P4 waves

• a progressive wave transfers energy from one point to another without transferring material, they are made up of oscillating particles of a medium/field
• electromagnetic waves are oscillating electric and magnetic fields which travel without need for a substance
• examples of electromagnetic waves are:

  radio waves, microwaves, infrared radiation, light, ultraviolet radiation, x-rays and gamma radiation
• longitudinal waves are waves in which the direction of vibration of the particles is parallel to the direction of travel of the wave
• longitudinal waves are made up of compressions and rarefactions
• examples of longitudinal waves are:

  sound waves, primary seismic waves, compression waves on a slinky
• transverse waves are waves in which the direction of vibration of the particles is perpendicular to the direction of travel of the waves
• transverse waves are plane-polarised when the vibrations stay in one plane only, and unpolarised when the vibrations change from one plane to another
• longitudinal waves cannot be polarised
• displacement = distance and direction of a vibrating particle from its equilibrium position
• amplitude = maximum displacement of a vibrating particle
• wavelength = least distance between two adjacent vibrating particles with the same displacement and velocity at the same time
• cycle = from one maximum displacement to the next maximum displacement
• period = the time for one complete wave to pass a fixed point
• frequency = number of cycles of vibration of a particle per second
• the equation for frequency is f = 1/t

  f = frequency in Hz

  t = time in s
• the equation for wave speed is c = f/lambda

  c = speed in ms^-1

  f = frequency in Hz

  lambda = wavelength in m
• the phase of a vibrating particle at a certain time is the fraction of the cycle it has completed since the start of the cycle
• the phase difference between two particles vibrating at the same frequency is the fraction of a cycle between the vibrations of the two particles, measured in degrees or radians
• 1 cycle = 360 degrees = 2pi radians
• reflection is when a wave hits a boundary between two media and stays in the original medium instead of passing into the next medium
• in reflection, the angle of incidence always equals the angle of reflection
• the angle of incidence is the angle between the incident wave and the boundary, the angle of reflection is the angle between the reflected wave and the boundary
• refraction is when a wave passes across a boundary between two media with different refractive indexes, so changes direction and speed in the new medium
• diffraction is when waves spread out after passing through a gap or round an obstacle
• in diffraction, the narrower the gap, the more the waves spread out, and the longer the wavelength, the more the waves spread out
• superposition is when waves meet and combine for an instant before they move apart
• the principle of superposition states that:

  when two waves meet, the total displacement at a point is equal to the sum of the individual displacements at that point
• two types of interference can occur during superposition:

  constructive interference

  destructive interference
• constructive interference is when the waves have displacement in the same direction, so a crest and a crest makes a supercrest, and a trough and a trough makes a supertrough
• destructive interference is when the waves have displacement in opposite directions, so a crest and a trough cancel out to an overall displacement of zero
• a stationary wave is formed from the superposition of two progressive waves, travelling in opposite directions in the same plane, with the same frequency, wavelength and amplitude
• stationary waves do not transfer any energy
• in a stationary wave, when waves meet in phase there will be constructive interference
• constructive interference produces antinodes which have maximum amplitude
• destructive interference produces nodes which have minimum amplitude
• in a stationary wave, when waves meet out of phase there will be destructive interference
• stationary waves are formed by:

  • waves travel to the boundaries and are reflected
  • two waves travelling in opposite directions interfere with each other and superpose
  • the boundaries are fixed so cannot move and become nodes
  • waves interfere destructively to produce nodes, and interfere constructively to produce antinodes
• frequency:

  in stationary waves, all particles except those at the nodes vibrate at the same frequency

  in progressive waves, all particles vibrate at the same frequency
• amplitude:

  in stationary waves, the amplitude is zero at the nodes and maximum at the antinodes

  in progressive waves, the amplitude is the same for all particles