Topic 6 - Waves Flashcards
Waves
> Waves carry energy from one place to another and can also carry information.
When waves travel through a medium, the particles of the medium oscillate and transfer energy between each other.
Only energy is transferred, the particles stay in the same place.
Types of wave
- Transverse
2. Longitudinal
Amplitude - definition
> The amplitude of a wave is the maximum displacement of a point on the wave from its undisturbed position.
Wavelength - definition
> The wavelength is the distance between the same point on two adjacent waves.
E.g. trough to trough.
Frequency
> Frequency is the number of complete waves passing a certain point per second.
It’s measured in Hertz.
1 Hz = 1 wave per second.
Period of a wave
> The period of a wave is the amount of time it takes for a full cycle of the wave to pass a point.
Period = 1 divided by Frequency.
Seconds = 1 divided by Hz.
Wave Speed
> The wave speed is the speed at which the energy is transferred (or the wave moves) through the medium.
Transverse Waves
> In transverse waves, the oscillations are perpendicular to the direction of energy transfer.
Most waves are transverse, inc. all electromagnetic waves (e.g. light), ripples and waves in water and a wave on a string, spring wiggled from side to side.
Mechanical Wave - definition
> A mechanical wave is a wave that is an oscillation of matter, and therefore transfers energy through a medium.
Water waves, shock waves and waves in springs and ropes are all examples of mechanical waves.
Longitudinal waves
> In longitudinal waves, the oscillations are parallel to the direction of energy transfer.
If you push a spring you get a longitudinal wave.
Examples: sound waves in air, ultrasound, shock waves e.g. seismic waves.
Longitudinal waves show areas of compression and rarefaction.
Rarefaction - definition
> A rarefaction is a region in a longitudinal wave where the particles are furthest apart.
Compression - definition
> A compression is a region in a longitudinal wave where the particles are closest together.
The wave equation
>Applies to all waves. >wave speed = frequency × wavelength v = f λ >wave speed, v, in metres per second, m/s >frequency, f, in hertz, Hz >wavelength, λ, in metres, m
Experiments with waves - measuring the speed of sound - steps
> By attaching a signal generator to a speaker you can generate sounds with a specific frequency.
You can use 2 microphones and an oscilloscope to find the wavelength of the sound waves generated.
1. Set up the oscilloscope so the detected waves at each microphone are shown as separate waves.
2. 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 one wavelength apart.
3. Measure the distance between the microphones to find one wavelength.
4. You can then use the wave speed formula to find the speed of the sound waves passing through air - the frequency is whatever you set the signal generator to.
Experiments with waves - measuring the speed of water ripples practical - steps
> Using a signal generator attached to the dipper of a ripple tank, you can create water waves at a set frequency.
- Dim the lights and turn on the lamp - you’ll see a wave pattern made by the shadows of the wave crests on the screen below the tank.
- The distance between each shadow line is equal to one wavelength. Measure the distance between the shadow lines that are 10 wave lengths apart, then divide this distance by 10 to find the average wavelength. This is a suitable method for measuring small wavelengths.
- If you’re struggling to measure the distance, you could always take a photo of the shadows and ruler, and find the wavelength from the photo instead.
- Use the wave speed formula to find 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.
Experiments with waves - finding the speed of a wave on string - steps
> You use a signal generator, but this time you attach it to a vibration transducer which converts the signals to vibrations.
- Set up the equipment shown in book, then turn on the signal generator and vibration transducer. The string will start to vibrate.
- You can adjust the frequency setting on the generator to change the length of the wave created on the string. You should keep adjusting the frequency of the signal generator until there appears to be a clear wave on the string. This happens when a whole number of half-waves fit exactly on the string (you want at least 4 or 5 ideally). The frequency you need will depend on the length of string between the pulley and the transducer, and the masses you’ve used.
- You need to measure the wavelength of the wave. Best way to do so accurately is to measure the length of all the half-wavelengths on the string in one go, then divide by the total number of half-wavelengths to get the mean half-wavelength. You can then double this value to get the full wavelength.
- The frequency is whatever you set the signal generator to.
- Find speed using formula.
Experiments with waves - finding the speed of a wave on string - set-up
> This set-up is suitable for investigating waves on a string because it’s easy to see and measure the wavelength and frequency.
Signal generator attached to vibration transducer which is attached to a string and pulley with masses on end of strong.
All on a bench with pulley and masses coming off edge of bench.
Waves - the three actions
> When waves arrive at a boundary between 2 different materials, 3 things can happen:
- Absorbed
- Transmitted
- Reflected
What can happen when waves arrive at a boundary between 2 different materials?
