SP4 Waves Flashcards
What do waves do?
- transfer energy and information
- without transferring matter
Give an example of how wavesdo not transfer matter
When buoys in sea stay still despite waves passing them
How can we describe waves?
As oscillations or vibrations around a fixed point
Define Wavelength
Distance between the same points on two consecutive waves
Define amplitude
Distance from the equilibrium line to the maximum displacement
Define frequency
the number of waves that pass a single point per second
Define period
Time taken for a whole wave to completely pass a single point
Define wavefront
The plane in which the wave travels. ie: the direction of the wave
A diagram shows a duck bobbing up and down on the surface of water as the waves pass below it. Explain how the toy duck demonstrates that waves do not transfer matter
- the plastic duck moves up and down but does not travel with the wave
- waves transfer energy but not the particles of the medium
- this means that when a wave travels between two points, no matter actually travels with it, the point on the wave just vibrate back and forth about fixed positions
- Objects floating on the water simply bob up and down when waves pass under them, demonstrating how there is no movement of matter in the direction of the wave, only energy
- can also describe the type of wave it is: eg: transverse wave
Define rest position
The undisturbed position of particles or fields when they are not vibrating
Define displacement
The distance that a certain point in the medium has moved from its rest position
**
Define peak
the highest point above the rest position
Define trough
The lowest point below the rest position
What is the symbol for amplitude and what is its unit?
- symbol: A
- unit: metres (m)
What is the symbol for wavelength and what is its unit?
- symbol: λ
- measured in metres (m)
What is the symbol for frequency and what is its unit?
- symbol: f
- unit: Hertz (Hz)
What is the symbol for time period and what is its unit?
- symbol: T
- measured in seconds (s)
What is the equation that links frequency and time period together?
f = 1/T
What is the equation for wave speed?
frequency x wavelength
What is the relationship between frequency and velocity?
As frequency increases, velocity increases
What is the relationship between wavelength and velocity?
As wavelength increases, so does velocity
What is the relationship between period and frequency?
- period is inversely proportional to frequency
- so as period increases, frequency decreases, and vice versa
What is the relationship between period, frequency and velocity?
period decreases, frequency + velocity increase
Define longitudinal waves
vibrations are parallel to the direction of travel
What are the key features of a longitudinal wave and what are their definitions?
- compressions: points of high pressure because particles are close togther
- rarefactions: points of low pressure because particles are spaced far apart
What mediums can longitudinal waves travel through?
- solids
- liquids
- gases
- not a vacuum
What are some examples of longitudinal waves?
- sound waves
- seismic P waves
- pressure waves caused by repeated movements in liquids and gases
Define transverse waves
Vibrations are perpendicular to the direction of travel
What mediums can transverse waves travel through?
- solids
- surfaces of liquids
- not inside liquids or gas
What mediums can electromagnetic transverse waves travel through?
- solids
- liquids
- gases
- vacuum
What are some examples of transverse waves?
- seismic s waves
- electromagnetic waves
- ripples on the surface of water
How do we measure the speed of sound in air?
Method 1: Make a noise ~50 m away from a wall and record the time for the echo to be heard. Then substitute the values into s=d/t
Method 2: Have two microphones connected to a datalogger at a large distance apart, and record the time difference between the sound passing from one to another - then use the equation s=d/t
How do we measure the velocity of ripples of water on its surface?
- Choose a calm flat water surface such as a lake or a swimming pool
- Two people stand a few metres apart using a tape measure to measure this distance
- One person counts down from three and then disturbs the water surface (using their hand, for example) to create a ripple
- The second person then starts a stopwatch to time how long it takes for the first ripple to get to them
- The experiment is then repeated 10 times and an average value for the time is calculated
- The average time and distance can then be used to calculate the wave speed using the equation: s=d/t
How do we measure wavelength in a transverse wave?
From one peak to the next peak
OR
From one trough to the next trough
How do we measure wavelength in longitudinal waves?
Centre of one compression to the centre of the next
What is the speed of sound in air?
330m/s
What are the four things that can happen to a wave once it hits the boundary?
- refraction
- reflection
- absorption
- transmission
When does reflection occur?
When a wave hits a boundary between two media and does not pass through, but instead stays in the original medium
How does the smoothness of a surface make its ability to reflect vary?
- flat surfaces are the most reflective
- rough surfaces are the least reflective, because the light scatters in all directions
- the smoother the surface, the stronger the reflected wave is
Describe reflection on opaque surfaces
- will reflect light not absorbed by the material
- the electrons will absorb the light energy and then reemit it as a reflected wave
State the law of reflection
angle of incidence = angle of reflection
Where are the angle of incidence and reflection measured?
Measured between the light ray and the normal
Define normal
An imaginary line which meets the boundary at right angles
When does refraction occur?
A wave changes speed at the boundary between two materials of different densities
What happens to the wave speed when the medium it is travelling into is denser than the one it is currently in?
