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.
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 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
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
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
Sound can bend around a door frame, but light cannot undergo diffraction because it always travels in straight lines.
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?
2 Which of the following conditions produces the most significant spreading of waves during diffraction?
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.
