A student sets up a ripple tank and generates water waves of a fixed frequency. The student observes that when the waves pass through a gap whose width is much larger than the wavelength, very little spreading of the waves occurs. The student then reduces the gap width until it is approximately equal to the wavelength. Describe what the student observes about the waves after they pass through the narrower gap, and explain why the amount of spreading changes when the gap width is reduced.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The waves spread out noticeably after passing through the narrower gap, forming a semicircular or curved pattern beyond the gap.
This increased spreading, or diffraction, occurs because the gap width becomes comparable to the wavelength. When the slit is much wider than the wavelength, each part of the wavefront behaves independently and the wave continues straight. As the slit narrows to about one wavelength, each point on the wavefront acts as a secondary source of waves (Huygens’ principle), producing a curved wavefront and a wider spread.
The frequency, wavelength and speed of the waves remain unchanged by diffraction; only the direction of propagation changes.
This increased spreading, or diffraction, occurs because the gap width becomes comparable to the wavelength. When the slit is much wider than the wavelength, each part of the wavefront behaves independently and the wave continues straight. As the slit narrows to about one wavelength, each point on the wavefront acts as a secondary source of waves (Huygens’ principle), producing a curved wavefront and a wider spread.
The frequency, wavelength and speed of the waves remain unchanged by diffraction; only the direction of propagation changes.
Examiner tips
- Use the word ‘diffraction’ and describe the semicircular pattern; link the change to the slit width being ~λ.
- Show that frequency, wavelength and speed are unchanged – this demonstrates understanding of wave properties.
Common mistakes
- Confusing diffraction with refraction or reflection.
- Saying the wavelength changes; it does not change during diffraction.
- Omitting the semicircular pattern or the fact that the gap width is comparable to λ.
Mark scheme (4 marks)
- The waves spread out (diffract) significantly / noticeably after passing through the narrower gap
- The waves spread out in a semicircular / curved pattern beyond the gap
- Diffraction is greatest when the gap size is of the same order of magnitude as the wavelength
- The frequency, wavelength, and speed of the waves are unchanged by diffraction
Key terms in this question
Related
- All Cambridge International IGCSE Physics (0625) revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
More General properties of waves questions
- Describe what happens to a wave when it undergoes diffraction through a gap, and…
- Explain what happens to the speed, frequency, and wavelength of a wave when it u…
- A student observes that when a wave undergoes reflection at a smooth, plane surf…
- A sound wave travelling through air enters a solid wall. State and explain what …