4.4.1 OCR A Level Physics (A) Wave Motion

4.4.1 OCR A Level Physics (A) Wave Motion

1. Prgressive Waves

   • A progressive wave is an oscillation of particles or fields that travels through matter (or a vacuum)
     • They transfer energy but do not move any matter
       • The particles or fields vibrate as the wave passes but don't move with the wave
   • Transverse Waves
     • Oscillations are perpendicular to the direction of energy transfer
       • Electromagnetic Waves
       • Seismic S-Waves
       • Water Waves
     • They have peaks and throughs that correspond with a maximum and minimum displacement
   • Longitudinal Waves
     • A wave where oscillations are paralell to the direction of energy transfer
       • Sound Waves
       • Seismic P-Waves
     • They have compressions and rarefactions
       • A compression is an area of high preassure and density
         • That corresponds to a peak on a transverse wave
       • A rarefraction is an area of low preassure and density
         • That corresponds to a trough on a transverse wave

2. Key Definitions

   • Amplitude
     • The maximum displacement on a wave
   • Displacement
     • The distance from any point on a wave to the equillibrium position
   • Wavelength
     • The distance between consecutive identical points
       • Usually peak to peak or trough to trough
   • Period
     • The time taken for 1 complete oscillation to occour
       • Inversely proportional to frequency
         • T = 1/f
   • Frequency
     • The amount of total oscillations that occour in 1 second
       • Inversely proportional to period
         • f = 1/T
   • Wave Speed
     • The distance travelled by a wave per unit time
       • v = fλ
         • v = wavespeed
         • f = frequency
         • λ = wavelength
   • Phase Differance
     • The difference in phase between two points on the same wave or 2 points on coherent waves

3. Graphical Representatons of Waves

   • Displacement Time
     • Equilibrium to any point on the wave is the displacement
     • Distance between consecutive identical points = time period
   • Displacement distance
     • Equilibrium to any point on the wave is displacement
     • Distance between consecutive identical points = wavelength

4. Oscilloscope

   • An oscilloscope is used to view waves and to take measurements of waves
     • Their displays have y-gain in the y-axis and time base in the x-axis
   • Method
     • Connect to oscilloscope to a wave source
       • Adjust the y-gain so the whole wave can fit on the display vertically
         • Adjust the time base to fit at least 3 cycles on the display
           • Measure the amount of boxes peak to trough
             • Halve the result to find the amplitude in volts
               • So if 1 box represents 1v and the wave spans 4 boxes
                 • 4x1 = 4v
                   • 4v/2 = 2v which is the amplitude
             • Measure across the most full cycles possible to find period
               • So if 1 horizontal box represents 5μs and 3 cycles span 6 boxes
                 • 6x5 = 30μs
                   • 30/3 = 10μs which is the period of 1 cycle
               • Using the period result and f=1/T calculate the frequency
                 • f = 1/10μs
                   • f = 10^5 Hz

5. Intensity

   • Intensity is the power per unit area perpendicular to the direction of energy transfer
     • I = P/A
       • Therefore I has the units WM^-2
   • For a wave the intensity is proportional to the amplitude squared
     • I = k x (amplitude)^2
       • Amplitude can't be written as a to avoid confusing it with area

6. Polarisation

   • Plane polarsed waves are oscillations in one single direction which is perependicular to the direction of wave travel
     • Only transverse waves can be polarised
   • Two rules govern what is transmitted by a polarising filter
     • Rule 1
       • As long as the angle between the polarisation of the wave and the transmission axis is not 90 degrees
         • The transmission axis is the axis of the filter
         • Then some of the wave will be transmitted
           • For example, if a wave is vertically then part of it will be transmitted by anything that's transmison axis is not horizontal
             • The transmitted intensity = the intensity before the filter x cos^2 (the angle between the wave and the filter)
     • Rule 2
       • Any transmitted waves are polarised in the same plane as the transmission axis
         • Irrespective of their previous polarisation

7. Reflection, Refraction and Diffraction

   • Reflection
     • Reflection occurs when light changes direction at a boundary between two mediums
       • While remaining in the original medium
     • The angle of incidence is equal to the angle of reflection
     • Frequency and wavelength remain unchanged
   • Refraction
     • Refraction occurs when a wave changes speed and therefore direction at the boundary between 2 medium
       • But entering the new medium
         • While in the new medium the frequency remains constant so because of v = fλ the wavelength must change when the wave speed changes
           • There will always be some partial reflection at the boundary however

Imágenes y archivos del mapa