T6: waves Flashcards
what happens 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.
what is amplitude?
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. Measured in hertz (Hz)
What is the equation for a period of wave?
period (seconds) = 1 ÷ frequency (Hz)
T = 1 ÷ f
what are the different types of waves?
all waves are either transverse or longitudinal.
In transverse waves what are the oscillations like?
the oscillations are perpendicular (90) to the direction of energy transfer.
What are 3 examples of transverse waves?
- all electromagnetic waves (e.g. light)
- ripples and waves in water.
- a wave on a string
In longitudinal waves what are the oscillations like?
parallel to the direction of energy transfer.
What are 2 examples of longitudinal waves?
- sound waves in air (e.g. ultrasound)
- shock waves (e.g. some seismic waves)
Are most waves longitudinal or transverse?
transverse
what is wave speed?
the speed at which the energy is being transferred (or the speed the wave is moving at)
what is the equation for wave speed?
wave speed (m/s) = frequency (Hz) × wavelength (m)
v = f λ
How can you generate sounds with a specific frequency and how can you measure the wavelength of the sound waves generated?
1) by attaching a signal generator to a speaker
2) by using 2 microphones and an oscilloscope
What are the steps to measure the speed of sound using an oscilloscope?
Speed of water ripples
waves on a string
What are the 3 different things that can happen when waves arrive at a boundary between 2 different materials?
1) the waves are ABSORBED by the material the wave is trying to cross into- this transfers energy to the material’s energy stores
2) The waves are transmitted; the waves carry on travelling through the new material. This often leads to refraction.
3) the waves are refracted?
What happens when a waves arrives at a boundary between 2 different materials depends on…?
the wavelength of the wave and the properties of the material involved.
what is the rule for all reflected waves?
angle of incidence=angle of reflection
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?
The normal is an imaginary line that is perpendicular to the surface at the point of incidence (the point where the wave HITS the boundary)
What is specular reflection?
specular reflection happens when a wave is reflected in a single direction by a smooth surface. e.g. when light is reflected by a mirror to give a clear reflection
What is diffuse reflection?
when a wave is reflected by a rough surface and the reflected rays are scattered in lots of different directions.
Why does diffuse reflection occur?
The normal is different for each incoming ray so the angle of incidence is different for each ray. The rule of angle of incidence=angle of reflection still applies.
What happens when light is reflected by a rough surface?
the surface appears matte and you don’t get a clear reflection of objects.
what are all Em waves?
transverse waves that transfer energy from a source to an absorber.
how do all EM waves travel?
all waves travel at the same speed through air or a vacuum (space)
what are the different electromagnetic waves in the continuous spectrum?
radio waves (1 - 10^4)
microwaves (10^-2)
infrared (10^-5)
visible light (10^-7)
ultraviolet (10^-8)
X-rays (10^-10)
gamma rays (10^-15)
what happens to frequency and wavelength down the electromagnetic spectrum?
increasing frequency and decreasing wavelength.
Why is there a large range of frequency in the electromagnetic spectrum?
EM waves are generated by a variety of changes in atoms in their nuclei. This also explains why atoms absorb a range of frequencies; each one causes a different change.
Why are different EM waves used for different purposes?
because of their different properties.
How much an angle refracted depends on ____ ?
how much the wave speeds up or slows down, which usually depends on the density of the 2 materials (usually the higher the density of a material, the slower a wave travels through it.)
What happens if a wave crosses a boundary and slows down?
bends towards the normal
What happens if a wave crosses into a material and speeds up?
bends away from the normal
What happens when a wave is refracted?
the wavelength of the wave changes but the frequency stays the same
What happens if a wave is travelling along the normal?
it will change speed but it’s not refracted
What is optical density?
a measure of how quickly light can travel through it; the higher the optical density the slower light waves travel through it.
When investigating light why is it best to do it in a dim room?
so you can clearly see the light rays?