> When waves arrive at a boundary between 2 different materials, 3 things can happen:
1. The waves are absorbed by the material the wave is trying to cross into - this transfers energy to the material’s energy stores. This is how microwaves work.
2. The waves are transmitted - the waves carry on travelling through the new material. This often leads to refraction.
3. The waves are reflected.
What actually happens depends on the wavelength of the wave and the properties of the materials involved.
Rule of reflection
> Angle of incidence = angle of reflection.
>See diagram in book for ray diagram.
Angle of incidence - definiton
> The angle of incidence is the angle between the incoming wave and the normal.
Angle of reflection - definition
> The angle between the reflected wave and the normal.
The normal - defintion
> The normal is an imaginary line that’s perpendicular to the surface at the point of incidence (point where the wave hits the boundary).
Usually shown by a dotted line.
Types of reflection
> Waves are reflected at different boundaries in different ways:
- Specular
- Diffuse
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 you get a nice clear reflection.
Diffuse reflection
> Diffuse reflection is 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 happens because the normal is different for each incoming ray, which means that the angle of incidence is different for each ray. The rule of angle of incidence = angle of reflection still applies.
When light is reflected by a rough surface, the surface appears matte (not shiny) and you don’t get a clear reflection of objects.
Refraction - definition
> When light rays are bent when they enter a new media.
>Waves changing direction at a boundary.
Refraction
> When a wave crosses a boundary between materials at an angle, it changes direction - it’s refracted.
How much it’s refracted by depends on how much the wave speeds up or slows down, which usually depends on the density of the 2 materials.
If a wave crosses a boundary and slows down, it will bend towards the normal. If it crosses into a material and speeds up it will bend away from the normal.
The wavelength of a wave changes when it is refracted, but the frequency stays the same.
If the wave is travelling along the normal it will change speed, but it’s NOT refracted.
What is the optical density of a material?
> The optical density of a material is a measure of how quickly light can travel through it - the higher the optical density, the slower the light waves travel through it.
Density and wave speed
> The higher the density, the slower a wave travels through it.
Constructing a ray diagram to show refraction
- First, draw the boundary between your 2 materials and the normal.
- Draw an incident ray that meets the normal at the boundary. The angle between the ray and the normal is the angle of incidence. If you are given this angle use protractor.
- Now draw the refracted ray on the other side of the boundary. If the material is optically denser than the first, the refracted ray bends towards the normal. The angle between the refracted ray and the normal is smaller than the angle of incidence. If the second material is less optically dense, the angle of refraction is bigger than the angle of incidence.
>See book.
Investigating light - background
> Light experiments need to be done in a dim room so you can clearly see paths of the rays of light.
They also both use either a ray box or a laser to produce thin rays of light. This is so you can trace the paths more accurately, meaning more exact angle measurements.
Investigating light - investigating refraction - steps
> The boundaries between different substances refract light by different amounts. You can investigate this by looking at how much light is refracted when it passes from air into different materials.
1. Place a transparent rectangular block on a piece of paper and trace around it. Use a ray box or a laser to shine a ray of light at the middle of one side of the block.
2. Trace the incident ray and mark where the light emerges on the other side of the block. Remove the block, and with a straight line, join up the incident ray and the emerging point to show the path of the refracted ray through the block.
3. Draw the normal at the pint where the light ray entered the block. Use a protractor to measure the angle between the incident ray and the normal (angle of incidence) and the angle between the refracted ray and the normal (angle of refraction).
4. Repeat this experiment using rectangular blocks made from different materials, keeping the incident angle the same throughout.
You should find that the angles of refraction changes with different materials as they have different optical densities.
Use transparent materials.
Investigating light practical - comparing how different surfaces reflect light - steps
> How light reflects depends on the smoothness of the surface.
- Take a piece of paper and draw a straight line across it. Place an object so one of its sides lines up with this line.
- Shine a ray of light at the object’s surface and trace the incoming and reflected light beams.
- Draw the normal at the point where the ray hits the object. Use a protractor to measure the angle of incidence and the angle of reflection and record these values in a table. Also make a note of the width and brightness of the reflected light ray.
- Repeat this experiment for a range of objects.
Investigating light practical - comparing how different surfaces reflect light - results
> You should see that smooth surfaces like mirrors give clear reflections (the reflected ray is as thin and bright as the incident ray).
Rough surfaces like paper cause diffuse reflection which causes the reflected beam to be wider and dimmer (or not observable at all).
You should also find that the angle of incidence always equals the angle of reflection.
