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Waves
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- 1What are waves?When waves travel through a medium, the particles of the medium oscillate and transfer energy between each other. But overall, the particles stay in the same place - only energy is transferred
- 2AmplitudeMaximum dispacement of a point on the wave from its undisturbed position
- 3WavelengthDistance between the same point on 2 adjacent waves, e.g. between the trough of one wave and the trough of the wave next to it
- 4FrequencyNumber of complete waves passing a certain point per second. Frequency is measured in hertz (Hz). 1 Hz is 1 wave per second
- 5PeriodT = 1/f
Period = 1/frequency
s = 1/Hz - 6TransverseOscillations are perpendicular to the direction of energy transfer
Most waves are transverse including:
All electromagnetic waves
Ripples and waves in water
A wave on a string - 7LongitudinalOscillations are parallel to the direction of energy transfer
Compressions
Rarefactions
Examples
Sound waves in air, e.g. ultrasound
Shock waves, e.g. some seismic waves - 8Wave SpeedWave Speed = Frequency x Wavelength
m/s = Hz x m - 9Measure Speed of SoundBy attaching a signal generator to a speaker you can generate sounds with specific frequency. You can use 2 microphones and an oscilloscope to find the wavelength of the sound waves generated
Set up the oscilloscope so the detected waves at each microphone are shown as separate waves
Start with both microphones next to the speaker, then slwoly move 1 away until the two waves are aligned on the display, but have moved exactly one wavelength apart
Measure the distance between the microscopes to find one wavelength
Wavespeed = frequency x wavelength. Frequency is whatever you set the signal generator to
Speed of sound in air is around 330 m/s, so check your results roughly agree with this - 10Measure Speed of Water RipplesUsing a signal generator attached to the dipper of a ripple tank you can create water waves at a set frequency
Use a lamp to see wave crests on a screen below the tank. Make sure the size of the waves' shadows are the same size as the waves
Distance between each shadow line is equal to one wavelength. Measure the distance between shadow lines that are 10 wavelengths apart, then divide this distance by 10 to find the average wavelength. Good method for measuring small wavelengths
Could take a photo of the shadows and ruler and find the wavelength from the photo instead
Use wavespeed = frequency x wavelength - 11Waves on Strings: ImageSet up the equipment then turn on the signal generator and vibration transducer. String will start to vibrate
Adjust the frequency of the signal generator until there's a clear wave on the string. The frequency you need will depend on the length of string between the pulley and the transducer and the masses you've used
You need to measure the wavelength of these waves. Measure lengths of 4 half wavelengths then difvide to get the mean half wavelength. Double to get full wavelenth
Freuqncy of wave is whatever the signal generator is set to
Wavespeed = frequency x wavelentgh - 12Waves on Strings: MethodSet up the equipment then turn on the signal generator and vibration transducer. String will start to vibrate
Adjust the frequency of the signal generator until there's a clear wave on the string. The frequency you need will depend on the length of string between the pulley and the transducer and the masses you've used
You need to measure the wavelength of these waves. Measure lengths of 4 half wavelengths then divide to get the mean half wavelength. Double to get full wavelength
Frequency of wave is whatever the signal generator is set to
Wavespeed = frequency x wavelength - 13Investigating RefractionPlace a transparent rectangular block on a piece of paper and trace around it. Use a ray box or a laser to shine a ray at the middle of one side of the block
Trace the incident ray and mark where the light ray emerges on the other side of the block. Remove the block and, with a straight line, join up the incident ray and the emerging point to show the path of the refracted ray through the block
Draw the normal at the point where the light ray entered the bock. Use a protractor to measure the angle between the incident ray and the normal (the angle of incidence, I) and the angle between the refracted ray and the normal (the angle of refraction, R)
Repeat this experiment using rectangular blocks made from different materials, keeping the incident angle the same throughout
You should find that the angle of refraction changes for different materials - this difference is due to their different optical densities - 14Investigating ReflectionTake a piece of paper and draw a straight line across it. Place an object so one of its sides line up with this line
Shine a ray of light at the object's surface and trace the incoming and reflecting light beams
Draw the normal at the point where the ray hits the object. Use a protractor to measure the angle of incidence and the angle of reflection and record these values in a table. Also made note of the width and brightness of the reflected light ray
