6- WAVES Flashcards
what happens when waves travel through a medium
particles of medium oscillate and transfer energy between each other but overall particles stay in one place and energy is transferred
what is the amplitude of a wave
the maximum displacement of a point on the wave from its undisturbed position
what is wavelength
the distance between the same point on 2 adjacent waves e.g. between trough of 1 wave and the trough of the wave next to it
what is frequency
the number of complete waves passing a certain point per second
it is measured in hertz
1Hz= 1 wave per second
how can you find the period of a wave
what is a period
period (s) = 1/ f(frequency Hz)
– the time taken for a whole wave to completely pass a single point
explain tranverse waves
oscillations are perpendicular to the direction of the energy transfer
e.g. electromagnetic waves(light)
ripples and waves in water, a wave on a string
-Vibrations are at right angles to the direction of travel
-Have peaks and troughs
explain longitudinal waves
oscillations are parallel to the direction of energy transfer
e.g. sound waves in air (ultrasound)
shock waves (some seismic waves)
it makes compressions and rarefractions
Vibrations are in the same direction as the direction of travel
what is wavespeed
speed at which energy is being transferred or the speed at which waves are moving at
how to calculate wavespeed
wavespeed= frequency x wavelength
explain measuring velocity =
sound in air
attach signal generator to speal to generate sound with specific frequency
set up oscilliscope so detected waves at each microphone are shown as separate waves
start with both mics next to speaker, slowly move 1 away until two waves are aligned on displayed but have exactly moved only 1 wavelenghth apart
measure distance between mics to find 1 wavelenghth
use frequencyxwavelength to find speed of sound waves passing through air
frequency is what you set signal generator to (1khz is sensible )
(speed of sound is around 330m/s)
measuring velocity=
explain ripples on water surface
signal generator attached to ripple tank, create water waves at set frequency
use lamp to see wave crests on screen below tank
make sure size of waves shadows= size of waves
distance between each shadow line= 1 wavelength
measure distance between shadow lines that are 10 wavelengths then divide this distanc by 10 to find average wavelenght
or take photo of shadows and ruler and find wavelength from photo
aloows u to measure wavelength without disturbing waves
measuring waves on a string
on bench, theres the signal generator connected to vibration transducer which connected to string connected to pulley holding the masses
turn on signal generator and vibration transducer, string will vibrate
adjust frequency of signal generator until theres a clear wave on the string, frequency needed depends on length of string between pulley and transducer and masses
measure wavelength of waves
(measure 4-5 half wavelengths in one go then divide to get the mean half wavelength - then double it to get a full wavelength)
frequency of wave is what signal generator is set to
find speed with frequency x wavelength
what happens when waves arrive at a boundary between 2 different mateirals
1-waves are absorbed by the material the wave is trying to cross into, this transfers energy to materials energy store
2- theyre transmitted, they keep travelling hrough new material often leading to refraction
3- they reflect
explain how the reflection of waves depends on the surface
Waves will reflect off a flat surface
- The smoother the surface, the stronger the reflected wave is
- Rough surfaces scatter the light in all directions, so they appear
matt and not reflective.
waves are reflected at different boundaries in different ways
angle of incidence=?
angle of incidence= angle of reflection
what is the angle of incidence
the angle bewteen the incoming wave and the normal
what is the angle of reflection
the angle between the reflected wave and the normal
in angles of reflection and indicidence
what is the normal ?
