waves

• optical fibres are made of an optically dense core surrounded by cladding with a lower optical density, allowing TIR to occur
• the cladding in an optical fibre protects the core and prevents signal degradation through light escaping
• modal dispersion

  • caused by light rays entering the fibre at different angles
  • the rays all take different paths and so a different length of time
  • reduced by making core narrow
• material dispersion

  • caused by light of different wavelengths
  • the rays all have different speeds and so a different length of time
  • reduced by using monochromatic light
• pulse broadening is where the received signal is broader than the original signal transmitted, causing overlap an loss of information
• absorption is when the signals energy is absorbed by the fibre, reducing the amplitude and so causing a loss of signal
• both absorption and dispersion can be reduced with an optical fibre repeater
• in single slit diffraction, the central fringe is double the width of the others and much more intense
• bright fringes occur where waves meet in phase and interfere constructively
• dark fringes occur where waves meet completely out of phase and interfere destructively
• for single slit diffraction of white light, there is a central maximum of white light with the alternating bright fringes being a spectra of violet to red
• diffraction gratings can be used to

  • create a line absorption spectra from stars to see the elements present there
  • x ray crystallography (measures the atomic spacing in certain materials)
• TIR occurs when light is passing from a material with a greater refractive index to a material with a lower refractive index
• the polarising filter in sunglasses is usually aligned vertically to block the horizontal vibrations of light
• natural sources are usually vertically polarised and glare from flat surfaces is usually horizontally polarised
• diffraction is most clear when the size of the gap is the same as the wavelength of the wave
• increasing wavelength means wider fringes
• increasing slit width means narrower fringes