Topic 6 - Waves Flashcards
What happens when waves travel through a medium?
The particles oscillate and transfer energy between each other. Overall the particles stay in the same place and only energy is tranferred.
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 (eg. between each trough).
What is frequency?
Number of complete waves passing a certain point per second, measure in hertz (Hz).
1 Hz =…
One wave per second.
What is the period of a wave?
The amount of time it takes for a full cycle of the wave.
How do you calculate the period?
Period (s) = 1/frequence (Hz)
What are the two types of wave?
Transverse or longitudinal.
What is a transverse wave?
One which the oscillations (vibrations) are perpendicular to the direction of energy transfer.
What are examples of transverse waves?
Electromagnetic waves (eg. light), ripples and waves in water and waves on a string.
What are longitudinal waves?
Oscillations are parallel to the direction of energy transfer.
What are examples of longitudinal waves?
Sound waves in air, ultasound, shock waves eg. some seismic waves.
What is wave speed?
The speed at which energy is being transferred or the speed at which the wave is moving at.
How can you calculate wave speed?
Wave speed (m/s) = frequency (Hz) x wavelength (m) v=fλ.
Descrribe how you can measure the speed of sound?
Attach a signal generator to speaker generating sounds with a specific frequency. Two microphones and an oscilliscope can find the wave length of the sound waves generated.
1) Set up the oscilliscope so the detected waves at each microphone are shown as seperate waves.
2) Start with both microphones next to the speaker, then slowly move away until the two waves are aligned on the display, but have moved one wavelength apart.
3) Measure the distance between the microphones to find one wavelength.
4) Use v=fλ to find speed. The frequency is whatever you set the signal generator to.
What is the rough speed of sound in air?
330 m/s.
How can you measure the speed of water ripples?
1) using a signal generator attached to the dipper of a ripple tank creates water waves at a set frequency.
2) use a lamp to see wave crests on a screen below the tank. make sure the size of the waves shadows are the same size of the waves.
3) the distance between each shadow is equal to one wavelength.
measure the distance between shadow lines that are 10 wavelengths apart, then divide distance by 10 to find the average wavelength. - good for measuring small wavelengths.
4) you can take a photo and measure shadows with a ruler.
5) use v=fλ to calculate the speed.
Why is this set up suitable for investigating waves (water)?
Allows you to measure the wavelength without disturbing the waves.
What do you need to make sure to do when measuring the spped of water ripples using a lamp?
Make sure you dim the lights.
How can you use the wave equation for waves on strings?
1) Set up experiment, turn on the signal generator and vibration transducer. the strings will start to vibrate.
2) Adjust the frequency of the signal generator until there’s a clear wave on the string.
3) Measure the wavelengths of the waves.
4) the frequence is whatever the signal generator is set to.
5) v=fλ
Wave equation for waves on strings: What does the frequency you need depend on?
Length of string between pully and transducer, masses you’ve used.
Wave equation for waves on strings: Why is the set up suitable for measuring waves on a string?
Easy to see and measure the wavelength.
What can happen when waves arrive at a boundary between two different materials?
They are absorbed, transmitted or reflected.
What happens when waves are absorbed?
It transfers energy to the materials energy stores.
What happens when waves are transmitted?
The waves carry on travelling through the new material - often leading to refraction.
What happens when waves are absorbed, refelected and transmitted depends on what?
The wavelength of the wave and properties of the materials involved.
What is the one rule about refleted 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?
An imaginary line that is perpendicular to the surface at the point of incidence. - normally shown with a dotted line.
What is the point of incidence?
The point where the wave hits the boundary.
What is specular reflection?
Happens when a wave is reflected in a single direction by a smooth surface.
What is diffuse reflection?
When a wave is reflected by a rough surface and the reflected waves are scatted in lots of different directions.
Why does diffuse reflection occur?
Because the normal is different for each incoming ray so the angle of incidence is different for each ray.
What happens when light is reflected across a rough surface?
The surface appears matte and you don’t get a clear reflection of objects.
What type of waves are EM waves?
Transverse - transfer energy from a source to an absorber.
Why can electromagnetic waves travel through a vacuum?
