Waves

• wave

  an oscillation that transfers energy without transmitting any matter but making the particles of the medium vibrate
• amplitude

  the maximum displacement of the wave from its rest position
• frequency


  number of complete waves passing a certain point per second

  measured in Hz (hertz)
• period 


  the amount of time taken for a full cycle of the wave to be completed

  measured in seconds
• transverse waves

  oscillations are perpendicular to the energy transfer

  examples: electromagnetic spectrum, ripples in water, waves on a spring
• longitudinal waves


  oscillations are parallel to the energy transfer

  consists of rarefactions and compressions

  examples: sounds, shock waves
• the period is inversely proportional to the frequency
• angle of incidence = angle of reflection
• specular reflection


  reflection of a smooth surface such as a mirror

  the wave is reflected in a single direction
• diffuse reflection 


  reflected off a rough surface such as paper

  the normal line is different for each ray so the angle of incidence is different therefore the angle of reflection is too

  no clear reflection
• refraction


  happens when a wave crosses a boundary between materials at a direction

  the change in direction is refraction
• the 'level' of refraction depends on

  the density of the two materials:

  • a higher density means the wave will move through it slower
  • if it slows down, the wave will bend towards the normal line

  in the image: i = the angle of incidence; r = the angle of refraction

  
• in refraction, the wavelength changes but the frequency stays the same
• electromagnetic waves

  transverse waves

  caused by vibrations of electric and magnetic fields

  all travel the at the same speed

  travel at the speed of light ( 3x10^8 m/s) in a vacuum
• radio waves


  made by oscillating electric and magnetic fields

  uses: television, radio, bluetooth
• microwaves 


  communication to and from satellites as microwaves can easily pass through the earth's atmosphere

  microwave ovens heat the water, which absorbs the energy and then transmits it to the food
• infra red (electromagnetic spectrum)

  emitted by all hot objects

  a toaster releases infra red which increases the temperature of the bread

  used in electric heaters and infrared cameras as well
• ultra violet

  used in:

  • fluorescent lights which are energy efficient
  • tanning beds
  • the sun releases UV light

  dangers: damages the surface level of cells which can cause early skin aging, blindness and cancer
• visible light

  used in fiber optic communications - the waves are reflected off a glass filbre that they run through
• gamma and x-rays


  danger: both ionising radiation so can cause cell mutation, damage and cancer

  medical uses - both in radiotherapy

  x-rays:

  • xrays which easily pass through soft tissue but not bone or metal so they create a 'negative' image on the other side

  gamma rays:

  • medical tracer
• order of electromagnetic waves (increasing in frequency)

  Radio waves

  Microwaves

  Infra red

  Visible light

  Ultra violet

  X-rays

  Gamma
• Infra red - more detail

  all objects emit and absorb infra red

  an object that is hotter than its surroundings emits more infra red then it absorbs, making it cool down

  black, matte surfaces absorb and emit more infra red then white shiny surfaces
• black body radiation

  perfect black bodies absorb all radiation and are the best emitters of it

  as the temperature of an object increase, the intensity of each wavelength and the peak wavelength increases

  
• different lenses produce different kinds of images
• real image

  light from an object comes together to form an image on a screen
• virtual image

  rays are diverging so the light appears to be coming from a different place

  you cannot project a virtual image onto a screen
• convex lens - image diagram steps

  starting point = the top of the object

  1. draw a ray from the starting point through the center of the lens and out the other side
  2. draw a ray from the starting point to the lens (line is parallel to the axis of the lens), then to the primary focus
  3. (optional) draw a ray from the starting point through the focal point until you hit the lens, then continue the ray parallel to the axis
• convex lenes

  a more curved surface makes the rays converge closer to the lens

  used in magnifying glasses

  • the object needs to be nearer the lens then the focal point is
  • it will create a virtual image
• concave lenses

  always create virtual images

  the further away the object is, the smaller the image is

  the image is always:

  • smaller
  • the right way up
  • on the same side of the lens
• drawing concave lens diagrams

  draw a ray through the center, from the top of the object

  draw a ray from the top of the object, parallel to the axis

  draw a dotted ray (virtual ray) from the focal point to the point where ray 2 intersects the lens

  
• primary colours


  red, green, blue
• when all the colours are mixed together, it forms white

  red refracts the least, violet refracts the most

  
• white reflects all wavelengths of visible light

  black absorbs all wavelengths of visible light - we see the lack of colour
• the colour of an opaque object depends on which wavelength of light are most strongly reflected

  eg - a red apple lightly absorbs all the other colours and most strongly reflects red

  colour is based on what wavelengths are reflected
• seismic waves: P waves


  longitudinal

  travel through solids and liquids

  are faster than S waves
• seismic waves: S waves


  transverse waves

  cannot pass through liquids
• seismic waves

  both travel faster in denser materials

  in an earthquakes:

  • seismic waves detected to learn about the inner earth
  • detected using seismometers

  seismic waves gradually change speed as they travel though the earth because the density of the material gradually changes
• ears

  ear drums: caused to vibrate because of sound

  ossicles: little bones in the ear which then vibrate

  vibrations are passed on to semicircular canals and to the cochlea

  cochlea: turns vibrations into electrical signals which allow your brain to 'hear' sound

  limited to hearing range 20Hz - 20kHz

  
• ultrasound

  create frequencies above 20kHz

  are partially reflected at boundaries

  - the time taken for it to get back to the receiver measures how far away the boundary is

  they are used medical imaging (to see babies) and industrial imaging to find flaws in materials