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Physics AS - Waves

AS Level EdExcel Physics (Unit 2)


on 1 June 2011

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Transcript of Physics AS - Waves

Waves Superposition & Standing Waves The Basics Amplitude - the max displacement from the equilibrium position Wavelength - the shortest distance between two adjacent points that are in phase Frequency - the number of oscillation per second Period - the time taken for one complete oscillation Speed of a wave is given by In a displacement/time graph In a displacement/distance graph Transverse and longitudinal waves Transverse - oscillations at right angles to the direction of the wave Longitudinal - oscillations along the direction of travel Sound waves are longitudinal Particles of air are displaced from their equilibrium positions and produce regions of compression and rarefaction A is a line representing a series od equivalent points on the waves (e.g. all the compressions) wavefront This equation appliesto all types of wave In Phase - when two waves crests (or troughs) occur in the same place In Antiphase - when the crest of one wave coincides with the trough of another Coherence - two sets of waves with the same frequency and constant phase difference Superposition - when two or more waves arrive at the same place. If the waves are inphase the superposition is constructive. Or if the waves are in antiphase the superposition is destructive. Question: Explain what an interference pattern is? Model answer: A source of coherent light is defracted through grating as the light spreads out some of the waves will be superposed. Some of the waves will be inphase and constructive interference will occur, and some of the waves will be out of phase and destructive interference will occur Superposition of a continous wave reflected from a boundary with its incident wave will produce an interference pattern called a standing wave How are standing waves produced? Standing waves on strings and in pipes String: Ends of string: both nodes Open end tube: One end open and one end closed tube: Ends of pipe: max. displacement - antinodes At closed end:
no displacement -
node Diffraction ...is the spreading of wavefronts as a wave passes through a gap or around an obstacle. The amount of diffraction is greatest when the wavelength is similar to the size of the gap or obstacle one slit diffraction two source interference When there are two sources the waves superpose as interference is taking place... ...but when another slit is made the diffraction of the waves interfere with each other... Electron diffraction The Electromagnetic Spectrum Visible light is in range 400-700nm all of the waves travel at the same speed in a vacuum (3x10^8m/s) all of the waves are transverse all of the waves consist of an oscillating electric and magnetic field Pulse-echo
techniques Waves reflect from a boundary between two media The greater the distance in density between the two materials, the stronger the reflection Pulse-echo techniques are used to detect the position and/or the motion of a boundary between two materials A pulse is required so that the time interval between the incident pulse and the reflected pulse (echo) can be mesured The Doppler Effect When waves are emitted from a moving source or detected by a moving receiver, the detected frequency differs from the emitted frequency. Ultrasound
Scanning Reflected pulses of ultrasound are used to determine where the boundaries are between different tissues then build up an image of the foetus Resolution - the smallest level of detail that can be seen Two factors that affect a scans resolution the wavelength of the sound used if reduced - resolution improved the time intervals of the pulses if shortened - resolution improved Refraction When waves meet a boundary between two materials, some of the wave is reflected and some is transmitted.
The transmitted wave changes speed and may change direction. This is refraction. Total internal relfection Not the type you see at the beach! If a wave passes from a more dense to a less dense material, it is possible for all the light to reflect and none to refract at that interface. This is called total internal reflection. This happens if the ange of incidence within the material is greater than the critical angle C. At smaller angles, some of the light may be reflected. Polarisation Light is a transverse wave.
In unpolarised light, these variations take place in all planes at right angles to the direction in which the ray of light is travelling.
In plane polarised light, the variations in electric field take place only in one plane. The variations in magnetic field are in a plane at right angles to this.
Longitudinal waves cannot be polarised Plane polarised light can be shone through different concentrations of sugar solution and from that the angle of rotation of the light can be mesured. A linear graph can then be drawn so unknown concentrations of sugar solution can be identified in relation to the angle of rotation
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