They aren’t vibrations of particles, they’re vibrations of electric and magnetic fields.
All EM waves travel at the same speed through ____ or ______.
air or a vacuum.
How are electromagnetic waves grouped?
Based on wavelength and frequency.
What is the wavelength of radio waves?
1m - 10⁴ m
What is the wavelength of micro waves?
10⁻²m
What is the wavelength of infared?
10⁻⁵m
What is the wavelength of visible light?
10⁻⁷m
What is the wavelength of ultra violet?
10⁻⁸m
What is the wavelength of x rays?
10⁻¹⁰m
What is the wavelength of gamma rays?
10⁻¹⁵m
Why is there a large range of frequencies between types of EM waves?
EM waves are generated by a variety of changes in atoms and their nuclei. - also explains why atoms can absorb a range of frequencies, each one causes a difference change.
Why are EM waves used for different purposes?
They have different properties.
What happens when a wave crosses a boundary between materies at an angle?
It changes direction / is refracted.
What does how much a wave is refracted by when crossing a boundary between materials depend upon?
How much the wave speeds up or slows down, which usually depends on the density of the two materials. The higher the density of the material, the slower a wave travels through it.
What will happen if a wave crosses a boundary and slows down?
It will bend towards the normal.
What will happen if the wave crosses into a material and speeds up?
It will bend away from the normal.
What happens to the wavelength and frequency when a wave is refracted?
The wavelength changes but the frequency stays the same.
Is the wave refracted when it travels along the normal and changes speed?
No.
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 light waves travel through it.
How can you investigate how different substances refract light by different amounts?
- Place a transparent rectangular block on a piece of paper and trace around it. Shine a ray to the middle of one side of the block using a ray box or laser.
- Trace the incident ray and mark where the light ray emerges on the other side of the block. Remove the block and join the two rays with a straight line to show the path of the refracted ray.
- Draw the normal at the point where the light ray entered the block. Use a portractor to measure the angle of incidence and angle of refraction.
- Repeat the experiement using rectangular block made from different materials.
Investigating how different substances refract light by different amounts: What should you observe?
The angle of refraction changes for different materials due to their different optical densities.
How can you investigate how different substances reflect light by different amounts?
- Draw a straight line across a piece of paper. Place an object so one of its sides lines up with the line.
- Shine a ray of light at the object’s surface and trace the incoming and relected light beams.
- Draw the normal at the point where the ray hits the object. Use a portractor to measure the angle of incidence and reflection and record these values in a table. Also make note of the width and brightness of the reflected light ray.
- Repeat the experiment for a range of objects.
Investigating how different substances reflect light by different amounts: What should you observe?
Smooth surfaces like mirrors give clear reflections - the reflected ray is as thin and bright as the incident ray. Rough surfaces like paper diffuse reflection which causes the reflected beam to be wider and dimmer (or unobservable).
What are EM waves made up of?
Oscillating electric and magnetic fields.
How can alternating currents make em waves?
Ac is made up of ocillating charges. As the charges oscillate, they produce oscillating electric and magnetic fields ie. eletromagnetic waves. The frequency of the waves produced will be equal to thwe frequency of the alternating current.
How can you produce radio waves?
Using alternating current in an electric circuit. The object in which charges/electrons oscillates to create the radio waves is called a transmitter. When transmitted radio waves reach a reciever, the radio waves are absorbed. The energy carried by the waves is transferred to the electrons in the material of the reciever. The energy casues the electrons to oscillate and, if the reciever is part of a complete circuit, it generates an alternating current. This current has the same frequency as the radio waves that generate it.
What are radio waves?
EM radiation with wavelengths longer than about 10cm.
How can long-wave radio (wavelengths of 1-10km) be transmitted and recieved halfway around the world?
Long wavelength diffract (bend) around the curved surface of the Earth. Can also diffract around hills, into tunnels…
What do long-wave radio wavelengths make possible?
Radio signals to be recieved even if the reciever isn’t in line of sight of the transmitter.
Why can short-wave radio signals (10m-100m) be recieved at long distances from the transmitterr?
They are reflected from the ionosphere - an electrically charged layer in the Earth’s upper atomsphere.
