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Diffraction (CIE A Level Physics)

Revision Note

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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 through a gap

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 by a barrier

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

Examiner 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

  • The extent of diffraction depends on the width of the gap compared to the wavelength of the waves
  • For gaps that are much much smaller than the wavelength of the wave, no diffraction occurs

  • For gaps that are much much bigger than the wavelength of the wave, no diffraction occurs

  • When the wavelength of the wave and the width of the gap are similar in size, then diffraction occurs:
    • When the wavelength is bigger than the gap, more diffraction occurs,
      • The wave spreads out more after passing through
    • When the wavelength is smaller than the gap, less diffraction occurs
      • The wave spreads out less after passing through

 Size of the gap and diffractionDiffraction 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

 

  • Examples of diffraction include:
    • Radio waves moving in between or around buildings
    • Water waves moving through a gap into a harbour

Ripple tanks

  • Ripple tanks are a common experiment used to demonstrate diffraction of water waves
    • Through a gap
    • Around a barrier
  • The amount of diffraction that occurs can be changed by changing the wavelength of the waves compared to the gap size

 

Changing the gap size for diffraction in a ripple tank

ripple-tank-diffraction-gap-size

When the gap size is bigger than the wavelength less diffraction occurs and the waves spread out less after passing through

 

Examiner Tip

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

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Ashika

Author: Ashika

Expertise: Physics Project Lead

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.