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Waves (Refraction (TIR (Fibre Optics (Advantages (Not effected by…

- - - - Fibre optic cables contain a thin flexible glass core, surrounded by a thick cladding of slightly lower optical density
      - Light that enters the fibre will totally internally reflect off the walls of the fibre, if they strike the walls at a great enough angle to the normal
      - This transmits the light down the core, which can be used to transfer data down a fibre
      - The cladding:
        
        Protects the core from scratches which would allow light to escape from the core
        
        Prevents cross talk between fibres as light that escapes the glass core will dissipate in the cladding
        
        Provides a lower optical density for light to totally internally reflect
        
        Has an optical density only slightly below that of core, so that the critical angle is large, so that only light rays on straight paths are reflected, minimising modal dispersion
      - The core:
        
        Is narrow to keep the light on a straight path, reducing modal dispersion, and reducing absorption by minimising path length
        
        Straight path minimises number of reflections to limit energy loss at reflection
        
        Keeps light rays striking the core-cladding interface at a high angle, so that as much light refracts as possible
      - Absorption
        
        Some light rays do not strike the core-cladding boundary at a great enough angle and so are refracted out
        
        Energy of wave is absorbed by material in the core and by reflections and so overall signal amplitude decreases
        
        Signal retains it's shape
      - Dispersion
        
        Material
        
        Different wavelength waves travel at different speeds in the material and so spread out
        
        Use monochromatic light
        
        Pulse broadens
        
        Modal
        
        Light rays take different length paths through core, and so arrive at the end of the core in different time
        
        Pulse broadens
        
        Use narrow core and cladding with only slightly lower optical density
        
        As then indirect routes will be refracted out and light will be kept on straighter paths
        
        Use signal repeaters
      - Advantages
        
        Not effected by electromagnetic interference
        
        Do not heat up much
        
        Are cheap to make
        
        Light has high frequency, so lots of data can be transferred quicker than in electrical cables
- - - - Bright central maximum surrounded by alternating side minima and maxima, moving symmetrically outwards.
      - Side maxima diminish in intensity moving outwards
      - Central maximum is twice the width of side maxmima
      - Central maximum is at least twice the intensity of the first side maximum
    - - Central maximum is bright white
      - Waves of different wavelengths diffract why different amounts when they diffract through the slit and so they spread out
      - Each side maxima is a continuous spectrum with blue on the inside and red on the outside.
      - Overall pattern is less intense
      - Side maxima get wider and dimmer moving outwards and start to overlap far out
      - Maxima generally wider with white light
      - Clear minima not seen
    - - Wider slit leads to less diffraction so central maximum narrower, side maxima closer in, and pattern more intense as light is spread out by less
      - Opposite for smaller slit width
  - - - For two point sources
      - Waves from each source travel different distances to reach a given point on a screen
      - When path difference is whole number of wavelengths of waves from the sources, the waves will meet the screen in phase, superpose and interfere constructively to produce a point of maximum intensity.
      - When path difference is odd multiple of half wavelength waves will meet the screen in anti phase, superpose and interfere totally destructively to produce a point of zero intensity on the screen
      - Series of equally spaced maxima and minima, moving outwards from centre symmetrically (with maximum in centre)
      - All maxima are same width and same intensity
    - - Slits do not act as perfect point sources as the slits do have a width
      - The intensities of the peak is modulated by the single slit diffraction pattern
    - - When monochromatic, coherent source is used a definite monochromatic pattern produced
      - High frequency alternating maxima and minima, moving outwards from centre of pattern in symmetrical fashion
      - Intensity of maxima decreases slightly moving outwards, and technically will increase again, but this is rarely seen
      - White Light
        
        Central maximum is bright white
        
        Each side maxima is a continuous spectrum with blue on inside and red on outside
        
        Overall pattern is less intense
        
        Maxima get wider and dimmer moving outwards
        
        Maxima start to overlap when far from centre, until screen appears uniformly illuminated with white light
        
        Avg. wavelength of white light used to find fringe spacing
      - \(\lambda=\frac{w\,s}{D}\)
    - - The distance between single and double slits can be kept constant so that the phase difference between waves from each slit is kept constant
      - Since slits illuminated from same monochromatic, coherent source, two slits will act as coherent sources of light :
  - - - As multiple sources are added, the interference pattern becomes sharper
      - The bright fringes become narrower and the dark fringes become wider and darker
      - Very sharp pattern produced with central zero order line with higher orders moving symmetrically outwards.
      - Usually only a few orders are observed
      - The intensities of the orders is modulated by the single slit diffraction envelope and so intensities decrease slightly moving outwards
      - Angle of diffraction for bright fringes very large for diffraction gratings
      - Bright orders are separated by large dark minima
      - Only a finite number of orders can be observed as angle of diffraction cannot be greater than 90 degrees
        
        \(n_{max}=\frac{d}{\lambda}\)
    - - Zero order is intense white
      - Higher orders become continuous spectra, with blue on the inside and red on the outside
      - Overall pattern is less intense
      - Orders get wider and dimmer moving outwards
      - Orders generally do not overlap when produced using a diffraction grating, as pattern is so sparsely populated
  - - - Coherent
        All waves in the light have same frequency and wavelength, and have a constant phase difference between them
      - Monochromatic
        Made up from waves of the same frequency (and therefore wavelength)
      - Polarised
        All waves oscillate in same plane
      - Collimated
        Laser produces an approximately horizontal beam
  - - - When a wave passes through a gap of comparable size to it's wavelength, the energy of the wave spreads out, and the waves spreads out through the gap.
        
        When size of gap much bigger than wavelength then very little diffraction occurs
        
        When size of gap is within 10x as big as wavelength then noticeable diffraction occurs
        
        When slit size equals the wavelength of the wave then maximum diffraction occurs and the energy of the wave is spread out over 180 degrees
        
        When slit size is smaller than wavelength, then the wave is just reflected back off the slit
      - A wave will diffract around an obstacle in the same way as through a slit, to produce a shadow which closes moving away from the object
        
        More diffraction leads to a shorter shadow
        
        When the wavelength is bigger than the object the wave passes straight through the object
- - - - Intensity
        Power per unit area
  - - - When talking about two waves it is the difference in phase between a point on each wave that is at the same location in space
- - - - A resonant frequency is a frequency of wave that will form a stationary wave, and is a frequency such that a whole number of waves are transmitted by the generator in the time it takes on wave front to move through medium, be reflected, and move back to generator.
      - Frequency of \(n^{th}\) harmonic
        Given by \(nf_{0}\)
      - \(f_{0}\) is lowest possible frequency stationary wave that can form in a medium
  - - - If formed from reflection of a transmitted wave at medium boundary, then there must be little loss in energy at the boundary