it is an imaginary line that is perpendicular to the surface at the point of incidence (where the waves hit the boundary)
it is usually shown as a dotted line
what are the conditions for light to reflect off a surface
Light will reflect if the object is opaque and is not absorbed by the material
The electrons will absorb the light energy, then reemit it as a reflected wave
what is specular reflection
when a wave is reflected in a single direction by a smooth surface
e.g. when light is reflected by a mirror you get a nice clear reflection
explain diffuse reflection
when a wave is reflected by a rough surface (e.g. a piece of paper ) and the reflected rays are scattered in lots of different directions
this is bc the normal for each incoming ray is different so angle of incidence is different for each ray
angle of incidence still = angle of reflection
what happens when light is reflected by a rough surface (diffuse reflection)
the surface appears matte (not shiny) and you do not get a clear reflection of objects
explain transmission
Waves will pass through a transparent material
- The more transparent, the more light will pass through the material
- It can still refract, but the process of passing through the material and still emerging is
transmission
explain absorption
If the frequency of light matches the energy levels of the electrons
- The light will be absorbed by the electrons and not reemitted
- They will be absorbed, and then reemitted over time as heat
- So that particular frequency has been absorbed
- If a material appears green, only green light has been reflected, and the rest of the
frequencies in visible light have been absorbed
explain how sound waves work with solids
they can travel through the solids causing vibrations in the solid
explain how our ears interpret sound
-outer ear collects sound & channels it down the ear canal
- it travels down, it’s still a pressure air wave
- sound waves hit eardrum which is a tightly stretched membrane which vibrates as the incoming pressure waves reach it
-eardrum vibrates at same frequency as sound wave
-small bones connected to this vibrate at same frequency (stirrup bone)
-Vibrations of bones transmitted to fluid in the inner ear
-Compression waves are transferred to fluid (in the cochlea)
-small bones act as amplifier of sound waves the eardrum receives
-As fluid moves due to compression waves, small hairs that line
cochlea move too
=Each hair is sensitive to different sound frequencies, so some move more than others for certain frequencies(hairs each come from a nerve cell)
-when a certain frequency is received, hair attuned to that specific frequency moves a lot, releasing electrical impulse to brain, which interprets this into sound
what are limitations to human hearing
Humans cannot hear below 20Hz or above 20kHz=We have evolved to hear this range of frequencies as it gives us the greatest survival
advantage
In the cochlea, the hairs attuned to the higher frequencies die or get damaged
We cannot hear ultrasound as we do not use sonar to hunt etc. we have accurate vision instead
In the cochlea, the hairs attuned to the higher frequencies die or get damaged
how can this happen
Can be due to constant loud noise damaging these hairs over the years
o Or can be due to the changes in the inner ear as you grow older
o Smoking, chemotherapy, diabetes are also all causes
o So higher frequencies cannot be heard as we get older
when happens when ultrasound reaches a boundary between two media
they are partially reflected
back
- The remainder of the waves continue and pass through
- So a receiver next to the emitter can record the reflected waves
what can ultrasound be used for
speed of the waves are constant, so measuring the time between emission and detection can show distance from the source they are
- Or can be used for imaging under surfaces
-A crack in a metal block will cause some waves to reflect earlier than the rest, so will show up
-Scan of human foetus also use ultrasound for their non-invasive imaging
what is infrasound
opposite of ultrasound – it is a sound wave with a frequency lower than 20Hz – also known as seismic waves. There are two: P and S waves
-This is used to explore the Earth’s core
what are p waves
P waves are longitudinal, and can pass through solids and liquids
what are s waves
S waves are transverse, only passing through solids (these move slower too)
explain waves on the opposite side of the Earth to an earthquake
only P waves are detected which suggests that the core of the Earth is liquid – hence no S waves can penetrate it
explain how ultrasound is used with ships
Pulse of ultrasound is sent below a ship, and the time taken for it to reflect and
reach the ship can be used to calculate the depth
o This is used to work out whether there is a shoal of fish below the ship
o Or how far the seabed is below the ship
name electromagnetic waves in increasing frequency and decreasing wavelength
name their wavelengths
radio (1m-10^4)
micro (10^-2)
infrared (10^-5)
visible light (10^-7)
ultra violet(10^-8)
x rays (10^-10)
gamma rays(10^-15)
explain properties of electromagnetic waves
These are transverse waves
- Do not need particles to move
- In space, all waves have the same velocity (speed of light)
- They can transfer energy from a source to absorber
o Microwave source to food
