P6: Waves

• Transverse waves


  Waves where the oscillations (vibrations) are perpendicular (at 90) to the direction of energy transfer
• Longitudinal waves


  Waves where the oscillations are parallel to the direction of energy transfer
• Waves transfer energy in the direction they are travelling
• All waves are either transverse or longitudinal
• Transverse waves

  • Most waves are transverse, including all electromagnetic waves, ripples and waves in water, waves on a string
• Longitudinal waves

  • Examples are sound waves in air, ultrasound, shock waves
• Wave speed

  The speed at which energy is being transferred (or the speed the wave is moving at)
• Measuring speed of sound using oscilloscope

  Attach speaker to signal generator, move headphones away until waves are in phase, use formula to calculate speed
• Measuring speed of water ripples using lamp

  Use signal generator to create water waves, shine lamp to create shadows, measure distance between shadow lines to find wavelength, then calculate speed
• Measuring speed of waves on a string

  Set up string with vibration transducer, adjust frequency until standing wave, measure wavelength, then calculate speed
• Reflection

  When a wave hits a boundary, it can be absorbed, transmitted or reflected
• Angle of incidence


  The angle between the incoming wave and the normal
• Angle of reflection


  The angle between the reflected wave and the normal
• Specular reflection


  Reflection where the wave is reflected in a single direction by a smooth surface
• Diffuse reflection

  Reflection where the wave is reflected by a rough surface and the reflected rays are scattered in many directions
• Electromagnetic waves form a continuous spectrum with different frequencies and wavelengths
• Refraction

  When a wave crosses a boundary between materials at an angle, it changes direction
• Optical density

  A measure of how quickly light can travel through a material - the higher the optical density, the slower light waves travel
• Investigating refraction using transparent blocks

  Trace path of light ray through block, measure angles of incidence and refraction, compare for different materials
• Investigating reflection using objects

  Draw line, place object, shine light at object and trace incoming and reflected rays
• Experiment to measure light refraction

  1. Place a block in the middle of one side

  2. Shine a light through the block, show the path of the refracted light and mark where it emerges

  3. Draw the normal at the point where the light ray entered the block, measure the angle of incidence and angle of refraction

  4. Repeat with blocks made of different materials
• Different materials reflect light by different amounts
• Experiment to measure light reflection

  1. Draw a straight line on paper, place an object so one side lines up

  2. Shine a light at the object's surface and trace the incoming and reflected light beams

  3. Draw the normal, measure the angle of incidence and angle of reflection

  4. Repeat for a range of objects
• • Smooth surfaces like mirrors give clear reflections, rough surfaces cause diffuse reflection
  • The angle of incidence always equals the angle of reflection
• Radio waves

  Electromagnetic radiation with wavelengths longer than about 10 cm
• How radio waves are produced

  1. Made by oscillating charges, producing electric and magnetic fields

  2. Can be produced by an alternating current in a circuit

  3. The frequency of the radio waves equals the frequency of the oscillating charges
• Transmitter


  The object that generates the oscillating charges to produce radio waves
• How radio waves are received

  1. Radio waves are absorbed by the electrons in the receiver material

  2. This causes the electrons to oscillate, generating an alternating current

  3. This current has the same frequency as the original radio waves
• Radio waves

  • Long wavelength radio waves can diffract around the curved surface of the Earth and obstacles
  • Short wavelength radio waves can be reflected from the ionosphere
• Uses of radio waves

  • Long-wave radio for long distance communication
  • Short-wave radio for satellite communication
  • Bluetooth for short-range wireless data transfer
  • FM and TV for line-of-sight transmission
• Microwaves

  Electromagnetic radiation used for satellite communication and microwave ovens
• How microwaves are used for satellite communication

  1. Microwaves are transmitted from Earth to a satellite in orbit

  2. The satellite receives and retransmits the signal back to Earth
• How microwaves are used in microwave ovens

  1. Microwaves are absorbed by water molecules in food

  2. The water molecules heat up, transferring energy to the rest of the food
• Infrared radiation

  Electromagnetic radiation given off by hot objects, can be used to detect and increase temperature
• How infrared radiation is used

  1. Infrared cameras detect infrared radiation and display it as a temperature image

  2. Absorbing infrared radiation causes objects to heat up, e.g. in electric heaters and toasters
• Ultraviolet radiation

  Electromagnetic radiation that causes fluorescence and tanning
• Uses of ultraviolet radiation

  • Fluorescent lights and security marking
  • Tanning lamps and sunlight
• 1. rays and gamma rays

  High energy electromagnetic radiation used in medicine
• Medical uses of X-rays and gamma rays

  • X-ray imaging to see bones
  • Radiotherapy to treat cancer
  • Gamma ray tracing to follow substances in the body
• 1. rays and gamma rays can be harmful, so precautions are taken by medical staff