CIE A Level Physics (9702) 2019-2021

Revision Notes

9.2.1 Diffraction

What is Diffraction?

  • Diffraction is the spreading out of waves when they pass an obstruction
    • This obstruction is typically a narrow slit (an aperture)
  • The extent of diffraction depends on the width of the gap compared with the wavelength of the waves
    • Diffraction is the most prominent when the width of the slit is approximately equal to the wavelength

Diffraction diagram, downloadable AS & A Level Physics revision notes

Diffraction: when a wave passes through a narrow gap, it spreads out


  • Diffraction is usually represented by a wavefront as shown by the vertical lines in the diagram above
  • The only property of a wave that changes when its diffracted is its amplitude
    • This is because some energy is dissipated when a wave is diffracted through a gap
  • Diffraction can also occur when waves curve around an edge:

Diffraction on an edge, downloadable AS & A Level Physics revision notes

When a wave goes past the edge of a barrier, the waves can curve around it


  • Any type of wave can be diffracted i.e. sound, light, water

Worked Example

When a wave is travelling through air, which scenario best demonstrates diffraction?

A.   UV radiation through a gate post
B.   Sound waves passing a steel rod
C.   Radio waves passing between human hair
D.   X-rays passing through atoms in a crystalline solid

Answer:    D

  • Diffraction is most prominent when the wavelength is close to the aperture size
  • UV waves have a wavelength between 4 × 10-7 – 1 × 10-8 m so won’t be diffracted by a gate post
  • Sound waves have a wavelength of 1.72 × 10-2 – 17 m so would not be diffracted by the diffraction grating
  • Radio waves have a wavelength of 0.1 – 106 m so would not be diffracted by human hair
  • X-rays have a wavelength of 1 × 10-8 – 4 × 10-13 m which is roughly the gap between atoms in a crystalline solid
    • Therefore, the correct answer is D

Exam Tip

When drawing diffracted waves, take care to keep the wavelength constant. It is only the amplitude of the wave that changes when diffracted.

Diffraction Experiments

  • As discussed above, the effects of diffraction are most prominent when the gap size is approximately the same or smaller than the wavelength of the wave
  • As the gap size increases, the effect gradually gets less pronounced until, in the case that the gap is much larger than the wavelength, the waves are no longer spread out


Diffraction gap size, downloadable AS & A Level Physics revision notes

The size of the gap (compared to the wavelength) affects how much the waves spread out


  • Ripple tanks are used a common experiment to demonstrate diffraction of water waves


Ripple tank, downloadable AS & A Level Physics revision notes

Wave effects may all be demonstrated using a ripple tank


  • The diagram below shows how the wavelengths differ with frequency in a ripple tank
    • The higher the frequency, the shorter the wavelength
    • The lower the frequency, the longer the wavelength


Ripple tank vibrations, downloadable AS & A Level Physics revision notes

Ripple tank patterns for low and high frequency vibration


Exam Tip

Familiarising yourself with the wavelength of electromagnetic waves is essential for identifying which wave will cause the greatest diffraction effect for a giving gap width.

Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.

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