Why is a ray box or a laser used?
to produce thin rays of light so you can easily see the middle of the ray when tracing it.
what are EM waves made up of?
oscillating electric and magnetic fields
What are alternating currents made up of?
Oscillating charges, as the charges oscillate they produce oscillating electric and magnetic fields e.g. electromagnetic waves.
What will the frequency of the waves produced be equal to?
the frequency of the alternating current
How can you produce radio waves?
Using and alternating current in an electrical circuit. The object in which charges (electrons) oscillate to create the radio waves is called a transmitter.
What is the energy carried by the waves transferred to?
the electrons in the material of the receiver.
What does the energy transferred to the electrons cause?
it causes the electrons to oscillate and if the receiver is part of a complete electrical circuit, it generates an alternating current. The current has the same frequency as the radio waves that generated it.
What are radio waves?
EM radiation with wavelengths longer than about 10cm
Why can long wave radio (wavelengths of 1-10km) be transmitted from London and received halfway around the world?
As long wavelengths diffract (bend) around the curved surface of the area. They can also diffract around hills into tunnels and all sorts.
Why can short-wave radio signals be received at long distances from the transmitter?
because they are reflected from the ionosphere; an electrically charged layer in the earth’s upper atmosphere.
What does bluetooth uses short wave radio waves for?
to send data over short distances between devices without wires
What do medium wave signals reflecting from ionosphere depend on?
the atmospheric conditions and the time of day
Why do radio waves used for TV and FM radio transmissions have very short wavelengths?
To get reception you must be in direct sight of the transmitter; the signal does not have to bend or travel far through buildings
How does communication to and from a satellite work?
using microwaves. It is best to use microwaves which can easily pass through earth’s watery atmosphere.
How does a satellite TV work?
1) A signal from a transmitter is transmitted into space
2) where it is picked up by the satellite receiver dish orbiting thousands of kilometres above the earth.
3) The satellite transmits the signal back to earth in a different direction
4) where it is received by a satellite dish on the ground.
Why is there a delay in satellite TVs?
There is slight time delay between the signal being sent and received due to the long distance the signal has to travel.
In microwave ovens, what are the microwaves absorbed by?
water molecules in the food
How do microwaves heat up food?
- microwaves can penetrate a few cm into 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 objects give out infrared radiation?
all hot objects; the hotter the objects, the more IR radiation it gives out.
What do infrared cameras do?
- detect infrared radiation
- monitor temperatures
How do infrared cameras work?
they detect the infrared radiation and turns it into an electrical signal which is displayed on a screen as a picture. The hotter the object the brighter it appears.
Why can food be cooked using infrared radiation?
Absorbing infrared radiation causes objects to get hotter so the temperature of the food increases when it absorbs infrared radiation.
How do electric heaters heat up a room?
- electric heaters contain a long piece of wire that heats up when current flows through it.
- the wire then emits a lot of infrared radiation (and a bit of visible light hence it glows).
- the emitted IR radiation is absorbed by objects and air in the room.
- energy is transferred from the IR waves to the objects thermal energy store causing their temperature to increase.
What are optical fibres?
thin glass or plastic fibres that can carry data over long distances as pulses of visible light.
Why do optical fibres work?
due to reflection. The light rays are bounced back and forth until they reach the end of the fibre.
What happens to light as it travels along a fibre?
it is not easily absorbed or scattered
How does fluorescence work?
fluorescence is a property of certain chemicals; where ultraviolet radiation is absorbed and visible light is emitted. ( that’s why fluorescent colour look so bright, they emit light)
How do fluorescent lights work?
Fluorescent lights 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. (energy efficient so used when light is needed for long periods)
How do security pens work?
under UV light the ink will glow but will invisible otherwise.
What enables x-rays to work?
X-rays can pass easily through flesh but as easily through denser materials like bones or metals. This means that it is the amount of radiation absorbed (or not absorbed) that gives you an xray image.
How are X-rays and gamma rays used to treat cancer?
high doses of these rays kills all living cells- so they are carefully directed towards cancer cells to avoid killing healthy cells.
