P6 - waves Flashcards
what is transferred in a wave?
Energy
how are waves transferred through a medium?
the particles of the medium oscillate and transfer energy between each other
do waves transfer particles?
no
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 two adjacent waves
what is frequency?
the number of complete waves passing a certain point per second
what is frequency measured in?
hertz
what is 1 Hz
one wave per second
what is the formula to find the period of the wave from the frequency?
T = 1/f
what is the period of a wave?
the amount of time it takes for a full cycle of the wave
what are the two types of wave?
transverse and longitudinal
are there any waves that aren’t either transverse or longitudinal?
no
what are transverse waves?
waves where the oscillations (vibrations) are perpendicular to the direction of energy transfer
are most waves longitudinal or transverse?
transverse
what are three examples of transverse waves?
- all electromagnetic waves (e.g. light)
- ripples and waves in water
- a wave on a string
what are longitudinal waves?
waves where the oscillations are parallel to the direction of energy transfer
what is an example of a longitudinal wave?
sound waves
what is the equation for wave speed?
wave speed = frequency x wavelength
what is wave speed?
the speed at which energy is being transferred (or the speed the wave is moving at)
what can you use to measure the speed of sound?
an oscilloscope
how can you use an oscilloscope to measure the speed of sound?
by attaching a signal generator to a speaker you can generate sounds with a specific frequency. you can use two microphones and an oscilloscope to find the wavelength of the sound waves generated
what are the 5 steps of using an oscilloscope to measure the speed of sound?
- 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 slowly move one away until the two waves are aligned on the display, but have moved exactly 1 wavelength apart
- measure the distance between the microphones to find one wavelength
- you can then use the formula wavespeed = frequency x wavelength to find the speed of the sound waves passing through the air - the frequency is whatever you set the signal generator to (around 1 kHz is sensible)
- the speed of sound in air is around 330m/s, so check your results roughly agree with this
what piece of equipment do you need to measure the speed of water ripples?
a ripple tank
what are the 6 steps to measuring the speed of water ripples using a ripple tank
- using a signal generator attached to the dipper of a ripple tank you can create waver waves at a set frequency
- dim the lights in the lab and turn on the lamp. you should see the wave crests as shadows on the screen below the tank
- the distance between each shadow line is equal to one wavelength. measure the distance between shadow lines that are 10 wavelengths apart, and then divide this distance by 10 to find the average wavelength
- this is a good method for measuring the wavelength of moving waves of small wavelengths
- use the formula wave speed = frequency x wave length to calculate the speed of the waves
- this set-up is suitable for investigating waves, because it allows you to measure the wavelength without disturbing the waves
describe the set-up of a ripple tank
a signal generator creates small waves in a shallow tank of waver. there is a lamp above this tank, shining through the water onto the white screen below, causing the crests of the waves to show up as shadows. there is a ruler on the white screen to measure the length of the waves
what set-up do you need when measuring the speed of waves on a string?
a string stretched in the air over the end of a bench, attached to a vibration transducer on one side, and going over a pulley and off the end of the bench on the other side. the vibration transducer is attached to a signal generator, and the end of the string going off the bench is pulled down by weights.
what are the 5 steps to finding the speed of a wave on a string?
- set up the equipment right, then turn on the signal generator and vibration transducer. the string will start to vibrate
- adjust the frequency of the signal generator until there’s a clear wave on the string.
- you need to measure the wavelength of these waves. the best way to do this accurately is to measure the lengths of as many half-wavelengths as you can in one go, then divide to get the mean half-wavelength. you can double this mean to get a full wavelength
- the frequency of the wave is whatever the signal generator’s set to
- you can find the speed of the wave using wave speed = frequency x wavelength
what will the frequency you need to get a clear wave on the string depend on?
the length of the string between the pulley and the transducer, and the masses you’ve used
what waves can be absorbed, transmitted or reflected?
all waves
what are the three things that can happen when a wave meets a boundary between two materials?