Electromagnetic Waves - speicfication
> Electromagnetic waves are transverse waves that transfer energy from the source of the waves to an absorber. E.g. a hot object transfers energy by emitting infrared radiation, which is absorbed by the surrounding air.
Electromagnetic waves form a continuous spectrum and all types of electromagnetic wave travel at the same velocity through a vacuum (space) or air.
The waves that form the electromagnetic spectrum are grouped in terms of their wavelength and their frequency. Going from long to short wavelength (or from low to high frequency).
The groups are: radio, microwave, infrared, visible light (red to violet), ultraviolet, X-rays and gamma rays.
Our eyes only detect visible light and so detect a limited range of electromagnetic waves.
Why is there such a large range of frequencies of EM waves?
> There is such a large range of frequencies because EM waves are generated by a variety of changes in atoms and their nuclei. E.g. changes in the nucleus of an atom creates gamma rays.
This also explains why atoms can absorb a range of frequencies - each one causes a different change.
Because of their different properties each one has different purposes.
Uses of EM waves
> Electromagnetic waves have many practical applications. For example:
• radio waves – television and radio
• microwaves – satellite communications, cooking food
• infrared – electrical heaters, cooking food, infrared cameras
• visible light – fibre optic communications
• ultraviolet – energy efficient lamps, sun tanning
• X-rays and gamma rays – medical imaging and treatments
Electromagnetic spectrum. Order + wavelength
- Radio waves. 1m-10^4m.
- Micro waves. 10^-2m.
- Infra red. 10^-5m
- Visible light. 10^-7m.
- Ultra violet. 10^-8m.
- X-rays. 10^-10m.
- Gamma rays. 10^15m.
> As you go down, frequency increases and wavelength decreases.
Radio Waves - spec brief
> Radio waves can be produced by oscillations in electrical circuits.
When radio waves are absorbed they may create an
alternating current with the same frequency as the radio wave itself, so radio waves can themselves induce oscillations in an electrical circuit.
Making EM waves.
> EM waves are made up of oscillating electric and magnetic fields.
Alternating currents are made up of oscillating charges. As the charges oscillate, they produce oscillating electric and magnetic fields, i.e. electromagnetic waves.
The frequency of the waves produced will be equal to the frequency of the alternating current.
Making radio waves
- You can produce radio waves using an alternating current in an electrical circuit. The object in which charges (electrons) oscillate to create the radio waves is called a transmitter.
- When transmitted radio waves reach a receiver, the radio waves are absorbed.
- The energy transferred 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 had the same frequency as the radio waves that generated it.
>Diagram in book.
Radio Waves - uses
> Radio waves are EM radiation with wavelengths longer than about 10cm.
Long-wave radio waves (wavelengths of 1-10 km) can be transmitted from one place and be received half way around the world.
That’s because long wavelengths diffract (bend) around the curved surface of the Earth. Long-wave radio waves wavelengths can also diffract around hills, into tunnels and all sorts.
This makes it possible for radio signals to be recieved even if the receiver isn’t in line of sight of the transmitter.
Short-wave radio signals (wavelength 10m-100m) can, like long-wave, be received at long distances from the transmitter. That’s because they are reflected from the ionosphere.
Bluetooth uses short-wave radio waves to send data over short distances between devices without wires.
Medium-wave signals can also reflect from the ionosphere, depending on atmospheric conditions and the time of day.
The radio waves used for TV and FM radio transmissions have v. short wavelengths so to get reception, you must be in direct sight of the transmitter - the signal doesn’t bend or travel far through buildings.
Ionosphere - definition
an electrically charged layer in the Earth’s upper atmosphere.
Microwaves- satellites
> Communication to and from satellites (inc satellite TV signals and satellite phones) uses microwaves. But you need to use microwaves which can pass easily through the Earth’s watery atmosphere.
For satellite TV, the signal from a transmitter is transmitted into space where it’s 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’s a slight time delay between the signal being sent and received because of the long distance the signal has to travel.
Microwaves - cooking
> In communications, the microwaves used need to pass through the Earth’s watery atmosphere.
In microwave ovens, the microwaves need to be absorbed by water molecules in food - so they use a different wavelength to those used in satellite communications.
The microwaves penetrate up to a few cm 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.
Microwaves - uses
- Satellite communications
2. Cooking
Infrared - uses
> Infrared radiation is given out by all objects - and the hotter the object, the more IR radiation it gives out.
Uses:
1.Monitor temperature - IR cameras
2. Increase temp - e.g. electric heaters
Infrared - monitoring temperatures
> Infrared cameras can be used to detect infrared radiation and monitor temperature.
The camera detects 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 house’s thermal energy store can be detected using infrared cameras.
Different colours represent different amounts of IR radiation being detected.
