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: 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
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.