- the wave is absorbed by the second material
- the wave is transmitted through the second material
- the wave is reflected
what happens when a wave is absorbed?
the wave transfers energy to the material’s energy stores. Often, the energy is transferred to a thermal energy store, which leads to heating (this is how a microwave works)
what happens when a wave is transmitted through a material?
the wave carries on travelling through the new material. this often leads to refraction and can be used in communications as well as in the lenses of glasses and cameras
what happens when a wave is reflected?
where they incoming ray is neither absorbed nor transmitted, but instead is ‘sent back’ away from the second material. this is how echoes are created
what determines what actually happens when a wave meets a boundary between two materials?
the wavelength of the wave and the properties of the materials involved+
are electromagnetic (EM) waves transverse or longitudinal?
transverse
what do EM waves do?
transfer energy from a source to an absorber
how does a camp fire transfer energy to its surroundings?
it emits infrared radiation. these infrared waves are absorbed by objects and transfer energy to the object’s thermal energy store, causing the object to warm up
how are radio waves used in radios?
radio waves transfer energy to the kinetic energy stores of electrons in radio receivers, which generates an electric current
do EM waves travel at the same speed through air and a vacuum?
yes
what speed do ALL EM waves travel at?
300,000,000 (3 x 10^8) m/s
do EM waves travel at different speeds in different materials?
yes - this can lead to refraction
How much do EM waves vary in wavelength?
from around 10^-15m to more than 10^4m
how are EM waves grouped?
based on their wavelength and frequency
how many basic types of EM waves are there?
7
what type of spectrum do the different groups of EM waves merge to form?
a continuous spectrum
how much of the EM spectrum can our eyes detect?
only the small section of visible light
what are the 7 types of EM wave in order of longest wavelength to shortest wavelength?
- Radio waves
- microwaves
- infrared
- visible light
- ultra-violet
- X-rays
- Gamma rays
do microwaves have a high or low frequency?
low
do infra-red waves have a high or low frequency?
low
do X-rays have a high or low frequency?
high
why is there such a large range of frequencies in EM waves?
because EM waves are generated by a variety of changes in atoms and their nuclei, e.g. changes in the nucleus of an atom create gamma rays
why can atoms absorb such a range of frequencies?
each one causes a different change
what are EM waves generated by?
changes in atoms and their nuclei
what is a mnemonic to remember the order of the EM waves?
Rock Music Is Very Useful for eXperiments with Goats
what happens when a wave crosses a boundary between two materials?
it changes speed
what happens if a wave is travelling along the normal when it passes through a boundary between two materials?
it changes speed, but it is not refracted
what is refraction?
waves changing direction at a boundary
what happens if a wave hits the boundary between two materials at an angle?
it changes direction - it is refracted
what happens when a wave slows down?
it bends towards the normal
what happens when a wave speeds up?
it bends away from the normal
what does how much a wave is refracted by depend on?
how much the wave speeds up or slows down, which usually depends on the density of the two materials
does a wave travel faster through a material with a higher or lower optical density?
lower optical density
what part of the wave changes when it is refracted?
the wavelength
does the frequency of a wave change when the wave is refracted?
no
what do ray diagrams show?
the path of a wave
what are rays?
straight lines that are perpendicular to wave fronts
what do rays show?
the direction a wave is travelling in
how do you construct a ray diagram for a refracted light ray? (6 steps)
- first, start by drawing the boundary between your two materials, and the normal
- draw an incident ray that meets the normal at the boundary
- the angle between the incident ray and the normal is the angle of incidence
- now draw the refracted ray on the other side of the boundary
- the angle of refraction is the angle between the refracted ray and the normal
- if the second material is optically denser than the first, the refracted ray bends towards the normal, and the angle of refraction is larger than the angle of incidence
what is the normal?
an imaginary line that’s perpendicular to the point where the incoming wave hits the boundary
what is a wave front?
a line showing all of the points on a wave that are in the same position as each other after a given number or wavelengths (if you draw semi-circular sound waves spreading out from a speaker, the semi-circular lines are the wavefront.)
why does the wave slowing down cause it to refract?
when a wave crosses a boundary at an angle, only part of a wave crosses the boundary at first. If it’s travelling into a denser material, that part travels slower than the rest of the wave front, so by the time the whole wave front crosses the boundary, the faster part of the wave front will have travelled further than the slower part of the wave front. This difference in distance travellled (caused by the difference in speed) by the wave front causes the wave to bend (refract)
what are EM waves made up of?
oscillating electrical and magnetic fields
what are radio waves made by?
oscillating charges
why do alternating currents produce electromagnetic waves?
alternating currents are made up of oscillating charges. As the charges oscillate, they produce oscillating electric and magnet fields, i.e. electromagnetic waves
when an alternating current produces electromagnetic waves, what will the frequency of the waves be down to?
