waves

• Light travels at different speeds through different materials, depending on their refractive index.
• The speed of light is the same in all directions.
• When light passes from one material to another with a higher refractive index, it bends towards the normal (refraction).
• Waves are one of the ways in which energy may be transferred between stores
• Waves can be described as oscillations, or vibrations about a rest position. For example:

  • sound waves cause air particles to vibrate back and forth
  • ripples cause water particles to vibrate up and down
• The direction of these oscillations is the difference between longitudinal or transverse waves
• In longitudinal waves, the vibrations are parallel to the direction of wave travel. In transverse waves, the vibrations are at right angles to the direction of wave travel.
• All waves transfer energy but they do not transfer matter.
• Parts of a wave

  • rest position
  • displacement
  • peak
  • trough
  • amplitude
  • wavelength
  • time period
  • frequency
• rest position


  the undisturbed position of particles or fields when they are not vibrating
• Wave period and wave speed

  The time period of a wave can be calculated using the equation:

  time period =1/frequency

  This is when:

  • the period (T) is measured in seconds (s)
  • frequency (f) is measured in hertz (Hz)
• displacement


  the distance that a certain point in the medium has moved from its rest position
• peak


  the highest point above the rest position
• Calculating wave speed

  The speed of a wave can be calculated using the equation:

  wave speed = frequency × wavelength

  This is when:

  • wave speed (v) is measured in metres per second (m/s)
  • frequency (f) is measured in Hertz (Hz)
  • wavelength (λ) is measured in metres (m)
• trough


  the lowest point below the rest position
• The air is made up of many tiny particles. When sound is created, the air particles vibrate and collide with each other, causing the vibrations to pass between air particles. The vibrating particles pass the sound through to a person's ear and vibrate the ear drum.
• amplitude


  the maximum displacement of a point of a wave from its rest position
• Light travels much faster than sound through air
• wavelength


  distance covered by a full cycle of the wave, usually measured from peak to peak, or trough to trough
• time period


  the time taken for a full cycle of the wave, usually measured from peak to peak, or trough to trough
• frequency


  the number of waves passing a point each second
• Longitudinal waves

  In longitudinal waves, the vibrations are parallel to the direction of wave travel.

  Examples of longitudinal waves include:

  • sound waves
  • ultrasound waves
• Longitudinal waves show areas of compression and rarefaction:

  • compressions are regions of high pressure due to particles being close together
  • rarefactions are regions of low pressure due to particles being spread further apart
• Transverse waves

  In transverse waves, the vibrations are at right angles to the direction of wave travel.

  Examples of transverse waves include:

  • ripples on the surface of water
  • electromagnetic waves - eg light waves, microwaves, radio waves
• Electromagnetic waves are transverse waves
• • radio waves
  • microwaves
  • infrared
  • visible light
  • ultraviolet
  • x-rays
  • gamma rays
• radio waves- longest wavelength, lowest frequency, lowest energy

  gamma rays- shortest wavelengt, highest frequency, highest energy
• • red light has the lowest frequencies of visible light
  • violet light has the highest frequencies of visible light
• Radio waves are used for communication such as television and radio.
• Microwaves are used for cooking food and for satellite communications.
• High frequency microwaves have frequencies which are easily absorbed by molecules in food. The internal energy of the molecules increases when they absorb microwaves, which causes heating
• Radio waves can be produced by oscillations in electrical circuits. When radio waves are absorbed by a conductor, they create an alternating current. This electrical current has the same frequency as the radio waves. Information is coded into the wave before transmission, which can then be decoded when the wave is received. Television and radio systems use this principle to broadcast information.
• Infrared light is used by electrical heaters, cookers for cooking food, and by infrared cameras which detect people in the dark.
• Infrared light has frequencies which are absorbed by some chemical bonds. The internal energy of the bonds increases when they absorb infrared light, which causes heating
• Visible light is the light we can see
• We cannot see ultraviolet light but it can have hazardous effects on the human body. Ultraviolet light in sunlight can cause the skin to tan or burn. 
• Fluorescent substances are used in energy-efficient lamps - they absorb ultraviolet light produced inside the lamp, and re-emit the energy as visible light.
• Changes in atoms and their nuclei can cause electromagnetic waves to be generated or absorbed
• Gamma rays- they are a form of nuclear radiation.
• This makes them ideal for internal imaging. X-rays are absorbed by dense structures like bones, which is why X-ray photos are used to help identify broken bones.