# Ocr Physics A condensed notes on Waves

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WAVES

GENERAL INFO

·        Transfer energy from one place to another.

·        Waves moving away from a source are called progressive waves.

·        Particles oscillate vertically but do not move forwards or backwards.

·        Waves with regular and smooth oscillations are pure waves.

·        In longitudinal waves areas of high pressure are called compressions and areas of low pressures are called rarefactions.

·        Phase difference concerns relationship of pattern of vibration at 2 points. Measured in rads. Waves in antiphase are π out of phase.

·        Speed of waves can depend on what they travel through and can be temperature dependant.

·        Intensity = Power/Area

·        Intensity is proportional to (Amplitude²)

POLARISATION

·        Oscillations of a transverse wave can be at any direction as long as its 90˚ to direction of travel.

·        However polarising filters can cause oscillation in only one plane causing a plane polarised wave.

·        Cannot happen in longitudinal waves since they do not oscillate at right-angles to direction of travel.

·        When an already Polarised beam of light (ie oscillates in only one plane) goes through another perfect polariser called an analyst which is at angle to the initial polariser then plane of polarisation can be rotated.

·        The intensity of light passing through the analyst is equal to the value of the intensity of the wave approaching the analyst from the initial polariser × cos² (the angle of the analyst to the polariser.)    This is called Malus’ Law.                                                                                                                                     I  =  IMAX cos²  θ

·        Shows that if analyser is 90˚ to Polariser then no light will pass through – Crossed Polaroids.

·        Reflected light is partially polarised.

·        Polarisation can be used in strain analysis and transmission of radio waves so vertically transmitted waves do not interact with horizontal waves.

INTEFERENCE

·        Principle of Superposition – When 2 or more waves exist in the same place the resultant wave can be found by adding displacements of each individual wave.

·        Coherence is when there is constant phase difference between two waves. (Wave-length and frequency also same although amplitude is irrelevant)

·        Path difference – proportion of a wavelength one wave has travelled more than the other.

·        If path difference is a whole number of wavelengths ie one wave is a multiple of a wavelength away from the other then constructive interference occurs.  Waves reinforce each other. Amplitude increases.

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