Chapter 12 Flashcards
what are electromagnetic waves?
vibrations that don’t travel through a medium
what are mechanical waves?
vibrations that travel through a substance
examples of mechanical waves (2)
sound waves
seismic waves
examples of electromagnetic waves (2)
radio waves
light waves
what is the amplitude of a wave?
the distance from the distance line to the peak
the maximum displacement from its undisturbed position
what is the wavelength?
the distance between the same two point on two consecutive waves
what is frequency?
the number of waves passing through a certain point per second
what is frequency measured in?
Hertz (Hz)
what are the two types of waves?
longitudinal and transverse
equation for period of a wave
frequency
what is the period of a wave?
the time it takes to complete one full cycle of the wave
characteristics of transverse waves
oscillations are perpendicular to the direction of energy transfer
(they go up and down)
where do waves transfer energy?
in the direction they are travelling
characteristics of longitudinal waves
oscillations are parallel (along) the direction of travel
they go right and left
examples of transverse waves
electromagnetic waves
examples of longitudinal waves
sound waves
what does a large amplitude show?
the waves are carrying a lot of energy
what is the wave speed?
the distance travelled by a wave each second through a substance
how to calculate wave speed?
wave speed = frequency x wavelength
what is a rarefaction?
when the waves become more spread out
what is a compression?
when the waves get closer together
how to investigate reflection of waves?
use a ripple tank
what is refraction?
the change in direction of a wave when it crosses between medium
what is important to remember about the angle of incidence and reflection?
they are THE SAME
what are plane ?
they are straight
what is the angle of incidence?
the angle between the incoming wave and the normal
what is the angle of reflection?
the angle between the reflected wave and the normal
what is the normal?
an imaginary line that is perpendicular to the surface at the point of incidence
what does the amount by which a wave is refracted by depend on?
how much the wave speeds up or slows down
density of the two materials
what happens when a wave crosses a boundary and slows down?
it bends towards the normal
what happens when a wave crosses a boundary and speeds up?
it bends away from the normal
what happens to the frequency when a wave is refracted?
it stays the same
what is the optical density of a substance?
how well light can travel through it
what happens to the wavefronts in a denser material?
the waves slow down so they get closer together
what are the three things that can happen to a wave when it arrives at a boundary?
absorbed, transmitted, reflected
what are the two types of reflection
specular
diffuse
what is specular reflection?
when a wave is reflected in a single direction by a smooth substance
what is diffuse reflection?
when light is reflected by a rough surface and the different rays are scattered in many different directions
why does diffuse reflection occur?
the normal will be different for each ray because of the uneven surface
this means the angle of incidence and therefore reflection will be different for each ray
what are sound waves caused by?
vibrations
what are seismic waves?
the shock waves that are caused by an earthquake
where are earthquakes generated?
in the earth’s crust
what happens to waves when they arrive at a boundary? 5
it can be completely reflected partially reflected refracted absorbed slow down
what is the focus of an earthquake?
where it originates
what is the epicentre of an earthquake?
the nearest point to the focus on the surface
what are the 3 types of seismic waves?
Primary waves
Secondary waves
(Long waves)
what do P waves do? 2
cause initial tremors lasting about a minute
push and pull on material as they move
what do S waves do? 2
cause the aftershocks
move material side to side
what do L waves do and where are they found?
earth’s crust
arrive last and shake material both up and down and side to side
properties of P waves 3
longitudinal
travel very fast
travel through solids and liquids
properties of S waves 3
transverse
slower than P waves
can’t travel through gases or liquids
why do P and S waves bend as they travel through the mantel?
their speeds change with the increasing depth so their directions do too
why do P waves refract at the boundary between the mantle and outer core?
the two have very different densities so the P waves’ speed changes abruptly and so does their direction
why can’t S waves travel through the outer core?
they are transverse and can’t travel through liquid
what is the shadow zone?
where no P or S waves are detected
how was the boundary between the crust and mantle discovered?
when seismometers discovered that the speed of seismic waves changed at around 50 km below the surface
how do we know that the inner core is solid?
weak P waves are detected in the shadow zone so we know that the core is solid and refracts P waves into this zone
how does the existence of the shadow zone prove the existence of a liquid outer core? 2
S waves can’t travel through the outer core so aren’t detected
P waves are refracted at the boundary as they enter and leave the outer core. The second refraction is further around so they can’t reach the shadow zone
what happens when seismic waves reach a boundary? 2
some will be absorbed and some will be refracted, so they change speed gradually
what is the range of human hearing?
20 Hz - 20 KHz
what are ultrasound waves?
waves above the highest frequency we can hear
process of producing ultrasound
electrical devices produce electrical oscillations over a range of frequencies
these are converted into mechanical vibrations
this makes sound waves that are beyond our range of hearing
what is partial reflection?
when a wave passes from one medium to another, so,e is reflected and some is transmitted or refracted
how do ultrasounds measure internal boundaries?
when there is a boundary, some ultrasound is reflected back. The time taken is recorded and this is used to work out how far the boundary is
how are ultrasound waves used for medical imaging?
wherever there is a boundary, some of the wave is reflected back and detected
the timing and distribution of these echoes are recorded and turned into an image
how are ultrasound waves used in industrial imaging?
ultrasound waves will normally be reflected by the far side of the material
if there is a crack/flaw, they will be reflected sooner
what is echo sounding and how does it work?
it uses high frequency sound waves to measure the depth of water under a vessel or to locate an object
2 advantages of ultrasound used for medical scanning
reflected at boundaries (can scan soft tissues)
non ionising - not harmful
what is non ionising radiation?
radiation that doesn’t have the energy to ionise an atom (remove an electron)
how to calculate distance travelled by a wave (m)?
speed of ultrasound waves x time taken
how to calculate the depth of a boundary below a surface? (m)
0.5 x speed of wave x time taken
why are musical notes easy to listen to?
they are rhythmic with a regular wave pattern
what is general noise made of?
sound waves that randomly vary in frequency
what are sound waves caused by?
vibrating objects
what type of wave is sound?
longitudinal
how are vibrations passed through a medium?
through a series of rarefactions and compressions
why do sound waves refract when they enter different media?
they speed up
their wavelength changes but frequency remains the same so speed must also change
how do we hear?
sound waves reach ear drum
vibrations passed through ear drum into cochlea
cochlea turns these vibrations into electrical signals to brain
why is the frequency of human hearing limited?
the conversions of sound waves to vibrations only works over a limited range
how does echo sounding work?
high frequency sound waves are transmitted
they reflect from the hard, flat surface
these echoes are detected by a receiver
why do sounds move slower in gases (and don’t travel in a vacuum)?
there are no/few particles to vibrate
how does a vibration work?
the compressions and rarefactions make the particles vibrate and move
these vibrations make others move and so the sound travels
what to use to investigate water waves?
a ripple tank
what to use to investigate sound waves?
a loudspeaker and signal generator
what to use to investigate waves in a solid?
a stretched spring
how to explain rarefaction?
the wavefronts move slower in a denser medium (or vice versa)
so they are closer together and are at a smaller angle to the boundary than the angle of incidence
they have the same frequency but different speeds, so have different wavelengths
what happens to a substance when it absorbs waves and why?
it heats up because it takes the energy from the waves
what happens to waves travelling through a solid and why?
their amplitude decreases - the solid absorbs the energy
what happens when waves cross a boundary perpendicularly?
they don’t change direction but their speed changes
What happens when a material absorbs radio waves?
Creates an alternating current with the same frequency as the radio wave
