Physics - Unit 2

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  • Created by: charloj2
  • Created on: 31-05-15 09:43

The Nature of Waves

  • A progressive wave moves energy away from it's source.
  • Can be reflected and Refracted 
  • Intensity is a measure of How much energy a wave is Carrying (The rate og flow of energy per unit area at right angles to the direction of travel of the wave. It's measured in Wm-2)
  • Intensity is proportional to the amplitude of the wave. 
  • Frequency is the inverse of the period. 1Hz = 1 S-1 
  • Speed = Distance/Time OR Speed = Wavelength xFrequency 
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Longitudinal & Transverse Waves.

  • In Transverse Waves the Vibration is at Right Angles to the direction of travel 
  • Longitundinal waves the Vibrations are along the direction of travel. (Compressions - The waves clump together, Rarefaction - the waves are spread out)
  • A polarised wave only ossiclates in one direction - the other direction waves are filtered out. Can pnly happen to transverse waves. 
  • When light reflects it's partially polarised. The intensity changes when partially polarised light is reflected. 
  • Materials can rotate the plane of polarisation (the pplane in which a wave moves and vibrates) EXPERIMENT: put the material between two polarising filters. 
  • Rotating the Plane of Polarisation affects the intensity.  
  •           Amplitude = A0Cos0 - A0 is the amplitude of the initial wave, Cos0 is the angle the plane has rotated. 
  • The intensity is proportional the the amplitude squared. 
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The Electromagnetic Spectrum

  • In a vacuum, electromagnetic waves travel at 2.998x10^8 ms2, but slower in other medias.
  • They have electric and magnetic fields are at right angles to each other and the direction of travel
  • The longer the wavelength the more obvious it's characteristics. 
  • Energy is proportional to the Frequency.
  • The higher the energy, the more dangerous it is. 
  • UVA has the lowest frequency and is the least damaging. Can age skin. 
  • UVB - more dangerous, absorbed by DNA molecules causing mutations & Cancer. SUNBURN.
  • UVC - Ionising, causes cell mutations and destruction and cancer - blocked from the ozone layer. 
  • UVA triggers the release of Melanin - tanning agent.
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Superposition and Coherence.

  • Happens when two waves pass through each other. Grow or shrink. The displacement Combines. 
  • Constructive = Growing 
  • Distructive = Flat line
  • To get interference pattern the two sources must be coherent
  • COHERENT = Same wavelenght, frequence and a fixed phase difference.
  • Constructive interference depends on path difference. 
  • Constructive interference happens when there's a whole number of wavelenghts
  • Destructive is when there's a half wavelenght. 
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Standing/Stationary Waves.

  • Progressive Wave reflected off a Boundary
  • The superposition of two progressive waves with the same wwavelenght moving in opposite directions. 
  • NO energy is transmitted 
  • Resonant frequencies is when you get a standing wave.
  • Node = No amplitude
  • Antinode = Yes amplitude.
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Diffraction

  • The amount of diffreaction depends on the wavelenght of the wave compared to the size of the gap.
  • You get the most diffraction when the gap is the same size as the wavelenght. 
  • If the gap is smaller the waves are just reflected back 
  • Wave meets obstacle - shadow behind the obstacle where there are no waves. The wider the obstacle the longer the shadow.
  • when the Wavelenght = hole you get a diffraction pattern of dark and light. The narrower the slit, the wider the diffraction pattern. 
  • You need a coherent light source
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Two Source Interference.

  • Double Slit Experiment - LAser light is coherent and monochromatic (only one wavelength). THe slits have to be around the same size as the wavelength. 
  • Dark & Light depends on whether it's constructive or destructive. 
  • Work out the wavelength with YOung's Double slit formula
  • Fringe spacing = distance from the slits to the screen x wavelength/spacing of the slits.
  • The experiment was evidence for the wave nature of light. 
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Diffraction Gratings.

  • Interference Patterns get sharper when you diffract through more slits. 
  • Monochromatic light gives sharp lines 
  • slit distance x sin0 = nth prder maximum x wavelength
  • The larger the wavelength the more the pattern will spread out,
  • The coarser the grating the less the pattern will spread out.
  • If sin0 is more than one that order doesn't exist. 
  • Shining white light light through grating produces a spectra
  • White light is made up of different colours so when it's diffracteed the different colour wavelenghts spread out 
  • Each order is a spectrum - red on the outside, violet on the inside.
  • Zero order stays white.
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