It decreases, the wave slows down
What does the change in speed at the boundary cause?
A change in direction
What happens to the wavelength of a refracted wave?
It decreases
Why does the wavelength of a refracted wave decrease?
The energy of a wave is constant. Since the energy is directly linked to the frequency of the wave, it means that the frequency also stays constant. So if the frequency is constant and the speed decreases, this means that the wavelength must also decrease
When does a wave bend towards the normal?
When the wave is going into a denser medium that the one it is currently in, eg: air —> glass
When does a wave bend away from the normal?
When the wave is travelling into a less dense medium than the one it is currently in. Eg: glass —> air
What is the amount the wavelength of a refracted ray changes dependant on?
The amount that the direction changes
What type of material does transmission usually occur through?
Transparent materials. The more transparent, the more light will pass through that material
When does transmission occur?
When a wave passes through a substance
How can a wave action be classified as transmission?
The wave must pass through the material and emerge on the other side
Why might the transmitted wave have a lower amplitude as compared to before its transmission?
When passing through a material, waves are usually partially absorbed, which results in a lower amplitude
State some common examples of transmission
- light passing through a window continues with around 95% of its original energy
- ultrasound scanning a baby pass from the flesh into the bone and continue with enough energy for the machine to detect the echo
When does absorption occur?
Energy is transferred from the wave into the particles of a substance
Describe what happens if the frequency of the light matches the energy levels of the electrons
- light will be absorbed by the electrons and not reemited
- then reemited over time as heat
- which indicates that it has been absorbed
What does it mean if an object appears red (in terms of absorption and reflection)?
- only red light has been reflected
- the rest of the colours have been absorbed
Why does the wave change direction when it enters a denser medium?
- Different parts of the wave enter at different times, leading to a change in speed.
- the difference in speed between parts of the wave in the first medium and parts of it in the second medium causes the wave to bend: change in direction
Why do materials interact differently with waves?
- depends on wavelength
- some wavelengths may be transmitted, whereas others may be reflected or absorbed
- eg: Glass transmits and/or refracts visible light
- absorbs UV
- reflects IR
Describe how the human ear detects sound
- Sound waves enter the ear canal
- sound wave hits the eardrum, which vibrates at the same frequency as the wave
- tiny bones vibrate and amplify the vibrations
- vibration of bones transmitted to the fluid in the inner ear
- tiny hairs within cochlea detect the vibrations and create electrical impulses
- impulses travel along auditory nerve to the brain, giving the sensation of sound
Why is a sound wave a pressure wave?
The compressions and rarefactions cause changes in pressure, which vary in time with the wave
What happens when sound waves hit a solid?
The fluctuating pressure causes the solid to vibrate
What is a natural frequency?
The frequency at which a solid naturally vibrates when a sound wave hits it
What causes larger vibrations within solids?
When the frequency of a sound wave is similar to the solid’s natural frequency
How is the frequency of sound related to pitch?
- high frequency = high pitched
- low frequency = low pitched
How is the amplitude related to the volume of the sound?
- high amplitude = loud sound
- low amplitude - quiet sound
What is the normal human hearing range?
20 Hz to 20,000 Hz
What does a high frequency correlate to?
High energy of the wave
Define ultrasound
Sounds with a frequency greater than 20,000Hz
What are the uses of ultrasound?
- Sonar
- Foetus Scanning
How is ultrasound used in sonar?
- Ultrasound sent out
- Ultrasound is reflected off of the ocean bottom
- the time it takes for the echo to return is used to calculate the depth (this is twice the amount, because the ultrasound has to travel there and back)
How is ultrasound used in foetal scanning?
- An ultrasound detector produces and detects a beam of ultrasound waves into the body
- The ultrasound waves are reflected back to the detector by different boundaries between tissues in the path of the beam
- For example, the boundary between fluid and soft tissue or tissue and bone
Why does ultrasound work for foetal scanning?
- the waves partially reflect at each boundary
- so can be used to work out distances
- and therefore an image of the foetus
Define infrasound
sounds with a frequency lower than 20Hz
What are the two types of seismic waves?
P waves and S waves
What are seismic waves?
Waves produced by earthquakes
What type of waves are P waves?
Longitudinal
What types of waves are S waves?
Transverse wave
Which type of seismic wave can be detected on the other side of an Earthquake and why?
P waves, because they are longitudinal waves and therefore can travel through solids and liquids, whereas S waves are transverse, and they only travel through solids
What are the two main discoveries made due to seismic waves?
- On the opposite side of the Earth to an earthquake, only P-waves are detected, not S-waves, this suggests:
- The mantle is solid – this is because both types of wave can pass through it
- The outer core of the Earth is liquid – hence no S-waves can penetrate it - Refractions between layers cause two shadow zones, where no P-waves are detected, this suggests:
The inner core is solid – this is due to the size and positions of these shadow zones which indicate large refraction taking place
Define shadow zone
Area on the Earth’s surface where P waves nor S waves can be detected