the frequency of the alternating current
what is a transmitter?
the object in which charges (electrons) oscillate to create radio waves
how can you use radio waves to complete a circuit? (5 steps)
- you can produce radio waves using an alternating current in an electrical circuit
- when the transmitted radio waves reach a receiver, the radio waves are absorbed
- the energy carried by the waves is transferred to the electrons in the material of the receiver
- this energy causes the electrons to oscillate and, if the receiver is part of a complete electrical circuit, it generates an alternating current
- this current has the same frequency as the radio wave that generated it
what are radio waves mainly used for?
communication
what are radio waves?
electromagnetic radiation with wavelengths longer than about 10 cm
what are the wavelengths of long-wave radio waves?
1 - 10 km
why can long-wave radio signals be received at long distances from the transmitter?
long wavelengths diffract (bend) around the curved surface of the earth. Long-wave radio wavelengths can also diffract around hills, into tunnels, and all sorts. This makes it possible for radio signals to be received even if the receiver isn’t in line of sight of the transmitter
what are the wavelengths of short-wave radio signals?
10m - 100m
why can short-wave radio signals be received at long distances from the transmitter?
they are reflected from the ionosphere
what is the ionosphere?
an electrically charged layer in the earth’s upper atmosphere
how does bluetooth work?
bluetooth uses short-wave radio waves to send data over short distances between devices without wires
how long are the wavelengths of the radio waves used for TV and FM radio transmission?
they have very short wavelengths - to get reception, you must be in direct sight of the transmitter - the signal doesn’t travel far through buildings
what type of waves are used by satellites?
microwaves
why do satellites use microwaves?
they can pass easily through the earth’s watery atmosphere
how do satellite TVs work?
the signal from a transmitter is transmitted into space, where it is picked up by the satellite receiver dish orbiting thousands of kilometres above the Earth. The satellite transmits the signal back to earth in a different direction, where it’s received by a satellite dish on the ground. There is a slight time delay between the signal being sent and received because of the long distance the signal has to travel
name two uses of microwaves
satellites and microwaves
how do microwave ovens work?
- the microwaves are absorbed by water molecules in food
- the microwaves penetrate up to a few sentimetres into the food before being absorbed and transferring the energy they are carrying to the water molecules in the food, causing the water to heat up
- the water molecules then transfer this energy to the rest of the molecules in the food by heating - which quickly cooks the food
what can infrared radiation be used for?
to increase or monitor temperature
what can infrared cameras be used for?
to detect infrared radiation and monitor temperature
how do infrared cameras work?
the camera detects the IR radiation and turns it into an electrical signal, which is displayed on a screen as a picture. The hotter an object is, the brighter it appears. E.g. energy transfer from a houses thermal energy store can be detected using infrared cameras
what happens to objects that absorb IR radiation?
they get hotter - food can be cooked using IR radiation, e.g. in a toaster
when does an object give out lots of infrared radiation?
when it’s really hot
how do electric heaters work?
they contain a long piece of wire that heats up when a current flows through it. This wire then emits lots of infrared radiation (and a little visible light - the wire glows). The emitted Infrared (IR) radiation is absorbed by objects and the air in the room - energy is transferred by the IR waves to the thermal energy stores of the objects, causing their temperatures to increase
what are optical fibres? how do they work?
thin glass or plastic fibres that can carry data (e.g. from telephones of computers) over long distances as pulses of visible light.
They work because of reflection - the light rages are bounces back and forth until they reach the end of the fibre
why is visible light used in optical fibres?
because it is easy to refract light enough so that it remains in a narrow fibre, and light is also not easily absorbed or scattered as it travels along a fibre
what is fluorescence?
a property of certain chemicals, where ultra-violet (UV) radiation is absorbed and then visible light is emitted. That’s why fluorescent colours look so bright - they actually emit light
how do fluorescent lights work?
they generate UV radiation, which is absorbed and re-emitted as visible light by a layer of a compound called phosphor on the inside of the bulb. They’re energy-efficient so they’re good to use when light is needed for long periods (like in a classroom)
what can security pens be used for?
they can be used to mark property with your name - under UV light the ink will glow (fluoresce), but it’s invisible otherwise. This can help the police identify your property if it’s stolen
why can tanning salons be dangerous?
overexposure to UV radiation can be dangerous
what are three uses of ultra-violet radiation?