Infrared - increasing temperatures
> Absorbing IR radiation causes objects to get hotter. >Food can be cooked using IR radiation - the temperature of the food increases when it absorbs IR radiation, e.g. from a toaster’s heating element.
Electric heaters heat a room in the same way.
Electric heaters 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).
This emitted 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 temperature to increase.
Optical fibres - definition
> Optical fibres are thin glass or plastic fibres that can carry data over long distances as pulses of visible light.
How do optical fibres work?
> They work because of reflection.
The light rays are bounced back and forth until they reach the end of the fibre.
Light is not easily or scattered as it travels along the fibre.
Cladding to protect fibre.
Ultraviolet
> Fluorescence is a property of certain chemicals, where ultra-violet radiation is absorbed and then visible light is emitted. That’s why fluorescent colours look so bright - they actually emit light.
Fluorescent lights generate UV radiation, which is absorbed and re-emitted as visible light by a layer of phosphorus on the inside of the bulb. They’re energy-efficient so they’re good to use when light is needed for long periods.
Security pens can be used to mark property with your name (e.g. laptops.) 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.
UV radiation is produced by the Sun, and exposure to it is what gives people a suntan.
When it’s not sunny, some people go to tanning salons where UV lamps are used to give them an artificial suntan.
Overexposure to UV radiation can be dangerous.
Fluorescent lights emit very little UV- they’re totally safe.
X-rays and Gamma Rays
> Used in medicine.
Radiographers in hospitals take X-ray ‘photographs’ of people to see if they have any broken bones.
X-rays pass easily through flesh but not so easily through denser material like bones or metal. So it’s the amount of radiation that’s absorbed or not absorbed that gives you an X-ray image.
Radiographers use X-rays and gamma rays to treat people with cancer (radiotherapy). This is because high doses of these rays kill all living cells - so they’re carefully directed towards cancer cells, to avoid killing too many normal, healthy cells.
Gamma radiation can also be used as a medical tracer - this is where 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.
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.
EM radiation - dangers
> When EM radiation enters living tissue, it’s often harmless, but sometimes it creates havoc. The effects of each type of radiation are based on how much energy the wave transfers.
Low frequency waves, like radio waves, don’t transfer much energy and so mostly pass through soft tissue without being absorbed.
High frequency waves like UV and X-rays and gamma rays all transfer lots of energy and so can cause lots of damage.
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 rick of skin cancer.
X-rays and gamma rays are types of ionising radiation. (They carry enough energy to knock electrons off atoms). This can cause gene mutation or cell destruction, and cancer.
Measuring radiation
> Use EM radiation if benefit outweighs risk e.. risk of developing cancer from X-ray taken is much smaller than potential risk of not find and treating an injury.
Radiation dose (measured in sieverts) is a measure of the risk of harm from the body being exposed to radiation.
This is not a measure of the total amount of radiation that has been absorbed.
The risk depends on the on the total amount of radiation absorbed and how harmful the type of radiation is.
1000mSv = 1Sv.
Risk of EM - differences
> Risk can be different for different parts of the body.
A CT scan uses X-rays and a computer to build up a picture of the inside of the patient’s body.
If a patient has a CT scan on their chest, they are four times more likely to suffer damage to their genes than a head scan.
Lens
> A lens forms an image by refracting light.
In a convex lens, parallel rays of light are brought to a focus at the principal focus.
The distance from the lens to the principal focus is called the focal length.
Ray diagrams are used to show the formation of images by convex and concave lenses.
The image produced by a convex lens can be either real or virtual.
The image produced by a concave lens is always virtual.
Focal length - definition
> The distance from the lens to the principal focus.
>There is a principal focus on each side of the lens.
Convex lens
> A convex lens bulges outwards.
>It causes rays of light parallel to the axis to converge (bring together) at the principal focus.
Concave lens
> A concave lens caves inwards.
>It causes rays of light parallel to the axis to diverge (spread out).
Principal focus
> The principal focus of a convex lens is where rays hitting the lens parallel to the axis all meet.
The principal focus of a concave lens is the point where rays hitting the lens parallel to the appear to all come from - you can trace them back until they all appear to meet up at a point behind the lens.
The three rules of 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.
The three rules of refraction in a concave lens
- An incident ray parallel to the axis refracts through the lens, and travels in line with the principal focus (so it appears to have come from the principal focus).
- An incident ray passing through the lens towards 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.
Real images
> A real image is where the light from an object comes together to form an image on a ‘scree’ - like the image formed on an eye’s retina.
Virtual images
> A virtual image is when the rays are diverging, so the object appears to be coming from a completely different place.
