Waves

Waves

1. The nature of waves

   • Wave- A disturbance which carries through a medium
     • Carries energy
       • Waves allow us to transfer ENERGY and INFORMATION without transferring matter
   • Particles have a cyclic motion
     • Particles of water move up and down
   • Transverse wave
     • Direction of energy transfer perpendicular (at a right angle) to direction of particles' vibrations.
     • Longitudinal wave
       • Direction of energy transfer is along/parallel to direction of particles' vibrations i.e. goes in the same direction
       • Compression-region in medium for longitudinal wave where particles close together(compressed)
       • Rare faction-region in medium for longitudinal wave where particles far away from each other
       • Examples
         • Sound waves
         • Primary earthquake waves
     • Examples
       • All electromagnetic waves (light)-microwave,infra-red waves
       • Ripples-in a pond
   • The properties of waves
     • Amplitude-Maximum displacement of a particle on medium from its undisturbed position.It's distance from crest/trough to rest.
       • Bigger amplitude of waves,the more energy the wave carries
         • Saying something this way is inverse proportionality
     • Period
       • Period of 1 wave= 1/Frequency
       • Measured in seconds
       • time taken for each wave to pass a fixed point
     • Wavelength
       • the length of 1 complete wave cycle i.e. distance from 1 crest to the next crest or 1 trough to the next trough
       • Wavelength= Wave Speed/Frequency
         • measured in m
       • space between 2 wavefronts
     • Frequency
       • No. of waves passing fixed point every second
         • 1 wave crest passing each second=1Hz
       • measured in Hertz (Hz)
       • Bigger the frequency the lower the period
         • Another e.g. of inverse proportionality
       • F=Wave speed/wavelength
     • estimation of typical walking speed=1m/s-2m/s(13 yr old)
     • measured in m not cm
     • Straight waves are called plane waves
   • Reflection&Refraction
     • All electromagnetic waves travel at same speed
       • also travel at same speed in vacuum (empty space)
     • Mega means 1 million
       • Mega Hertz-MHz
         • MHz to Hz x1,000,000
           • E.g. F=2500Mhz=2,500,000,000
             • standard form=2.5x10 9
         • Hz to MHz divide by 1,000,000
           • E.g. F=2,500,000,000Hz=2500MHz
         • Can also write in standard form
     • Wavefront
       • Imaginary surface representing corresponding points of a wave that vibrate in unison
       • Wavelength=distance between wavefronts
         • Distance between wavefronts crest to crest i.e. the distance between one crest to another
     • Reflection-Plane(straight)waves in a ripple tank are reflected from a straight barrier at the same angles to the barrier as the incident waves as the speed and wavelength of the reflected wave doesn't change
       • Refraction-Refraction occurs at a boundary between two different materials because the speed and wavelength of the waves change at the boundary, so there's a non-zero angle when crossing the boundary meaning that the angle of the refracted wave is above 0 degrees from the boundary.Plane waves crossing a boundary between two different materials are refracted unless they cross the boundary at normal incidence i.e. the wavelength and speed doesn't change when crossing the boundary.The only time waves are refracted is when the speed of the wave changes when crossing the boundary between 2 mediums however the frequency is the same as the incident wave.
     • At a boundary between 2 different materials waves can be transmitted or absorbed

2. Reflection of Waves

   • Find where incident wave his surface
   • Draw normal ray from point where incident ray hits surface,90 degrees from the surface upwards
   • Use protractor to measure distance between normal ray and incident line,then normal ray and reflective wave
   • Draw reflective wave 90 degrees from the incident wave
   • wavelength of wave doesn't change when reflects
   • distance between wavefronts is an indication of wavelength
   • Tangent
     • where incidence line first hits surface
     • 90 degrees from incident ray
     • Incident ray is always given
     • always drawn outside of curve.Only 1 tangent on 1 point of a curve
   • angle of incidence=angle of reflection

3. How to measure the wavelength with a ripple tank

   • Create plain waves(plain waves are the straight ripples on the surface of the water)
     • Put ruler next to the pattern
       • Take pic of pattern with ruler visible
         • Count as many full wavelengths as possible
           • Measure total length and divide by the number of wavelengths.This gives you the length of 1 wavelength

4. More about waves

   • All sounds originate in vibrations of any material object. When an object vibrates,it produces sound.Sounds can travel in solids,liquids and gases by setting up vibrations in them.
     • Vibrating object pushes and pulls repeatedly on the air sending vibrations of air in waves as compressions and rarefactions which causes the layers of air near it to vibrate and the process repeats to layers of air near that layer of air etc..When the waves reach your ears they make your eardrums vibrate in and out so that you can hear the sound.
   • Sounds need a medium to propagate
     • Medium-Something to carry it
       • Propagate-To go/travel through
   • Sounds can't travel in a vacuum as sounds need a medium,and since there's nothing to compress or expand in a vacuum,there can't be any sound
     • Vacuum-Nothingness
   • Small objects vibrate with a large frequency,bigger objects with a small frequency
   • Speed of sound is approx. 340m/s at 15 degrees Celsius
   • Echo-reflection of sound
   • Measuring the speed of sound
     • 1)Stand 40m in front of a tall building+bang 2 blocks together
       • 2)Each time you hear an echo,bang blocks again
         • 3)Another person should be using a stopwatch to time how long it takes to hear 10 echoes
           • 4)Repeat this process at least 3 times then record in a table
     • When outside,echoes from a tall building are used to measure speed of sound in air
     • You don't hear the sound straight away as the speed of sound is slower than the speed of light
   • Sound waves-vibrations that travel through a medium.

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