How do medical tracers work?
- A gamma emitting source is injected into a persons body and its progress is followed around the body.
- Gamma radiation is well suited to this as it can pass out through the body to be detected
How do radiographers try to keep their exposure to X-rays and gamma rays to a minimum?
- wear lead aprons
- stand behind lead screens
- leave the room
Give an example of a negative image.
An xray: the brighter bits are where fewer xrays get through. The plate starts off all white.
What does the effect of each type of EM wave depend on?
how much energy the wave transfers.
What is the difference between the effects of low and high frequency waves?
- low frequency waves, like radio waves, do no transfer a lot of energy so mostly pass through soft tissue without being absorbed.
- high frequency waves, like UV, X-rays and gamma rays, transfer a lot of energy so cause a lot of damage.
What is the risk of UV radiation?
damages surface cells causing sunburn, skin to age prematurely, blindness and increased risk of skin cancer.
What are the risks of X-rays and gamma rays?
they are both ionising radiation (they carry enough energy to knock electrons off atoms)/ This can cause gene mutation or cell destruction and cancer
What is radiation dose?
a measure of the risk of harm from the body being exposed to radiation.
What is radiation dose measured in?
Sieverts or milli sieverts
1000 mSv = 1 Sv
What does the risk of radiation depend on?
- the total amount of radiation absorbed
- how harmful the type of radiation is
How do lenses form images?
by refracting light and changing its direction.
What are the 2 types of lenses?
convex and concave; they have different shapes and opposite effects on light rays.
What does a convex lens do?
A convex lens bulges outwards. It causes rays of light parallel to the axis to be brought together (converge) at the principal focus.
What does a concave lens do?
A concave lens caves inwards. It causes rays of light parallel to the axis to spread out (diverge).
What is the axis of the lens?
a line passing through the middle of the lens.
What is the principal of focus in a convex lens?
where rays hitting the lens parallel to the axis all meet.
What is the principal of focus in a concave lens?
the point where the rays hitting the lens parallel to the axis appear to all come from
What is the focal length?
There is a principal focus on each side of the lens. The distance from the centre of the lens to the principal focus is the focal length.
What are the 3 rules of refraction in a convex lens?
1) an incident ray parallel to the axis refracts through the lens and passes through the principal focus on the other side.
2) An incident ray passing through the principal focus refracts through the lens and travels parallel to the axis.
3) An incident ray passing through the centre of the lens carries on in the same direction.
What are the 3 rules for refraction in a concave lens?
1) An incident ray parallel to the axis refracts through the lens and travels in line with the principal focus
2) An incident ray passing through the lens towards the principal focus refracts through the lens and travels parallel to the axis.
3) An incident ray passing through the centre of the lens carries on in the same direction.
What is a real image?
where the light from an object come together to form an image on a ‘screen” : like the image formed on an eye’s retina.
What is a virtual image?
a virtual image is when rays are diverging, and so the light from the object appears to be coming from a completely different place.
Give 2 examples of virtual images.
1) when you look in the mirror you see a virtual image of your face as the object (your face) appears to be behind the mirror.
2) using a magnifying lens: the virtual image is bigger than the object actually is.
What are the 3 things you must mention to describe an image?
1) how big the image is compared to the object
2) whether it is upright or inverted relative to the object
3) whether it is real or virtual
On a ray diagram what will an object between F and 2F produce?
a real, inverted, bigger image than the object, beyond 2F
On a ray diagram what will an object at 2F produce?
an image that is real, inverted and same size as the object and at 2F
On a ray diagram what will an object between F and the lens produce?
a virtual, upright, bigger(than the object) image on the same side as the lens
What is the image produced by a concave lens?
The image produced by a concave lens is always VIRTUAL.
This means the image is always real, upright and smaller than the object and on the same side of the lens as the object.
What is the image produced by a convex lens like?
The image produced by a convex lens can be either real or virtual.