What type of radio wave does bluetooth use?
Short-wave radio waves- send data over short distances without wires.
What can medium-wave signals do?
Reflect from the ionosphere, depending on the atmospheric conditions and the time of day.
What radio waves are used for TV and FM radio?
Very short wavelengths - to get reception you must be in direct sight of the transmitter, the signal does not bend or travel far through buildings.
What type of wave does communication to and from satellites use?
Microwaves. - which pass easily through the Earth’s watery atmosphere.
How do satellite tvs work?
The signal from a transmitter is transmitted into splace where it is picked up by the satellite reciever dish orbiting thousands of kilmoeters above the earth. The satellite transmits the signal back to Earth in a different direction where it is recieved by a satellite dish on the ground. There is a slight time delay between the signal being sent and recieved because of the long distance the signal has to travel.
Why do microwave ovens use different wavelengths to those used in satellite communications?
In communications, the microwave needs to pass through the eaths watery atmosphere but in ovens the microwaves need to be absorbed by water molecules in food so use a different wavelength.
How do microwaves heat food?
The microwaves penetrate up to a few centimeters 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 is infared (IR) radiation?
Radiation that is given out by all hot objects and the hotter the object, the more IR radiation it gives out.
What detects infared radation and moniters temperature?
Infared cameras.
How do infared cameras work?
Carmera detects IR radiation, the hotter an object is, the brighter it appears.
What happens when objects absorb IR radiation?
They can get hotter.
What is a use of IR radiation?
Energy transfer from a houses thermal store, heating food.
How do electric heaters heat a room?
Contain a long piece of wire that heats up when a current flows through it. The wire then emits lots of infared radiaiton. The emitted IR radiation id absorbed by objects and the air in the room - the energy is transferred by the IR waves to the thermal energy stores of the objects, causing the temperature to increase.
What is a use of visible light?
Fibre optic cables - transmit data.
What are optical fibres?
Thin glass or plastic fibres that can carry data over long distance as pulses of visible light.
How do fibre optic cables use visible light to transmit data?
Reflection. The lights rays are bouced back and forth until they reach the end of the fibre. Light is not easily absorbed or scattered as it travels along the fibres.
What is fluorescence?
Property of certain chemicals, where ultraviolet radiation is absorbed and then visible light is emitted.
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 a phosphor on inside the bulb.
Why are fluorescent lights good for long periods?
They are energy efficient.
What are security pens?
Used to mark property with your name. Under UV light the ink will glow but is invisible otherwise - helping police identigy property if its stolen.
How does UV give people suntans?
UV is produced by the sun.
What can X-rays be used for?
X-ray photographs to identify broken bones.
Why do you get an x-ray image?
X-rays pass easily through flesh but not through denser materials like bone or metal so its the amount of radiation absorbed that gives the images.
What do radiographers use to treat people with cancer?
X-rays and gamma rays. High doses of these rays kill all living cells - directed towards cancer cells so avoid killing too many healthy cells.
What is a further use of gamma rays?
Medical tracer - gamma emitting source is injected into the patient and its progress is followed around the body. Can pass out of the body to be detected.
How do radiographers protect themselves against x-rays and gamma rays?
Wear lead aprons, stand behind a lead screen/ leave the room.
What are the effects of each type of radiation based on?
How much energy the wave transfers.
Why don’t low frequency waves cause much damage?
They don’t transfer much energy and so mostly pass through soft tissue without being absorbed.
Why do high frequency waves cause a lot of damage?
They all transfer lots of energy.
What are dangers of UV radiation?
Damages surface cells, which leads to sunburn and can cause skin to age prematurely. More serious effects are blindness and increased risk of skin cancer.
What type of radiation are X-rays and gamma rays?
Ionising radiation.- carry enough energy to knock electrons off of atoms. Can cause gene mutation or cell destruction and cancer.
What is radiation dose?
Measure (in sieverts) of the risk of harm from the body being exposed to radiation. The risk depends on the total amount of radiation absorbed and how harmful the radiation is.
How do lenses form images?
By refracting light and changing its direction.
How do convex lenses work?
- Convex lenses bulge outwards causing rays of light parallel to the axis to be brought together at the principal focus.
