelength $\sim 500$ nm, much smaller than openings.
4. Step 4
  Light diffraction negligible for everyday objects → travels in straight lines.
Answer
Sound $\lambda \sim$ gap → diffracts. Light $\lambda \ll$ gap → barely diffracts.

Key Definitions and Keywords — Diffraction

Definitions to memorise and the exact keywords mark schemes credit for diffraction answers — sharpened from recent examiner reports for the 2026 Cambridge International A Level 9702 sitting.

Diffraction
Examiner keyword
Spreading of a wave as it passes through a gap or round an obstacle.

Common Mistakes and Misconceptions — Diffraction

The traps other students keep falling into on diffraction questions — taken from recent Cambridge International A Level 9702 examiner reports and mark schemes — and how to avoid them.

✕Saying diffraction only happens with small gaps
9702 Examiner Reports 2022-2024
Why it happens
Effects most visible for narrow gaps.
How to avoid it
Diffraction ALWAYS happens. Visible when gap is comparable to wavelength.

Diffraction — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Diffraction

Short Notes - Diffraction

Detailed Study Notes

At a glance

What you’ll learn

Diffraction basics

Single-slit pattern

Quick recap

Memorise this

How it’s examined

Take this whole topic with you

1Diffraction at gap (5 marks)

2Single-slit pattern (6 marks)

3Diffraction of sound (5 marks)

Diffraction

✕Saying diffraction only happens with small gaps

Diffraction — frequently asked questions

Diffraction

Short Notes - Diffraction

Detailed Study Notes

At a glance

What you’ll learn

Diffraction basics

Single-slit pattern

Quick recap

Memorise this

How it’s examined

Take this whole topic with you

1Diffraction at gap (5 marks)

2Single-slit pattern (6 marks)

3Diffraction of sound (5 marks)

Diffraction

✕Saying diffraction only happens with small gaps

Diffraction — frequently asked questions