Physics - waves

• Amplitude (A)


  Maximum displacement from the equilibrium position
• Frequency (f)


  Number of complete oscillations per second
• Period(T)


  Time for one complete wave oscillation

  Period = 1/frequency
• Wavelength(λ)


  Distance between two consecutive points that are in phase
• The Wave Equation


  v =fλ

  V = Wavespeed (ms-1)

  f = Frequency (Hz)

  λ = Wavelength (m)
• Radians


  Used as an alternative unit for measuring angles (57.3 degrees)

  - 1 complete circle = 2π radians

  - 1 complete oscillation = 2π radians
• Calculating Radians


  Radians → Degrees = Multiply by 180, divide by π

  Degrees → Radians = Multiply by π, divide by 180
• Phase Difference


  The difference in phase between two different points on a wave, measured in radians.

  - Phase difference = (2xπxd) / λ

  - Where d = distance between 2 points

  - Where λ = wavelength
• Transverse wave


  The particles oscillate perpendicular to the direction of energy transfer (propagation)
• Longitudinal wave


  The particles oscillate parallel to and in the direction of energy transfer (propagation)
• Polarised Wave


  The oscillations of a polarised wave are all in a single plane which inlcudes the direction of propagation of the wave.

  
• Unpolarised Wave


  The oscillations of an unpolarised wave are in many planes.
• Polarising filter


  A polarising filter absorbs some of the waves hit based on their plane of orientation.

  
• Coherence


  Sources are coherent if they maintain a fixed-phase relationship (and are polarised in the same plane)
• Sunglasses


  Polarised sunglasses help cut out glare. The light that is reflected from these sunglasses at a large angle is horizontally polarised, so the sunglasses have a horizontal polarising filter in the lens, which absorbs this reflected light.
• TV Transmission


  Terrestrial digital TV signals are polarised. If there are two neighbouring transmitters transmitting the same channel, they can be polarised in different directions to avoid interference. The receiving aerials must be of the same polarisation axis in order to pick up the signals.
• EM Spectrum


  Created when charged particles oscillate. Made up of a magnetic field and a electric field. The fields are perpendicular to each other. All are transverse waves.
• Speed of light


  c = 3x10^8
• Principle of Superposition


  When two waves meet, the total displacement at any point is equal to the sum of the individual displacements at that point.
• Constructive Superposition


  The resultant amplitude will be 2a
• Destructive Superposition


  The resultant amplitude will be -2a
• Superpose


  When two waves meet, they can pass through each other. The displacements of the waves are said to superpose as they combine.
• Maxima


  Waves meet with 0° phase difference, reulting in constructive superposition occurs producing a point of maximum amplitude

  
  

  nλ = d sin∅
• Minima


  Waves meet with a 180° phase difference, resulting in destructive superposition occurs producing a point of minimum amplitude
• How to form a stationary wave

  they are set up as a result of the superposition of two waves with the same amplitude and frequency, travelling at the same speed, but in opposite directions.
• Stationary Wave


  - Stores energy

  - No movement of wave shape

  - Neighbouring particles have different amplitudes

  - Neighbouring points have nπ phase difference

  - Has nodes

  
• Progressive Wave


  - Transfers energy

  - Wave shape moves

  - Neighbouring points have the same amplitudes

  - Neighbouring points have 2πd / λ phase difference

  - Has no nodes
• Internodal Distance (Difference between two nodes)


  λ / 2
• Harmonics


  Different frequencies of stationary waves on a string are called harmonics.

  
  

  The number of the harmonic is equal to the number of anti-nodes on the string.

  
  

  Length of string = (mλ) / 2

  where m = the number of harmonic

  
• Varying Frequency of a Vibrating String

  The frequency of the first harmonic on a vibrating string depends on 3 factors:

  - Length of string, L, (m)

  - Tension of string, T, (N)

  - Mass per unit length of the string, μ, (kg ms-1)
• Reflection


  When a wave rebounds off an interface

  Angle of incidence = Angle of reflection

  
• Refraction


  Change in speed of a wave passing into a different medium causing a change in wavelength.

  Causes a change of direction of the wave if it is not travelling along the normal.

  
• Refraction through mediums


  Less dense → More dense; Velocity decreases, therefore the angle between the ray and the normal decreases.

  
  

  More dense → Less dense; Velocity increases, therefore the angle between the ray and the normal increases.

  
• Refractive Index


  The ratio of the velocity in the air and the velocity of the light in a substance.

  
  

  refractive index also equals, n= sin i / sin r

  
• Combination of refractive index formulas
• Snell's Law

  
• Critical Angle


  The angle of incidence that cause's the angle of refraction = 90°

  
• Total Internal Reflection


  If the incident angle exceeds the critical angle, total internal reflection will occur.

  
• Optical Fibres


  Uses the idea of total internal reflection to transmit light along the fibre.

  
• Focal Length, f


  Displacement along the principle axis from the optical centre of ta lens to the real/virtual focal point