How do concave lenses work?
- They cave inwards causing parallel rays of light to spread out (diverge).
What is the axis of a lens?
The line passing through the middle of the lens.
Where is the principle focus of a convex lens?
Where rays hitting the lens parallel to the axis all meet.
Where is the principal focus of a concave lens?
The point where rays hitting the lens parallel to the axis appear to all come from.
What are the three rules for refraction in a convex lens?
- An incident ray parallel to the axis refracts through the lens and passes through the principal focus on the other side.
- An incident ray passing through the principal focus refracts through the lens and travels parallel to the axis.
- An incident ray passing through the centre of the lens carries on in the same direction.
What are the three rules for refraction in a concave lens?
- An incident ray parallel to the axis refracts through the lens, and travels in line with the principal focus.
- An incident ray passing through the lens towards the principal focus refracts through the lens and travels parallel to the focus.
- 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 comes together to form an image on a ‘screen’.
What is a virtual image?
When the rays are diverging, so the light from the objects appears to be coming from a completely different place.
What are examples of virtual images?
When you look in a mirror and looking through a magnifying glass.
What do you need to say to describe an image properly?
- How big it is compared to the object.
- Whether it’s upright or inverted relative to the object.
- Whether it is real of virtual.
In what way do concave lens differ from convex lenses?
They always produce a virtual image. The image is the right way up, smaller than the object on the same side of the lens as the object,
What type of lens does a magnifying glass use?
Convex.
How do magnifying glasses work?
The object being magnified must be closer to the lens than the focal length.
Since the image produced is a virtual image, the light rays don’t actually come from the place where the image appears to be.
What is the magnification formula?
Image height / object height.
What is a useful phrase about virtual images to remember?
“You can project a virtual image onto a screen”.
What is the visible light spectrum?
The range of wavelengths that we percieve as different colours.
What does the range of colour wavelengths range from?
Violet (400nm) to Red (700nm).
What are the primary colours?
Red, green and blue.
What happens when all the colours are put together?
White light is created.
What happens when visible light hits opaque objects?
Does not trasmit light. Absorbs some wavelengths and reflects others.
What does the colour of opaque objects depend upon?
Which wavelengths are most strongly reflected.
What do white objects reflect?
All the wavelengths of visible light equally.
What do black objects absorb?
All the wavelengths of visible light.
How are transparent and translucent objects see through/ partially see through?
They transmit light, not all the light hitting the surface of the object is absorbed or reflected - some can pass through.
Why are colour filters used?
To filter out different wavelengths of light, so that only certain colours are transmitted and the rest are absorbed.
How do objects at a constant temperature emit infared raditation?
At the same rate they are absorbing it.
What colours and surfaces are better at absorbing radiation?
Black matte surfaces.
What colour and suface is best at emitting radiation?
White shiny surface.
What is a leslie cube?
A hollow, watertight, metal cube made of aluminium, whose four vertical faces have different surfaces. Can be used to investigate IR emission by different surfaces.
How would you use a leslie cube to invesigate IR emmisions 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 for the cube to warm up then hold a thermometer against each of the four vertical faces of the cube. They should all be the same temperature.
- Hold an infared detector a set distance away from one of the cube’s vertical faces and record the amound of IR radiation it detects.
- Repeat this for each of the cube’s vertcal faces. Make sure the detector is the same position and distance.
How would you use a leslie cube to invesigate IR emmisions by different surfaces: What would you find?
You detect more infared radiation from the black surface than the white ones, and more from the matte surface than the shiny ones.
What should you do to make sure your results are repeatable?
Do the experiment more than once.
How would you use a leslie cube to invesigate IR emmisions by different surfaces: What should you do to stay safe?
Don’t move the cube when it’s full of boiling water. Be careful when carrying a full kettle.
What is a perfect black body?
An object that absorbs all of the radiation that hits it. No radiation is reflected or transmitted. Best possible emitters of radiation.
Why do all objects emit EM radiation?
Due to their thermal energy stores.
What does the intensity and distribution of the wavelengths emitted by an object depend on?
The objects temperature.
What is intensity?
