Wave Phenomena (OCR A Level Physics)

• Wave phenomena are unique to waves and not particles. These include:
  • Reflection
  • Refraction
  • Polarisation
  • Diffraction

Reflection

• Waves reflect from all surfaces, this is seen in mirrors

• The key feature of reflection is the angle of incidence i is equal to the angle of reflection r

Refraction

• Refraction is when light travels at a different speed when travelling through another medium
  • This is seen when light passes through a boundary such as from air to glass

• White light splits into a spectrum due to refraction
  • violet light is slowed down more and refracts more than red light

Refraction of light through a prism

Polarisation

• Polarisation is a wave phenomenon that only occurs for transverse waves
  • This is when the vibrations of transverse waves are restricted in one direction

Polaroid sunglasses use polarisers to reduce glare

Diffraction

• Diffraction is the spreading out of waves when they pass an obstruction
  • This obstruction is typically a narrow slit known as an aperture

• It usually represented by a wavefront
• The only property of a wave that changes when it diffracts is its amplitude
  • This is because some energy is dissipated when a wave is diffracted through a gap

• 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

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

• The diffraction pattern of light can be represented as a series of light and dark fringes which show the areas of maximum and minimum intensity
• If a laser emitting blue light is directed at a single slit, where the slit width is larger than the wavelength of the light, it will spread out as follows:

The intensity pattern of blue laser light diffracted through a single slit

• If the laser were to be replaced by a non-laser source emitting white light:
  • The central maximum would be white
  • All maxima would be composed of a spectrum
  • The shortest wavelength (violet / blue) would appear nearest to the central maximum
  • The longest wavelength (red) would appear furthest from the central maximum
  • The fringe spacing would be smaller and the maxima would be wider

Qualitative treatment of the variation of the width of the central diffraction maximum with wavelength and slit width