Diffraction of Waves

5.5 Diffraction of Waves

PHYSICS • Form 4 • Chapter 5: Waves

5.5 Diffraction of Waves

Diffraction of waves is an essential wave phenomenon where waves spread out as they pass through a slit, gap, or around the edge of an obstacle or barrier. This phenomenon occurs with water waves, sound waves, and light waves.

Learning Objectives

Define diffraction of waves for water, light, and sound waves.
Determine the changes in wave characteristics after passing through a slit or obstacle.
Draw and describe patterns of diffracted wavefronts for water waves passing through gaps or obstacles.
Identify the factors affecting the diffraction of waves: the size of the slit/gap and the wavelength.
Explain how the size of the slit and wavelength influence the extent of wave spreading.
Provide and explain real-life examples and applications of the diffraction of water, light, and sound waves.

Factors Affecting Diffraction of Water Waves and Wavefront Shapes

Compare how slit width and wavelength affect the degree of diffraction and visualise the resulting circular versus flat wavefront patterns.

Wide Slit vs Narrow Slit

Keeping wavelength constant, a wide slit produces less significant spreading with a mostly flat wavefront in the centre. A narrow slit, close to the wavelength, produces more significant spreading with distinct circular wavefronts.

Wide slit

Narrow slit

Short Wavelength vs Long Wavelength

Keeping slit width constant, a short wavelength passes through with slight curving at the ends. A long wavelength produces more rounded, distinct circular wavefronts.

Short wavelength

Long wavelength

Short Explanation

What is Diffraction of Waves?

Diffraction of waves is the spreading of waves when the waves propagate through a slit or side of a barrier/obstacle.

Effects on Wave Characteristics

During diffraction, wavelength, frequency, and wave speed remain unchanged. However, amplitude decreases and the direction of propagation spreads out.

Factors Affecting Diffraction

Diffraction is influenced by the size of the slit/gap and the wavelength. A narrow slit or a long wavelength produces more significant spreading.

Most Significant Diffraction

Diffraction is most obvious when the gap size is roughly equal to or smaller than the wavelength, \(\text{slit size} \approx \lambda\), or when the wave has a long wavelength.

Wave characteristic	Change during diffraction	Reason
Wavelength, \(\lambda\)	No change	Wave speed does not change.
Frequency, \(f\)	No change	No change to the source of frequency.
Speed, \(v\)	No change	No change in the medium before and after diffraction.
Amplitude, \(A\)	Decreased	Wave energy diverges and spreads out to a wider region.
Direction of propagation	Spreads from one direction to many directions	The wavefront spreads after passing through the slit or obstacle.

Try to Answer First

Answer in your mind, then press “Check Answer”.

What wave characteristics change when a wave undergoes diffraction?

Check Answer

Answer: Amplitude decreases and the direction of propagation changes (spreads out). Frequency, wavelength, and wave speed remain unchanged.

Why are water waves in a harbour sheltered by an embankment calmer than the waves in the open sea?

Check Answer

Answer: When sea waves pass through the narrow gap of an embankment, diffraction occurs. The wave spreads out over a wider area, causing its energy to diverge and its amplitude to decrease, producing calm water inside the harbour.

Under what condition is the diffraction of a wave most significant?

Check Answer

Answer: Diffraction is most significant when the size of the slit/gap is almost similar to (or smaller than) the wavelength of the wave, or when waves have a long wavelength.

Common Mistakes

!Thinking that wave speed or wavelength changes after diffraction.
!Forgetting that amplitude decreases because energy spreads over a larger area.
!Drawing diffracted wavefronts with changing distances between adjacent lines. The wavelength must remain constant before and after diffraction.

Concept Misunderstandings

Misunderstanding

Sound can bend around a door frame, but light cannot undergo diffraction because it always travels in straight lines.

Correct Concept

Light waves definitely undergo diffraction. However, because visible light has an extremely short wavelength, diffraction of light can only be noticed when light passes through an extremely narrow slit or pinhole. Sound has a much longer wavelength, so its diffraction is easily observed in daily life.

