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Interaction of Light with Matter

Speed and Refraction

As light passes through a medium, it constantly interacts with atoms which causes them to absorb and re-emit them in the same direction.

  • As a result, the more dense the medium, the slower light travels through it.
Medium Speed (m/s) Refractive Index
Vacuum \(3\times10^8\) -
Air \(2.99\times10^8\) 1.00028
Water \(2.25\times10^8\) 1.33
Glass \(1.99\times10^8\) 1.5
Diamond \(1.25\times10^8\) 2.4

Analysis of Light

Many properties of light can be used for analysis

  • Emission
  • Blackbody radiation
  • The change in wavelength resulting from doppler shifts
    • The amount of shift is dependent on the wavelength being emitted and the relative speed of the light
    • Motion away from the observer causes a redshift (wavelength gets longer)

Properties of light

  • Reflection
    • No light is absorbed, all the light hits the object and bounces off
  • Refraction
    • Some radiation interacts, and as a result, the light is slowed down
  • Dispersion
    • Light is refracted at different rates depending on the wavelength and the medium
  • Interference
    • Peaks of the waves interact destructively or constructively
  • Diffraction
    • The wave like properties of light cause them to spread out when passing through a narrow aperture. Also causes interference
  • Scattering
    • All the radiation is absorbed, and is released in all directions
  • Polarisation
    • Light waves can travel on any rotational plane

Reflection and Refraction

  • When light hits the interface between two different substances with different refractive indexes, two things occur
    1. Some light is reflected
      • The light does not interact with the medium at all
    2. Some light is refracted
      • The higher the refractive index of the second medium, the closer the light will be to perpendicular the interface


  • Materials with higher refractive indices tend to hold onto absorbed light for longer before reemitting it.
  • Since refractive indices are \(\nu\) dependent, the lower \(\nu\) (higher energy), the higher the refractive index.

Wave-Particle Duality

See the dual slit experiment

  • What are particles?
    • Have well defined positions
    • Are indivisible
    • Quantised in structure. Defined edges
  • What are waves?
    • Waves are patterns
    • Don’t have boundaries or edges
    • Don’t come in packages
    • Have \(\nu\), \(\lambda\) and \(E\)
  • What is light?
    • Comes in quantised packages called photons
      • Each photon has \(\nu\), \(\lambda\) and \(E\)
      • Photons only come in integer values (no half photons)
    • The wave is the permutation in the electromagnetic field which exists only in quantised states
      • The photon is the wave packet of the resultant waveform

Speed of Light

  • All light travels at the same speed through a vacuum (\(3\times10^8\:m/s\))
\[ E=h\nu=\frac{hc}{\lambda} \]
  • If h and c are constant, than only \(\nu\) and \(\lambda\) can vary the energy