o Sun emits energy to Earth
why are there such large range of frequencies for EM waves
theyre generated by a variety of changes in atoms and their nuclei
e.g. changes in nucleus of atom creates gamma rays
this explains why atoms can absorb a range of frequencies as each one causes a different change
bc of different properties, different em waves are used for different purposes
what are the relationships to do with electromagnetic waves
As speed is constant for all EM waves
- As wavelength decreases, frequency must increase
- As frequency increases, energy of the wave increases
what can our retinas only detect
Our retina can only detect visible light, a small part of the entire EM spectrum
- This visible light is still an EM wave like X rays and microwaves,
explain refraction according to density
If entering a denser material, it bends towards the normal
- If entering a less dense material, it bends away from
normal
(how much the wave speeds up or slows down which depends on density so the higher the density=the slower the wave travels through it)
if wave is travelling along normal it will change speed but wont refract
wavelength of wave changes when it refracts but frequency stays same
what is the optical density of a material
a measure of how quickly light can travel through it, the higher the optical density, the slower the light waves travel through it
what happens when a wave enters a different medium
wave fronts are closer togehter showing a change in wavelength so a change in velocity
wave doesnt change angle
what happens when a wave hits a different medium at an angle
wave changes angle
wave fronts are closer togehter showing a change in wavelength so a change in velocity
what does absorption,transmission, refraction, reflection depend on for a substance
Substances will absorb, transmit, refract or reflect certain EM waves depending on wavelength E.g. Glass, will transmit/refract visible light, Absorb UV radiation, Reflect IR radiation
The material interacts differently for different parts of EM spectrum because the wavelengths (and frequencies) are different
how does differences in velocity change how wavelengths behave
Some effects are due to differences in velocity
When light enters a denser medium, it slows down
Shorter wavelengths slow down more than longer wavelengths
E.g. Blue light slows down more than red
Why does dispersion occurs of white light into a prism?
The different wavelengths refract a different amount,
and therefore spread out creating a rainbow effect
- When refracting, the speed decreases and wavelength
decreases too in denser material, the horizontal lines
show the “wave-fronts” of the waves (imagine each
line is each maxima of the transverse wave)
what are radiowaves
Radio waves are produced by oscillations in electrical circuits
- When radio waves are absorbed they create an alternating current, AC, at the same
frequency as the radio waves
what happens when electrons change orbit
When electrons change orbit (move closer or further from the nucleus)
When electrons move to a higher orbit (further from the nucleus)
The atom has absorbed EM radiation
When the electrons falls to a lower orbit (closer to the nucleus)
The atoms has emitted EM radiation
If an electron gains enough energy, it can leave the atom to form an ion
So gamma rays originate from changes in the nucleus of an atom
how can UV light, X-rays and gamma can have hazardous effects on human body tissue.
UV – skin ages prematurely, increasing risk of skin cancer
so Sun cream prevents over-exposure in summer
X-ray and gamma are ionisation radiation that can cause the mutation of genes – causing cancer
so Minimal exposure should be ensured
what is radiation dose
Radiation dose: how much exposure leads to harm for a person
uses of radiowaves
TV and radio
o Long wavelength, can travel far without losing quality
uses of microwaves
- Satellite communication, cooking food
o Can penetrate atmosphere to reach satellites
uses of infrared
Cooking food, infrared cameras
o Transfers thermal energy
uses of visible light
Fibre optics
o Best reflection/scattering in glass (others have too short/long wavelengths)
uses of ultra violet
Sun tanning, energy efficient lamps
o Radiates the least heat but more energy
uses of x ray
- Medical imaging and treatment (and gamma)
o Very high in energy, and can penetrate material easily
what are em waves made of
oscillating and electric and magnetic fields
what are alternating currents made of
oscillating charges
as charges oscillate they produce ocillating electric and magnetic fields (electromagnetic waves)
frequency of waves= frequency of alternating current
alternating currents produce radio waves
what is a transmitter
the object which electrons oscilate to to create radiowaves
when transmitted radio waves reach receiver, radiowaves are absorbed , energy carried by waves is transferred to electrons in material of receiver
energy causes electrons to oscillate, if receiver is part of complete electrical circuit it makes alternating current
current is same frequency as radio wave it generates
how can long wave radio signals be received half way around the world
they have wavelengths of around 1-10km , long wavelengths diffract around curved surface of earth, they diffract around hills, into tunnels
so radiosignals can be received even if receiver isnt in line of sight of transmitter