What enables magnifying glasses to work?
creating a magnified virtual image
How do magnified glasses/ lenses work?
- the object being magnified has to be closer to the lens than the focal length.
- the image is virtual so the Light rays don’t actually come from where they appear to be
- you cannot project a virtual image onto a screen
What is the equation for magnification?
magnification = image height ÷ object height
What is the part of the electromagnetic spectrum that we see?
the visible light spectrum; this is range of wavelengths that we perceive as different colours.
What is the difference between each colour of the visible light spectrum?
Each colour within the visible light spectrum has its own narrow band of wavelength and frequency; ranging from violets at 400nm to reds at 700nm.
What are the primary colours of light?
red, blue and green. You cannot make primary colours by mixing other colours.
What happens when all of the colours of visible light are put together?
white light is created
What happens when visible light hits an opaque object?
It absorbs some wavelengths of light and reflects others.
What does the colour of an opaque object depend on?
The colour of an opaque object is determined by which wavelengths of light are more strongly reflected.
What do white objects do?
reflect all of the wavelengths of visible light equally.
What do black objects do?
absorb all wavelengths of visible light (your eyes see blacks the lack of visible light)
What does the colour of a transparent or translucent object depend on?
the wavelength of light transmitted and reflected by it.
What are colour filters used for?
to filter out different wavelengths so only certain colours (wavelengths) are transmitted- the rest are absorbed
What do primary colour filters do and what does this lead to when white light is shone on it?
1) only the primary colour that the filter is will be let through the rest will be absorbed.
2) if the filter and the object is blue, the object would look blue as blue light is still reflected from the object’s surface and is transmitted by the filter.
3) but if the object was red in a blue filter, it would appear black as all of the light reflected by the object would be absorbed by the filter.
What wavelengths do filters that aren’t primary colours let through?
wavelengths of the light for that colour and the wavelengths of the primary colour that can be added together to make that colour.
What do all objects continually absorb and emit?
Infrared radiation: infrared radiation is emitted from the surface of an object
What do hot objects do in terms of IR?
An object that is hotter than its surroundings will emit more IR radiation than it absorbs as it cools down.
What do cold objects do in terms of IR?
An object that is cooler than its surroundings will absorb more IR radiation than it emits as it warms up.
What do objects at a constant temperature do in terms of IR?
they emit IR radiation at the same rate that they are absorbing it
What surfaces and colours are better at absorbing and emitting infrared radiation?
- matte
- black
What is a perfect black body?
it is an object that absorbs all radiation that hits it. No radiation is reflected or transmitted.
What all objects emit and why?
- all objects emit EM radiation due to energy in their thermal energy stores.
- this energy is not just a part of the infrared spectrum; it covers a range of wavelengths and frequencies.
What does the intensity and distribution of the wavelengths emitted by the object depend on?
the objects’s temperature.
What is intensity?
the power per unit area
What happens to the intensity as the temperature of an object increases and why?
As the temperature of an object increases the intensity of every emitted wavelength increases.
What type of wavelength does the intensity increase more rapidly for?
more rapidly for shorter wavelengths than longer wavelengths. This causes the peak wavelength to decrease.
What does the overall temperature of the earth depend on?
the amount of radiation that is absorbed, emitted and reflected into space.
How does the temperature vary throughout the day?
- During the day, more radiation (like light) is transferred to the earth from the sun and absorbed, causing an increase in local temperature.
- At night, more radiation is being emitted than absorbed causing a decrease in local temperature.
- Overall the temperature of the earth stays fairly constant.
What would increase the earth’s overall temperature and why?
- changes to the atmosphere.
- if the atmosphere starts to absorb more radiation without emitting the same amount, the overall temperature will rise until absorption and emission are equal again.
What are sound waves caused by?
Sound waves are caused by vibrations. These vibrations are passed through the surrounding medium as a series of compressions and rarefactions.
What will sound waves generally travel faster in?
solids than liquids, liquids than gas
How does a sound wave travel through a solid?
by causing the particles in the solid to vibrate.
