Optics

Optics

1. Refraction

   • Is the change of direction of light between the boundary of air and a transparent substance.
   • Refractive Index = sin i/ sin r
   • n1 sin i = n2 sin r
   • n2/n1 = sin i/ sin r
   • refractive index is equal to ratio of wave speed, ns = c/ cs

2. Diffraction grating

   • consists of a glass or plastic slide with many closely spaced slits
   • happens because light is diffracted by each slit in grating and diffracted light from adjacent slits reinforces only in a few directions
   • angle between beam increases if λ of light increases or width in slit decreases
   • d sin θ = n λ
   • d in metres = 1/mm
   • Used in a spectrometer and to analyse star spectra

3. Double slit interference

   • bright fringes caused by light waves that are coherent, in phase
   • dark fringes caused by light waves that are incoherent, are 180° out of phase
   • fringe separation, w = λD/s

4. Total internal reflection

   • incident substance has larger refractive index than other substance
   • angle of incidence exceeds critical angle
   • sin ic = n2/n1
   • In optical fibres cladding had a lower refractive index than core , allows total internal reflection
   • fibres must be narrow or it will cause multipath dispersion
   • light must be coherent or image would be scrambled