Waves

• What is a progressive wave?

  a wave that transfers energy and information from one place to another and can be split into 2 types of wave
• What is a transverse wave?

  • a wave by which particles oscillate perpendicular to the direction of energy transfer
  • e.g. water waves, EM waves
• What is a longitudinal wave?

  • A wave by which the particles oscillate parallel to the direction of energy transfer
  • e.g. sound waves, P-waves in earthquakes
• What are some wave properties?

  • Displacement - distance from a point on a wave to the equilibrium position
  • Amplitude - maximum displacement from the equilibrium position
  • Wavelength - minimum distance between 2 points on a wave in phase
  • could be peak to peak or trough to trough
• What is the time period?

  time taken for one complete oscillation (measured in seconds)
• What is the frequency of a wave?

  • Number of cycles per unit time (Hz)
  • Number of whole wave cycles per second passing a given point
• What is the wave speed?

  distance travelled by the wave per unit time
• Phase of a wave definition

  a measurement of the position of a certain point along the wave cycle
• What is phase difference?

  How far out of sync 2 points on a wave are
• Why can't longitudinal waves be polarised?

  Their oscillations are parallel to the direction of energy transfer so their oscillations are already limited to a single plane
• What is a polarised wave?

  a wave that oscillates in one direction only
• Polarising filters

  • These can be used to polarise light and other waves
  • In the diagram: The polarising filter absorbs the horizontal components and light is vertically polarised
  • 2 polarising filters at right angles to each other block all light
  • If 2nd filter not at right angles to the plane of polarisation then only intensity of light is reduced (but still some light is allowed through)

  
• When does reflection occur?

  occurs when a wave changes direction at a boundary white it remains in the original boundary

  • angles measured from the normal
• When does refraction occur?

  • occurs when a wave changes direction and speed as it moves from one medium to another
  • typically the frequency remains constant but the speed and wavelength change
• What is diffraction?

  • The spreading of a wave as it passes through a gap or around an obstacle
  • Gap size needs to be similar to the wavelength
• Properties of Electromagnetic waves

  • Can travel through a vacuum
  • Travel at the speed of light
  • They are transverse waves with oscillating electromagnetic fields
• Snell's law:

  Used to calculate refraction

  
• What is the refractive index?

  • The ratio of the speed of light in a vacuum to the speed of light in a substance
  • refractive index = speed of light in a vacuum/ speed of light in substance
  • n=c/v
• If the speed of light has changed, then due to v = f λ, if v decreases it means λ decreases because frequency remains constant
• If a wave is going to a medium with a higher refractive index light bends towards the normal
• If a wave is going to a medium with a lower refractive index, the light bends away from the normal
• What is the path difference?

  The difference in distance travelled by two waves from their sources to the point where they meet
• What is the critical angle?

  • It is the angle that causes the angle of reflection and the normal to form a 90 degree (right-angle)
  • Once this point is passed, total internal reflection occurs
• What is total internal reflection?


  Total internal reflection is the complete reflection of light at the boundary between two different media, occurring when the angle of incidence is greater than the critical angle.
• What is superposition?


  • happens when 2 or more waves pass through each other and at the instant the waves cross, displacements combine
• What is the principle of superposition

  When 2 or more waves cross, the resultant displacement is the vector sum of the individual displacements
• Waves are in phase:


  • If they are at the same point in the wave cycle e.g. 2π radians or π radians
  • when the phase difference of 2 points is 0 or a multiple of 360 e.g. 0 and 2π radians
• Points with a phase difference of odd number multiples of 180° are exactly out of phase

  e.g. π radians = 1/2 a cycle, 3π radians = 1 1/2 cycles, 5π = 3 1/2 cycles
• 2 sources are coherent if they have the same wavelength and frequency and a fixed phase difference between them
• To get a clear interference pattern, 2 or more sources must be coherent (and in phase)
• Constructive interference occurs when: path difference = nλ (where n is an integer)
• Destructive interference occurs when: path difference = (2n+1/2)λ = (n+1/2)λ