Why can sound not travel in space?
space is a vacuum so there is no particles to move or vibrate.
How do humans hear sound?
1) sound waves that reach your ear drum cause it to vibrate.
2) These vibrations are passed on to tiny bones in your ears called ossicles, through the semicircular canals and to the cochlea.
3) The cochlea turns these vibrations into electrical signals which are sent to the brain to allow you to sense/hear the sound.
Why are the different hearing ranges for different materials?
Different materials can convert different frequencies of sound waves into vibrations.
What is the range of normal human hearing?
the range of normal human hearing is from 20 Hz to 20 kHz.
Why is human hearing limited?
Due to the size and shape of the eardrum and the due to the structure of all the parts within the ear that vibrate to transfer the energy from the sound wave.
What are sound waves reflected by? (give an example)
- by hard flat surface
- echoes are reflected sound waves
What causes sound waves to refract and why?
- sound waves will refract will they enter a different medium
- as they enter denser material, their wavelength changes but its frequency remains the same so the speed must change.
What are ultrasounds?
Ultrasound waves have a frequency higher than the upper limit of hearing for humans; a sound with frequencies higher than 20000 Hz
How can an ultrasound be produced?
- electrical devices can be made using 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.
What is partial reflection/ when does it occur?
When a wave passes from one material to another, some of the wave is reflected off the boundary between the 2 media and some is transmitted (and refracted).
How is partial reflection used in an ultrasound?
- You can point a pulse of an ultrasound at an object and where ever there are boundaries between one substance and another, some of the ultrasound gets reflected back.
- Ultrasound waves are partially reflected when they meet a boundary between two different media.
What can the time taken for the reflections to reach a detector be used for?
The time taken for the reflections to reach a detector can be used to determine how far away such a boundary is.
What are the 2 different uses of ultrasounds?
In medical imaging and industrial imaging
How do pre natal scans of a foetus work?
- the ultrasound waves can pass through the body, but whenever it reaches a boundary between 2 media (like the fluid in the womb and the skin of the foetus) the wave is reflected back and recorded.
- the exact timing and distribution of these echoes are processed by a computer to produce a video image of the foetus.
How are ultrasounds used in industrial imaging?
- the ultrasound waves can be used to find flaws in objects like pipes or materials like wood and metal.
- ultrasound waves entering a material will be reflected by the far side of the material.
- if there is crack or a flaw in an object, the wave will be reflected sooner.
What is echo sounding?
Echo sounding, using high frequency sound waves (like ultrasounds) is used to detect objects in deep water and measure water depth.
How does the properties of a wave differ?
the properties of a wave depends on the material they are travelling through.
What are the 5 different things that can happen when a wave arrives at a boundary between materials?
- completely reflected
- partially reflected
- refracted
- absorbed
- continue in the same direction but at a different speed.
What happens when there is an earthquake?
seismic waves are produced which will travel out the surface of the earth.
What is used to detect seismic waves all over the surface of the planet?
seismometer
What do seismologists work out?
the time it takes for shock waves to reach each seismometer and they also note which parts of the planet don’t receive chock waves at all.
What happens when seismic waves reach a boundary between different materials inside the earth?
some waves are absorbed and some waves are refracted
What happens when a seismic wave is refracted?
- If the waves are refracted they will change speed gradually causing a curved path.
What happens to a seismic wave if the properties change suddenly?
- If the properties change suddenly the wave speed changes abruptly causing a kink
What are the 2 types of seismic waves?
P-waves and S-waves.
What do P-waves and S-waves provide evidence for?
for the (internal) structure and size of the Earth’s core.
What are P-waves?
- P-waves are longitudinal, seismic waves.
- P-waves travel (at different speeds) through solids and liquids.
- P waves travel faster than S waves.
What are S-waves?
- S-waves are transverse, seismic waves.
- S-waves cannot travel through liquids (or gases).
- They are slower than P-waves.
