Wave Behaviour (Cambridge (CIE) IGCSE Physics)

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

• In optics, a transparent material is called a medium

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

• Angles of light are measured from an imaginary line called the normal

  • The normal is always drawn perpendicular to the boundary between two media

Reflection

• Reflection occurs when:

A wave hits a boundary between two media at a plane surface and does not pass through, but instead stays in the original medium

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 transparent media and undergoes a change in speed

• When a wave refracts, as well as a change in speed, the wave also undergoes:

  • A change in wavelength (but frequency stays the same)

  • A change in direction

An example of refraction

Waves can change direction when moving between materials with different densities

• The direction of the incident and refracted rays are 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

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

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 the most prominent when the width of the slit is approximately equal to the wavelength

• 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

Effect of gap size on diffraction

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 to fill the gap behind the object

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

  • The greater the wavelength then the greater the diffraction

Effect of wavelength on diffraction around an edge

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