Optical properties

Optical properties

1. refer to materials reponse to exposure to electromagnetic radiation and, in particular, to visible light

2. electromagnetic radiation

   • it is considered to be wavelike, consisting of electric and magnetic field components that are perpendicular to each other and also to the direction of propagation
   • the optical behaviour of a solid material is a function of its interactions with electromagnetic radiation having wavelengths within the visible region of the spectrum (0.4-0.7 up)
   • All radiation has a wave-like nature with an electric field (E) component and a magnetic field (H) component in perpendicular planes (equation)
   • (quantum-mechanical perspective) electromagnetic radiation may be considered to be composed of photons (groups of packets of energy that are queantized- they can have only specific values of energy)
     • photon equation

3. Light interaction with solids

   • when light proceeds to one medium to another> some transmitted, absorbed and reflected (equation)
   • classification of materials
     • transparent: little absortion - see through them
     • translucent: light transmitted diffusely
     • opaque: impervious to the transmission th visible light
   • the absortion coefficient depends on the wavelenght of light (absorption spectrum). UNIQUE for each material

4. materials

   • semiconductors
     • photon with an energy greater than the band gap energy are strongly absorbed
       • the photon energy promotes an electron from the valence band to the conduction band and leaves a hole behind the valence band
   • metals
     • appear opaque as a result of the absorption and then reemission of light radiation with a thin outer surface layer
     • absorption occurs via the excitation of electrons from occupied energy states to unoccupied ones above the Fermi energy level. Reemision takes place by decay electron transition in the reverse direction
     • The perceived color of a metal is determined by the spectral composition of the reflected light
     • e in the conduction band have a lot of available energy levels and thus metals strongly absorb light over a wide range of the spectrun. They also reflect it over a wide range of spectrum
     • large absorption coeeficients> light interacts with the metal very close to the surfacce and the surface quality of the metal strongly influence optical appereance
   • Insulators
     • completly transparent in the visible
     • with impurities> can change their optical properties and make them coloured in the visible (diamonts)