Topic 6 - Waves

• Waves transfer energy without transferring matter
• Longitudinal waves - the direction of oscillation is parallel to the direction of energy transfer. Consists on rarefactions and compressions
• Transverse waves - the direction of oscillation is perpendicular with the direction of energy transfer
• Examples of longitudinal waves:

  • Sound
  • Seismic P waves
• Examples of transverse waves:

  • water waves
  • seismic S waves
  • light
  • EM waves
• Wavelength - λ (m) - the length of one complete wave (peak to peak)
• Time Period - T (s) - the time taken for one wave to pass
• Frequency - f (Hz) - the number of waves that pass a point every second
• For sound: higher frequency = higher pitch so greater amplitude = louder
• Soundwaves cause your eardrum to vibrate, which sends a signal to your brain. Human hearing range: 20Hz to 20kHz.

  If frequency>20Khz: Ultrasound
• When sound meets a boundary between two mediums (materials), some sound is transmitted, while some is reflected. The resulting echoes can be timed to build up an image of what's out of view e.g a baby in a womb or schools of fish underneath a boat (sonar)
• The fact that seismic P-waves (longitudinal) pass through theEarth's centre while S-waves (transverse) do not suggests it has a molten core. This is because P-waves can travel through liquids while S-waves can't
• If light reflects off a smooth surface (e.g a mirror), this is a specular reflection. The angle of reflection is equal to the angle of incidence, relative to the normal. A rough surface will scatter light - this is diffuse reflection
• The angle of incidence is equal to the angle of reflection
• All angles are measured from the normal
• Electromagnetic waves don't need a medium to travel through
• Electromagnetic Spectrum:

  • Radio Waves - Phones, wifi, Tv
  • Microwaves - Cooking (absorbed by water)
  • Infrared - Cooking (absorbed by surface), Remote controls
  • Visible Light - Vision
  • Ultraviolet - Tanning, can cause skin cancer
  • X-rays - Medical scans, also dangerous
  • Gamma Rays - sterilising, medical treatments, also dangerous
• All EM waves are emitted and absorbed by electrons (apart from gamma - they are emitted by nuclei)
• If the energy of the wave is high enough, it can cause an electron to leave its atom, leaving an ion (ionising).
• UV, X-rays and gamma are ionising radiation
• Refraction: when waves enter a new medium, their speed changes, as does their angle (direction)
• If speed decreases, wavelength also does while frequency stays constant
• If the wave slows down, it bends towards the normal. This means the angle of refraction is smaller then the angle of incidence
• To measure the angles, you place your protractor on the normal line
• When the light exits, it speeds sup again and it bends away from the normal
• Lenses use refraction to make rays of light converge (meet) or diverge (spread out)
• Convex Lens: make rays converge
• Concave Lens: makes rays diverge
• To see where an image is formed, draw two rays from the top of the object

  1. Straight through the centre
  2. Parallel in, then through the principle focus (backwards if it is a concave lens)

  
• Convex Lens symbol:

  
• Concave lens symbol:

  
• Concave/Convex lens symbol and shape

  
• Convex lenses can make real AND virtual images
• Virtual image diagram

  
• Real images can be projected, virtual images cannot (as the rays meet on the wrong side of the lens:
• Concave lenses can only produce virtual images
• Concave objects diagram

  
• We perceive different colours when different wavelengths of light are absorbed by the retina
• An object will appear a certain colour as it reflects those wavelengths and absorbed others
• A blackbody is a theoretical object that perfectly absorbs and emits all wavelengths of radiation (it does not reflect radiation). We can apply this model to stars and planets as an approximation