Summary

Diffraction: The spreading of waves when propagating through a slit or past the edge of a barrier.
Unchanged characteristics: Wavelength (\(\lambda\)), frequency (\(f\)), and speed (\(v\)).
Changed characteristics: Amplitude (\(A\)) decreases; direction of propagation spreads from one direction to many directions.
More significant diffraction occurs when: Slit width is narrow (close to the wavelength) or wavelength is long.
Daily life applications: Water: breakwaters/embankments create calm docking areas due to reduced amplitude. Light: holograms on credit cards and bank cards for security. Sound: long-wavelength infrasound allows long-distance elephant communication; hearing sounds around corners.

Short Activity

Objective: Identify diffraction, wave characteristics, factors affecting diffraction, and daily life applications.

A. Objective Quiz

1 What happens to the frequency and amplitude of a water wave after it passes through a narrow gap and undergoes diffraction?

Frequency decreases and amplitude remains unchanged Frequency remains unchanged and amplitude decreases Both frequency and amplitude decrease

2 Which of the following conditions produces the most significant spreading of waves during diffraction?

A wide slit with a short wavelength A wide slit with a long wavelength A narrow slit with a long wavelength

B. Fill in the Blanks

3 Diffraction of waves is the spreading of waves when the waves propagate through a slit or the side of a .

4 During diffraction, the wavelength remains unchanged because the wave does not change in the same medium.

C. Matching / Drag and Drop

Drag each term to the correct description. If using a phone, tap the answer first, then tap the matching box.

Term / Application

Amplitude

Wavelength

Infrasonic waves

Embankment barrier

Description

Decreases because wave energy diverges and spreads over a wider regionCorrect ✓Try againEmpty

Remains unchanged because the medium and wave speed do not changeCorrect ✓Try againEmpty

Long-wavelength sound produced by elephants to communicate over long distancesCorrect ✓Try againEmpty

Creates calm water regions with reduced wave amplitude suitable for docking shipsCorrect ✓Try againEmpty

Keywords

Diffraction of waves Wavefront Amplitude Wavelength Infrasonic waves Embankment

Diffraction of Waves

5.5 Diffraction of Waves

PHYSICS • Form 4 • Chapter 5: Waves

5.5 Diffraction of Waves

Learning Objectives

Define diffraction of waves for water, light, and sound waves.
Determine the changes in wave characteristics after passing through a slit or obstacle.
Draw and describe patterns of diffracted wavefronts for water waves passing through gaps or obstacles.
Identify the factors affecting the diffraction of waves: the size of the slit/gap and the wavelength.
Explain how the size of the slit and wavelength influence the extent of wave spreading.
Provide and explain real-life examples and applications of the diffraction of water, light, and sound waves.

Factors Affecting Diffraction of Water Waves and Wavefront Shapes

Compare how slit width and wavelength affect the degree of diffraction and visualise the resulting circular versus flat wavefront patterns.

Wide Slit vs Narrow Slit

Wide slit

Narrow slit

Short Wavelength vs Long Wavelength

Keeping slit width constant, a short wavelength passes through with slight curving at the ends. A long wavelength produces more rounded, distinct circular wavefronts.

Short wavelength

Long wavelength

Short Explanation

What is Diffraction of Waves?

Diffraction of waves is the spreading of waves when the waves propagate through a slit or side of a barrier/obstacle.

Effects on Wave Characteristics

During diffraction, wavelength, frequency, and wave speed remain unchanged. However, amplitude decreases and the direction of propagation spreads out.

Factors Affecting Diffraction

Diffraction is influenced by the size of the slit/gap and the wavelength. A narrow slit or a long wavelength produces more significant spreading.

Most Significant Diffraction

Diffraction is most obvious when the gap size is roughly equal to or smaller than the wavelength, \(\text{slit size} \approx \lambda\), or when the wave has a long wavelength.

Wave characteristic	Change during diffraction	Reason
Wavelength, \(\lambda\)	No change	Wave speed does not change.
Frequency, \(f\)	No change	No change to the source of frequency.
Speed, \(v\)	No change	No change in the medium before and after diffraction.
Amplitude, \(A\)	Decreased	Wave energy diverges and spreads out to a wider region.
Direction of propagation	Spreads from one direction to many directions	The wavefront spreads after passing through the slit or obstacle.

Try to Answer First

Answer in your mind, then press “Check Answer”.

