Waves

• What is a transverse wave

  Oscillations are perpendicular to the direction of energy transfer
• Examples of transverse waves

  • All electromagnetic waves (light)
  • Waves on a string
  • Ripples and waves in water
• What is a longitudinal wave

  Oscillations are parallel to the direction of energy transfer
• Examples of longitudinal waves

  • Sound waves in air
• What are the two types of waves

  Longitudinal and transverse
• All longitudinal waves require a medium to travel eg air, a liquid or a solid.
• What is the amplitude

  Maximum displacement of a point from its undisturbed position
• What is the wavelength

  Distance between the same point on two different waves
• For both ripples on a water surface and sound waves in air, it is the wave that travels and not the water or the air.
• What is frequency?

  • The number of waves passing a point per second.
  • 1 Hz = 1 wave per second
  • Unit = Hz
• What is period

  • Time (in seconds) for one wave to pass a point
  • Unit = seconds
• Equation for period

  • period = 1 / frequency
  • T = 1 / frequency
• Equation for wave speed

  • wave speed = frequency x wavelength
  • v = f x λ
  • Units --> m/s - Hz x m
• Describe how to measure the speed in ripples of sound waves in air

  1. Attach a signal generator to a speaker. Set up the oscilloscope so detected waves are shown as separate waves
  2. Start with both microphones next to speaker then slowly move one away until the two waves are aligned but have moved one wavelength apart
  3. Measure distance between microphones to find one wavelength
  4. Use v = f x λ to find speed
• RP20: Measure the wavelength, speed and frequency of water ripples

  1. To measure wavelength place a metre rule at the side of screen perpendicular to wave fronts
  2. Use the ruler to measure length of screen. Take a photo of shadow lines. Count the number of complete waves on the screen and to determine wavelength divide the length of waves by number of complete waves
  3. To measure frequency count number of waves that pass a point. Time how long the number of complete waves take to pass a point using stopwatch
  4. To measure wave speed use v = f x λ. v is wave speed, f is frequency and λ is wavelength.
• What are electromagnetic waves

  They are transverse waves that transfer energy from the source of the waves to an absorber
• Name the EM Spectrum

  • Radio waves
  • Microwaves
  • Infrared
  • Visible light (red to violet)
  • Ultra violet
  • X-ray
  • Gamma ray
• Which waves has a low frequency

  Radio waves onwards

  
• Which waves has the highest frequency

  Gamma rays onwards
• Our eyes can only detect visible light and so detect a limited range of electromagnetic waves
• Radio waves transfer energy to the kinetic energy stores of electrons in radio receivers, which generates an electric current
• How do sound waves travel in solids

  The particles in a solid vibrate and transfer kinetic energy through the material
• Give an example use for ultrasound waves

  Medical or industrial imaging
• How do the speeds of EM radiation differ in a vacuum and in air

  Electromagnetic waves all travel at the same speed in the air and vacuum
• What happens when a wave meets a boundary between two materials

  The wave is reflected, transmitted or absorbed
• What happens when a wave is 'absorbed' by a second material

  The wave transfers energy to the materials energy stores. The energy is transferred to a thermal energy store which leads to heating
• What happens when a wave is 'transmitted' through a second material

  • The wave carried on travelling through the second material. Often leads to refraction.
  • Can be used in communications, lenses of glasses and cameras
• What happens when a wave is 'reflected'

  This is where the incoming ray is neither absorbed or transmitted, but instead is sent back away from the second material
• What is refraction

  Refers to the change in direction of a wave, as it passes from one medium to another and is caused by its change in speed
• • Different medium (materials) have different densities
  • Higher the density = Slower the wave
• Light travels more slowly in dense materials
• Light travelling from a less dense material, into a more dense material will bend towards the normal
• When a wave travels into a more dense medium, the wave speed decreases. The frequency stays the same
• When a wave travels into a more dense medium, the wave speed decreases. The wavelength decreases.
• What is optical density

  A measure of how quickly light can travel through it. The higher the optical density, the slower the light waves travel through it
• Construct a ray diagram for a refracted light ray

  1. Draw boundary between two materials and normal. Normal should be 90 to boundary
  2. Draw incident ray that meets normal at boundary.
  3. Angle between incident ray and normal is angle of incidence
  4. Draw refracted ray on other side of boundary
  5. Angle of refraction is angle between refracted ray and normal
  6. If second material is optically denser than first, refracted ray bends towards normal and angle of refraction is smaller than angle of incidence. If second material is less optically dense the angle of refraction is longer than angle of incidence.
• What is a wave front

  A line showing all of the points on a wave that are in the same position as each other after a given number of wavelengths
• Speed and direction changes of light during refraction (FAST)

  • Fast
  • Away
  • Slower
  • Towards
• Law of reflection

  Angle of incidence = Angle of refraction
• RP20: Waves on a string

  1. Switch on the vibration generator. Adjust the frequency until there's a clear wave on the string
  2. Use a metre ruler to measure across as many half wavelengths as possible. Then divide total length by number of half waves. Multiplying this number by two will give the wavelength
  3. Use frequency from power supply
  4. Calculate wave speed using v = f x λ