Repeat this experiment for a range of objects - 15Investigating Reflection: ExplanationSmooth surfaces like mirrors give clear reflections - the reflected ray is as thin and bright as the incident ray. Rough surfaces like paper cause diffuse reflection which causes the reflected beam to be wider and dimmer, or not observable at all
Angle of incidence always equals the angle of reflection - 16Sound WavesSound waves are cause by vibrating objects. These vibrations are passed throught he surroudning medium as a series of compressions and rarefactions - longitudinal
Sound generally travels faster in solids than in liquids, and faster in liquids than in gases due to more particles
When a sound wave travels through a solid it does so by causing the particles in the solid to vibrate
Sound can't travel in space because it's mostly a vacuum - there are no particles to move or vibrate - 17Ear DrumSound waves that reach your ear drum can cause it to vibrate
These vibrations are passed on to tiny bones in your ear called ossicles, through the semi-circular canals and to the cochlea
The cochlea turns these vibrations into electrical signals which get sent to your brain and allow you to sense (i.e. hear) the sound
Different materials can convert different frequencies of sound waves into vibrations. For example, humans can hear sound in the range of 20 Hz - 20 kHz. Microphones can pick up sound waves outside of this range, but if you tried to listen to this sound, you probably wouldn't hear anything
Human hearing is limited by the size and shape of our ear drum, as well as the structure of all parts within the ear that vibrate to transfer the energy from the sound wave - 18Sound Waves Can Reflect and RefratSound waves wil be reflected by hard, flat surfaces. Echoes are just reflected sound waves
Sound waves will also refract as they enter different media. As they enter denser material, they speed up. This is because when a wave travels into a different medium, it's wavelength changes but its frequency remains the same so its speed must also change - 19UltrasoundElectrical devices can be made which produce electrical oscillations over a range of frequencies. These can easily be converted into mechanical vibrations to produce sound waves beyond the range of human hearing - ultrasound
- 20Ultrasound Waves Get Partially Reflected at BoundariesWhen a wave passes from one medium into another, some of the wave is reflected off the boundary between 2 media and some is transmitted (and refracted). This is partial reflection
You can point a pulse of ultrasound at an object, and wherever there are boundaries between one substance and another, some of the ultrasound gets reflected back
The time it takes for the reflections to reach a detector can be used to measure how far away the boundary is - 21Uses of Ultrasound: Medical ImagingMedical imaging, e.g. pre-natal scanning of a foetus
Ultrasound waves can pass through the body, but whenever they reach a boundary between 2 different media (like fluid in the womb and the skin of the foetus) some of the is reflected back and detected
Exact timing and distribution of these echoes are processed by a computer to produce a video image of the foetus
No one knows for sure if ultrasound is safe in all cases, but x-rays would definitely be dangerous - 22Uses of Ultrasound: Industrial ImagingUltrasound can also be used to find flaws in objects such as pipes or materials such as wood or metal
Ultrasound waves entering a material will usually be reflected by the far side of the material
If there is a flaw such as a crack inside the object, the wave will be reflected sooner - 23Echo soundingUses high frequency sound waves (including ultrasound). It's used by boats and submarines to find out the depth of the water they are in or to locate objects in deep water
- 24Earthquakes and Explosions Cause Seismic WavesEarthquake produces seismic waves which travel out through the Earth. We detect these waves all over the surface of the planet using seismometers. Seismologists work out the time it takes for the chock waves to reach each seismometer. They also note which parts of the Earth don't receive the shock waves at all
When seismic waves reach a boundary between different layers of material (which all have different properties, like density) inside the Earth, some waves will be absorbed and some will be refracted
Most of the time, if the waves are refracted they change speed gradually, resulting in a curbed path. But when the properties change suddenly, the wave speed changes abruptly and the path has a kink - 25P waves and S wavesP waves are longitudinal
Travel through solids and liquids
Travel faster than S waves
S waves are transverse and can't travel through liquids or gases
Slower than P waves
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- Fichas sobre Waves, creado por Jasmine Sillence el 19/02/2022.
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