Wave Behaviour (Cambridge International IGCSE Physics)

Reflection, refraction & diffraction

Spec point — 3.1.4 · Describe how waves can undergo: (a) reflection at a plane surface (b) refraction due to a change of speed (c) diffraction through a narrow gap

Reflection, refraction & diffraction

• All waves, whether transverse or longitudinal, can undergo:

  • reflection at a plane surface
  • refraction due to a change of speed
  • diffraction through a narrow gap

• The material a wave travels through is called a medium

  • When referring to more than one medium, these are called media

Reflection

• Reflection occurs when:

A wave hits the boundary between two media and bounces back, staying in the original medium instead of passing through

An example of reflection

An identical image of the tree is seen in the water due to reflection

Refraction

• When waves enter a different medium, their speed can change
• This effect is called refraction, and it occurs when:

A wave passes a boundary between two different media and undergoes a change in speed

• When a wave refracts, the wave undergoes:

  • a change in speed
  • a change in wavelength
  • a change in direction
  • no change in frequency

An example of refraction

Waves change direction when their speed changes on crossing a boundary between two media

• The direction of the incident and refracted rays is also taken from the normal line

• If the waves slow down, they will bunch together, causing the wavelength to decrease

  • The waves will also start to turn slightly towards the normal

• If the waves speed up then they will spread out, causing the wavelength to increase

  • The waves will also turn slightly away from the normal

Diffraction

• When waves pass through a narrow gap, the waves spread out
• This effect is called diffraction

Waves diffracting through a narrow gap

diffracted,

Exam Hint When drawing waves being reflected, take care to:

• Make sure that the angle of incidence is equal to the angle of reflection
• Keep the wavelength of the waves the same

Similarly, when waves are diffracted the wavelength remains constant.

Refraction is the only wave effect in which the wavelength changes.

Refraction is the name given to the change in the speed of a wave when it passes from one medium to another. The change in direction is a consequence of this.

Factors affecting diffraction

Spec point — 3.1.4 · Describe how wavelength and gap size affects diffraction through a gap Describe how wavelength affects diffraction at an edge

Factors affecting diffraction

Extended Tier Only

• The extent of diffraction depends on the width of the gap compared with the wavelength of the waves

  • Diffraction is greatest when the wavelength of the wave is similar to (or equal to) the width of the gap

• As the gap gets bigger, the effect gradually gets less pronounced until, in the case that the gap is very much larger than the wavelength, the waves no longer spread out at all

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

• Diffraction can also occur when waves curve around an edge or barrier

• The waves spread out into the region behind the object

• The extent of this diffraction also depends upon the wavelength of the waves

  • The greater the wavelength, the greater the diffraction

When a wave goes past the edge of a barrier, the waves can curve around it. Shorter wavelengths undergo less diffraction than longer wavelengths

Exam Hint When you are asked to draw wavefronts diffracting through a narrow gap, make sure that you draw semi-circular crests centred on the gap when its size is similar to the wavelength, and ensure that the wavelength is unchanged.