explain how short wave radiosignals work
10m-100m
received at long distances from transmitter bc they are reflected from ionosphere (electrically charged layer in earth’s upper atmosphere)
how does bluetooth work
uses short wave radio waves to send data over short distances between devices without wires (wireless headsets to use phone while driving)
explain how medium wave signals can work
can reflect from the ionosphere depending on atmospheric conditions and time of day
explain radio waves used for tv and fm radio transmissions
short wavelengths
to get reception, you must be in direct sight of transmitter, signal doesnt bend of travel far through buildings
how are microwaves used by satellites
microwaves that can pass easily through earth’s watery atmosphere
for satellite tv, signal from a transmitter is transmitted into space
where its picked up by satellite receiver dish orbiting thousands of kms above earth, satellite transmits signal back to earth in different direction where its received by satellite dish on ground , time delay between signal sent and received bc of long distance travelled
explain how microwave ovens use different wavelength from satellites
microwaves need to pass through earth’s watery atmosphere
microwaves absorbed by water molecules in food so they use different wavelength to ones used in satellite communication
microwaves penetrate up to a few cms into food before being absorbed and transferring energy carried to water molecules in food causinf water to heat, then water molecules transfer energy to rest of molecules in food by heating which cooks food
how is infrared radiation used to increase/ monitor temperature
infrared radiation given out by all hot objects, hotter object= more IR radiation given out
absorbing ir radiation causes objects to get hotter
what are infrared cameras
used to detect infrared radiation and monitor temperature , camera detects ir radiation and turns it into electrical signal which is displayed on a sceen as a picture , the hotter the object, the brighter it appears
e.g energy transfer from a house’s thermal energy store can be detected infrared cameras
how can ir radiation be used for cooking
absorbing ir radiation causes objects to get hotter, food can be cooked using ir radiation , temperature of food increases e.g. from a toaster’s heating element
how do electric heaters work
contains long piece of wire that heats up room when current flows through, wire emits lots of infrared radiation (some light causing it to glow) emitted IR radiation absorbed by objects and air in room= energy transferred by ir waves to thermal store of objects causing temp to increase
explain fibre optic cables
thin glass/ plastic fibres that can carry data (from telephones, computers) over long distances as pulses of visible liight
they work bc of reflection, light rays bounced back and forth untilt hey reach end of fibre
visible light used in optical fibres
light not easily absorbed or scattered as it travels along fibre
what is fluorescence
property of certain chemicals, uv is absorbed and then visible light is emitted, bright as they emit light
explain flourescent lights
generates uv radiation which is absorbed and re-emitted as visible light by layer of phosphorus on inside of blub , energy efficient so good to use when light is needed for long periods
explain security pens
used to mark prperty with name e.g. laptops
under uv light, ink will glow but otherwise invisible , helps police identify property if stolen
explain tanning
uv produced by sun=exposure gives suntan
when its not sunny, people go to tanning salons where uv lamps give artificial tan
overexposure to uv is dangerous
how do x rays work
x ray passes easily through flesh but not bones(too dense) amount of radiation absorbed gives x ray image
how do radiographers use x rays and gamma rays to treat cancer
radiotherapy
high doses of rays kill all living cells , carefully directed to cancer cells to avoid killing healthy cells
how is gamma radiation used as medical tracer
gamma emitting source injected in patient , progress followed around body , gamma radiation good bc it passes out of body to be detected
what do radiographers wear to prevent harm from x rays and gamma rays
lead aprons
stands behind lead screen , leave room to keep exposure to minimum
effects of radiation based on how much energy a wave transfers
explain how
low frequency= like radiowaves dont transfer much energy, [passes through soft tissue without being absorbed
high frequency= uv, xrays, gamma= transfer lots of energy and causes lots of damage
why are x rays and gamma rays dangerous
ionising radiation
carry enough energy to knock off electrons off of atoms
causing gene mutation, cell destruction, cancer
explain how benefits of radiation can outweight health risks
e.g. risk of person involved in car accident developoing cancer from x ray photograph taken is smaller than potential health risk of not finding/ treating their injuries
what is radiation dose measured in
sieverts
measures risk of harm from body exposed to radiation
not measure of total amount of radiation absorbed
risk depends on total amount of radiation absorbed nd how harmful the type of radiation is
1000 millisieverts = 1 Sv
explain ct scans
uses x rays and computers to build up picture of inside of patients body