- fluorescent lights
- security pens
3 sun tans - naturally emitted from the sun, and also used in UV lamps at tanning salons
which two EM waves are used in medicine?
X-rays and gamma rays
how do X-rays work?
X-rays pass easily through fless but not so easily through denser material like bones of metal. So it’s the amount of radiation that’s absorbed (or not absorbed) that gives you an X-ray image
which EM waves do radiographers use to treat people with cancer? (in radiotherapy)
X-rays and gamma rays
why are X-rays and gamma rays used in radiotherapy (treating cancer)?
high doses of these rays kill all living cells, so they are carefully directed towards cancer cells, to avoid killing too many normal, healthy cells
how can gamma radiation be used as a medical tracer?
a gamma-emitting source is injected into the patient, and its progress is followed around the body. Gamma radiation is well suited to this because it can pass out through the body to be detected.
how and why do radiographers keep there exposure to radiation at a minimum?
both X-rays and gamma rays can be harmful to people, so radiographers wear lead aprons and stand behind a lead screen or leave the room to keep their exposure to them to a minimum
name the two things that the amount of infrared radiation emitted from an object is dependent on
temperature and the material of its surface
what is a leslie cube?
a hollow, watertight, metal cube whose four vertical faces have different surfaces (e.g. matt black paint, matt white paint, shiny metal and dull metal). You can use them to investigate IR emissions by different surfaces
what are the 8 steps to using a Leslie cube to investigate IR emissions by different surfaces?
- place an empty leslie cube on a heat-proof mat
- boil water in a kettle and fill the Leslie cube with boiling water
- wait a while for the cube to warm up, then hold a thermometer agains each of the four vertical faces of the cube. you should find that all four faces are the same temperature
- hold an infrared detector a set distance (e.g. 10 cm) away from one of the cube’s vertical faces, and record the amount of IR radiation it detects
- repeat this measurement for each of the cube’s vertical faces. Make sure you position the detector at the same distance from the cube each time
- you should find that you detect more infrared radiation from the black surface than the white wone, and more from the matt surfaces than the shiny ones
- you should do the experiment more than once, to make sure your results are repeatable
- it is important to be careful with the boiling water when doing this experiment
what does the amount of infrared radiation absorbed by different materials depend on?
the material
how can you do an experiment to show that the amount of infrared radiation absorbed depends on the material?
- take two metal plates and use solid pieces of candle wax to stick a ball bearing to one side of each of them. The other sides of these plates (without the ball) are faced towards the flame of a bunsen burner
- the sides of the plates that are facing towards the flame each have a different surface colour - one is matt black and the other is silver.
- the ball bearing on the black plate will fall first as the black surface absorbs more infrared radiation - transferring more energy to the thermal energy store of the wax. This means that the wax on the black plate melts before the wax on the silver plate
what does the danger/harmlessness of EM radiation depend on?
how much energy the wave transfers
is EM radiation harmful to people?
some of it is
how harmful are low-frequency EM waves? Why? Give an example of a low-frequency EM wave
low frequency waves, like radio waves, don’t transfer much energy and so mostly pass through soft tissue without being absorbed
how harmful are high frequency EM waves (give 3 examples)? why?
high frequency waves like UV, X-rays and gamma rays all transfer lots of energy and so can cause lots of damage.
how is UV radiation damaging?
UV radiation damages surface cells, which can lead to sunburn and cause skin to age prematurely. Some more serious effects are blindness and an increased risk of skin cancer.
how are X-rays and gamma rays harmful?
they are types of ionising radiation (they carry enough energy to knock electrons off atoms). This can cause gene mutation or cell destruction, and cancer
what is considered before any type of EM radiation is used?
whether the benefits outweigh the health risks
what is radiation dose? what is it measured in?
radiation dose (measured in sieverts) is a measure of the risk of harm from the body being exposed to radiation. It is NOT a measure of the total amount of radiation that has been absorbed.
what does the risk of radiation depend on?
the total amount of radiation absorbed and how harmful the type of radiation is.
what is a milisievert? why are they used?
a sievert is pretty big, so you’ll often see doses in millisieverts (mSv) where 1000 mSv = 1 Sv
is the radiation dose higher for a patient when having a CT scan on their head or their chest?
their chest - they are four times more likely to suffer damage to their genes when having a ches tscan
