6 - Waves

• Transverse- the oscillations are perpendicular to the direction of the energy transfer

  • [Oscillations are up and down but direction of energy transfer is sideways]

    eg: ripples on surface of water  

  
• Longitudinal - the oscillations are parallel to the direction of energy transfer

  • These waves require a medium to travel in, eg: air, a liquid or a solid 
  • Compression: where the air particles are close together
  • Refraction: where air particles are spaced out

   eg: sound waves travelling in their air

  
• ! All waves transfer energy
• Amplitude: the maximum displacement of a point on a wave, away from its undisturbed position

  
• • Wave length: the distance from one point on one wave to the equivalent point on the adjacent wave - measured in λ (lambda)

  
• • Wave length: from one compression to the next compression. Or one rarefaction  wave per second to the next rarefaction

  
• Frequency: the number of waves passing a point each second

        - unit = Hertz (Hz)  1hz = 1 wave per second

  
•      Period: the time (in seconds) for one wave to pass a point 

  Period (s) = 1/frequency   OR   [T = 1/F]    
• Wave speed: the speed at which the wave moves through the medium (ie the speed at which energy is transferred) V = f x  λ  → wave speed (m/s) = frequency (Hz) x wave length (m) 
• method to measure the speed of sound waves in air

  1. Both people have a distance of 500m , Person A is holding a pair of cymbals, and person B is holding a timer
  2. Person B starts timing when she sees a person A clash the cymbals together. 
  3. Person B then stops timing when she hears the sound of the cymbals clashing

  • then calculate the speed of the sound waves by dividing the distance travelled by the time taken

  
• problem with cymbal method (1)

  • every person has a different reaction time It takes a fraction of a second between seeing the symbols and starting the timer It also takes a fraction of a second between hearing the sound and stopping the timer

   [reduce this error by having a large number of observers with timers. We take all of their results and discard any that are anomalous. We then calculate a mean value]
• problem with cymbal method (2)

  • the time between seeing the symbols clash and hearing the sound is very short. That makes it very difficult to press the timer at the correct times

  [We can reduce that problem by increasing the distance between person A and person B. The longer the distance, the longer the time. That makes it easier to start and stop the timer at the correct times] 
• Electromagnetic waves: they are transverse waves that transfer energy from the source of the waves to an absorber   

  • Eg: microwaves- transfer energy from the source (the oven) to the absorber 
  • Solar panels- transfer energy from the source (the sun) to the absorber (solar panels on the spacecrafts)
• Light is an example of an electromagnetic wave- if we pass white light through a prism then it splits into a spectrum 

  • Each colour of light has a different wavelength and frequency 
  • Red: low frequency, long wavelength, Violet: high frequency, short wavelength

  
• radio waves, microwaves, infrared light, visible light, ultraviolet light, X-rays, and gamma rays

  • Electromagnetic spectrum is a continuous spectrum- the cut off point between the waves is not always clear 
  • They do not need a medium to travel through, this they can travel through a vacuum (eg: space) 
  • All electromagnetic waves travel at the same speed in a vacuum (3x10^8 m/s)
  • Different materials absorb, transmit or reflect electromagnetic waves (what happens ott he waves depends on the wavelength) 

  
• Waves can change direction when they change speed, moving from one medium to another 
• When light passes from air into glass, the velocity of light decreases (light wave slows down)- this causes the direction of the waves to change 

  • When waves slow down, they bend towards ‘the normal’ the light waves pass through the glass block
  • When the waves pass from the glass back into the air, their velocity increases (speed up) when waves speed up they bend away from the normal this causes the image of the object to appear have shifted position 

  
• Refraction can happen when any wave changes speed as it passes from one medium to another 

  • Exception: if the wave enter or leave the medium at right angles to the surface (along the normal) then they do not not change direction
• The wavefront is an imaginary line that connects all the same points in a 

  set of waves (could be through the peaks, troughs etc)

  • Makes its easier to visualine lots of waves moving together 

  
• Using wavefront to explain why waves change direction when they pass through one medium to another… 

  • When first wave fronts start to move into the glass, those parts start to slow down, this causes those wavefronts to get closer together – the wavelength get smaller and this causes the waves to change direction towards the normal (they speed up)
  • When wave speed up they change direction away from the normal 

  
• When electromagnetic waves are generated or absorbed, changes take place in atoms or in the nuclei of atoms 

  • When we heat atoms we cause electrons to move from one energy level to a higher one, when the electron returns to its original energy level it generates and electromagnetic wave   
  • A change in the atom generates an electromagnetic wave   
• Change in the nucleus can generate electromagnetic waves eg: gamma rays 

  • When electromagnetic waves are absorbed, that can also cause changes to atom, eg: electrons can change energy levels 
  • Electromagnetic waves can be emitted and absorbed over a wide frequency range from radio waves to gamma waves
• Hazards of electro magnetic waves: ultra violet waves, x-rays and gamma rays are very hazardous to the human body

  • Ultra violet waves: cause body to age prematurely, increase risk of skin cancer
  • X-rays & gamma rays: ionising radiation- they knock electrons off atoms when they’re absorbed, this means they can cause mutation in the genes and this increases risk of cancer    
• The damage done by radiation depends on the type of radiation and dose // dose is measured in sieverts (Sv) or millisieverts (mSv)  

  How radio waves are produced:  when electrons oscillate (move backwards and forwards) in electrical circuits 

  • These radio waves can be absorbed, this causes electrons in the circuit to oscillate. This can create an alternating current with the same frequency as the radio waves 
• Radiowaves: used to transmit radio and terrestrial TV signals (received using an ariel) 

  • Used because they can travel long distances before benign absorbed [eg: by buildings and trees] 
  • Longer wavelength radio waves can also spread out between hills (diffraction) 
  • Can also reflect a layer of charged particles in the atmosphere (ionosphere) this allows us to send radio waves very long distances around the earth 
• Microwaves: used to heat up food

  • Most foods contain water molecules, and these molecules absorb the energy of microwaves.  The energy causes the temperature of food to increase 
  • Used to communicate with satellites in space- due to the fact that they can pass through the earths atmosphere without being reflected or refracted
• Infrared: emitted by electrical heaters and also used to cook food in ovens

  • This is because the energy of infrared is easily absorbed by surface objects, eg: infrared from a heater is absorbed by objects in the room thus making the room warmer 
  • Infrared cameras: check buildings for heat losses
• Visible light

  • Communication using fibre optics [very thin strands of glass]  - we can transmit pulses of light down these fibres and use the pulses to carry information: used to carry telephone and cable TV signals 
  • Has a shortwaves length and can carry lot of information
• Ultra violet:

  • Energy efficient lightbulb, its created inside the bulb. It has a short wavelength and thus it carries more energy than visible light. The energy is absorbed by the internal surface of the bulb and it is converted to visible light- requires much less energy compared to a normal light bulb  
  • Sun tanning [however it increases risk of skin cancer and causes premature skin ageing] 
• X-Rays & Gamma rays: Used for medical imaging- x-rays for visualising broken bones / gamma rays used to detect cancers

  • Both are penetrative meaning they can easily pass through body tissue, however X-rays are absorbed by bones which is why we can see them on X-ray images   
  • Can be used in medical treatment (eg: to treat cancer)