What wave characteristics change when a wave undergoes diffraction?

Check Answer

Answer: Amplitude decreases and the direction of propagation changes (spreads out). Frequency, wavelength, and wave speed remain unchanged.

Why are water waves in a harbour sheltered by an embankment calmer than the waves in the open sea?

Check Answer

Under what condition is the diffraction of a wave most significant?

Check Answer

Answer: Diffraction is most significant when the size of the slit/gap is almost similar to (or smaller than) the wavelength of the wave, or when waves have a long wavelength.

Common Mistakes

!Thinking that wave speed or wavelength changes after diffraction.
!Forgetting that amplitude decreases because energy spreads over a larger area.
!Drawing diffracted wavefronts with changing distances between adjacent lines. The wavelength must remain constant before and after diffraction.

Concept Misunderstandings

Misunderstanding

Sound can bend around a door frame, but light cannot undergo diffraction because it always travels in straight lines.

Correct Concept

Summary

Diffraction: The spreading of waves when propagating through a slit or past the edge of a barrier.
Unchanged characteristics: Wavelength (\(\lambda\)), frequency (\(f\)), and speed (\(v\)).
Changed characteristics: Amplitude (\(A\)) decreases; direction of propagation spreads from one direction to many directions.
More significant diffraction occurs when: Slit width is narrow (close to the wavelength) or wavelength is long.
Daily life applications: Water: breakwaters/embankments create calm docking areas due to reduced amplitude. Light: holograms on credit cards and bank cards for security. Sound: long-wavelength infrasound allows long-distance elephant communication; hearing sounds around corners.

Short Activity

Objective: Identify diffraction, wave characteristics, factors affecting diffraction, and daily life applications.

A. Objective Quiz

1 What happens to the frequency and amplitude of a water wave after it passes through a narrow gap and undergoes diffraction?

Frequency decreases and amplitude remains unchanged Frequency remains unchanged and amplitude decreases Both frequency and amplitude decrease

2 Which of the following conditions produces the most significant spreading of waves during diffraction?

A wide slit with a short wavelength A wide slit with a long wavelength A narrow slit with a long wavelength

B. Fill in the Blanks

3 Diffraction of waves is the spreading of waves when the waves propagate through a slit or the side of a .

4 During diffraction, the wavelength remains unchanged because the wave does not change in the same medium.

C. Matching / Drag and Drop

Drag each term to the correct description. If using a phone, tap the answer first, then tap the matching box.

Term / Application

Amplitude

Wavelength

Infrasonic waves

Embankment barrier

Description

Decreases because wave energy diverges and spreads over a wider regionCorrect ✓Try againEmpty

Remains unchanged because the medium and wave speed do not changeCorrect ✓Try againEmpty

Long-wavelength sound produced by elephants to communicate over long distancesCorrect ✓Try againEmpty

Creates calm water regions with reduced wave amplitude suitable for docking shipsCorrect ✓Try againEmpty

Keywords

Diffraction of waves Wavefront Amplitude Wavelength Infrasonic waves Embankment

Diffraction of Waves

5.5 Diffraction of Waves

Learning Objectives

Factors Affecting Diffraction of Water Waves and Wavefront Shapes

Wide Slit vs Narrow Slit

Short Wavelength vs Long Wavelength

Short Explanation

What is Diffraction of Waves?

Effects on Wave Characteristics

Factors Affecting Diffraction

Most Significant Diffraction

Try to Answer First

Common Mistakes

Concept Misunderstandings

Summary

Short Activity

A. Objective Quiz

B. Fill in the Blanks

C. Matching / Drag and Drop

Keywords

Diffraction of Waves

5.5 Diffraction of Waves

Learning Objectives

Factors Affecting Diffraction of Water Waves and Wavefront Shapes

Wide Slit vs Narrow Slit

Short Wavelength vs Long Wavelength

Short Explanation

What is Diffraction of Waves?

Effects on Wave Characteristics

Factors Affecting Diffraction

Most Significant Diffraction

Try to Answer First

Common Mistakes

Concept Misunderstandings

Summary

Short Activity

A. Objective Quiz

B. Fill in the Blanks

C. Matching / Drag and Drop

Keywords

Chapter : Wave

Topic : Diffraction of Waves

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