Power per unit area, how much energy is transferred to a given area in a certain amount of time.
Describe how the intensity and wavelength distribution of a black body depends on its temperature.
As the temperature of an object increases, the intensity of every emitted wavelength increases. However, the intensity increases 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 it reflects, absorbs and emits.
Describe the Earths temperature during the day?
Lots of radiation (like light) is transferred to the Earth from the Sun and is absorbed, causing an increase in local temperature.
Describe the Earths temperature at night.
Less radiation is being absorbed than emitted, causing a decrease in local temperature.
What will cause the Earth’s overall temperature to rise?
If the atmosphere starts to absorb more radiation without emitting the same amount. The temp will continue to rise until absorption and emission are equal again.
What causes sound waves?
Vibrating objects. These vibrations are passed through the surrounding medium as a series of compressions and rarefactions.
What type of wave is sound waves?
Longitudinal.
Describe the speed of sound waves in solids liquids and gases.
Travel faster in solids than liquids. Faster in liquids than gases.
How do sound waves travel through a solid?
The wave causes the particles in the solid to vibrate.
Why can’t sound waves travel in space?
It’s mostly a vacuum - there a no particles to move or vibrate.
What are ossicles?
Tiny bones in the ear.
Describe how you hear sound.
Sound waves that reach your ear drum can cause it to vibrate. These vibrations are passed onto the ossicles, through the semicircular canals and to the cochlea. The cochlea turns these vibrations into electrical signals which get sent to your brain and allow you to sense sound.
What range of frequencies can humans hear?
20 Hz to 20 kHz.
What is human hearing limited to?
The size and shape of our ear drum, as well as the structure of all the parts within the ear that vibrate to transfer energy from the sound wave.
What are echoes?
Reflected sound waves.
What surfaces reflect sound waves?
Hard flat surfaces.
What happens as sound waves enter denser material?
They speed up. When a wave travels into a different medium, its wavelength changes but its frequency remains the same so its speed must also change.
What is an ultrasound?
Electrical device made which produces 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 (ie frequencies above 20,000 Hz).
What is partial reflection?
When a wave passes from one medium into another, some of the wave is reflected off the boundary between the two media, and some is transmitted (and refracted.
How can you measure how far away the boundary is?
You can point a pulse of ultrasound at an object, and wherever there are boundaries between one substance and another, some of the ultrasound gets reflected back. The time it takes for the reflections to reach a detector can be used to measure how far away the boundary is.
In what ways are ultrasounds useful?
Medical imaging and industrial imaging.
How are ultasounds used in pre-natal scanning of a foetus?
When the ultrasound waves reach the boundary between the fluid in the womb and the skin of the foetus, some of the waves are reflected back and detected. The exact timing and distribution of these echoes are processed by a computer to produce a video image of the foetus.
How can ultrasounds be used to find flaws in materials?
Can find flaws in pipes or materials such as wood or metal. Ultrasound waves entering a material will usually be reflected by the far side of the material. If there is a flaw such a crack inside the object, the wave will be reflected sooner.
Why do boats and submarines use echo sounding?
To find the depth of the water they are in or to locate objects in deep water.
What happens when a wave arrives at a boundary between materials?
It is completely reflected or partially reflected. The wave may continue travelling in the same direction but at a different speed, or it may be refracted or absorbed.
How can we detect seismic waves?
Using seismometers.
What do seismologists do?
Work out the time it takes for the shock waves to reach each seismometer and what parts of the Earth don’t recieve the shock waves at all.
What happens when seismic waves reach a boundary between different layers of material inside the Earth?
Some waves will be absorbed and some refracted. Most of the time, if the waves are refracted, they change speed gradually, resulting in a curved path. But when properties change suddenly, the wave speed changes abruptly, and the path has a kink.
What are two different types of seismic waves?
P waves and S waves.
How have seismic waves allowed us to understand the structure of the earth?
By observing how seismic waves are absorbed and refracted, scientits have been able to work out where the propertied of the Earth change dramatically.
What are P-waves?
Longitudinal waves. Travel through solids and liquids. Faster than S-waves.
What are S-waves?
Transverse. Can’t travel through liquids or gases. Slower than P-waves.