When you look in a mirror you see a virtual image of your face - because your face appears to be behind the mirror.
You can get a virtual image when looking at an object through a magnifying lens - the virtual image looks bigger than the object actually is.
the 3 things to say when describing an image
- How big it is compared to the object.
- Whether it’s upright or inverted relative to the object.
- Whether it’s real or virtual.
Drawing a ray diagram for an image through a convex lens/concave lens
> SEE BOOK.
What affects the image?
> The distance from the lens affects the image.
>See book.
Magnification.
> The magnification produced by a lens can be calculated using the equation: magnification = image height divided by object height.
Magnification is a ratio and so has no units.
Image height and object height should both be measured in either mm or cm.
Magnifying glass
> Magnifying glasses work by creating a magnified virtual image.
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.
Remember ‘you can’t project a virtual image onto a screen’ - that’s a useful phrase to use in the exam if they ask you about virtual images.
Visible Light
> EM waves cover a very large spectrum. We can only see a tiny part of this - the visible light spectrum. This is a range of wave lengths that we perceive as different colours.
Each colour has its own narrow range of wavelengths (and frequencies) ranging from violet at 400nm up to reds at 700nm.
Different mixtures of these colours allow us to seen even more shades, e.g. red and blue wavelengths seen together appear purple.
When all of these different colours are put together, it creates white light.
What do colour and transparency depend on?
> Absorbed wavelength
>Different objects absorb, transmit and reflect different wavelengths of light in different ways.
Opaque objects
> Opaque objects are objects that do not transmit light. When visible light waves hit them, they absorb some wavelengths of light and reflect others.
The colour of an opaque object depends on which wavelengths of light are most strongly reflected.
Determining the colour of an opaque object
> E.g. an appear to be read because the wavelengths corresponding to the red part of the visible light spectrum are most strongly reflected. The other wavelengths are absorbed.
So white light hits the apple and red light is reflected - all the other colours are absorbed.
For opaque objects that aren’t a primary colour, they may be reflecting either the wavelengths of light corresponding to that colour or the wavelengths of the primary colours that can mix together to make that colour.
White objects
> White objects reflect all of the wavelengths of visible light equally.
Black objects
> Black objects absorb all wavelengths of visible light.
>Your eyes see black as the lack of any visible light (i.e. the lack of any colour).
White light
> A combination of all the colours.
Transparent - definition
See-through
Translucent - definition
Partially see-through
Transparent and translucent
> Transparent and translucent objects transmit light, i.e. not all light that hits the surface of the object is absorbed or reflected - some can pass through.
Some wavelengths of light may be absorbed or reflected by transparent and translucent objects.
A A transparent or translucent object’s colour is related to the wavelengths of light transmitted and reflected by it.
Colour filters
> Colour filters are used to filter out different wavelengths of light, so that only certain colours (wavelengths) are transmitted - the rest are absorbed.
A primary colour filter transmits that colour, e.g. if a white light is shone at a blue colour filter, only blue light will be let through - the rest of the light will be absorbed.
If you look at a blue object through a blue colour filter, it would still look blue. Blue light is reflected from the object’s surface and is transmitted by the filter.
However, if the object was red (or any other colour not made from blue light), the object would appear black when viewed through a blue filter. All of the light reflected by the object will be absorbed by the filter.
Filters that aren’t for primary colours let through both the wavelengths of light for that colour AND the wavelengths of the primary colours that can be added together to make that colour.
Rules of infrared
> All bodies (objects), no matter what temperature, emit and absorb infrared radiation.
The hotter the body, the more infrared radiation it
radiates in a given time.
An object that’s hotter than its surroundings emits more IR radiation than it absorbs as it cools down.
And an object that’s cooler than its surroundings absorbs more IR radiation than it emits as it warms up.
Objects at a constant temperature emit infrared radiation at the same rate that they are absorbing it.
Do some objects emit and absorb infrared better than others?
> Yes.
Some colours and surfaces absorb and emit radiation better than others.
E.g. a black surface is better at absorbing and emitting radiation than a white one, and a matt surface is better at absorbing and emitting radiation than a shiny one.
Investigating absorption - practical - background
> The amount of infrared radiation absorbed by different materials also depends on the material.
You can do an experiment to show this, using a Bunsen burner and some candle wax.
See diagram in book.
Investigating absorption - steps.
- Set up the equipment as shown in book. 2 ball bearings are each stuck to one side of a metal late with solid pieces of candle wax. The other sides of these plates are then faced towards the flame. The plates are placed the same distance away from the flame.
- 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 the wax on the black plate melts before the wax on the silver plate.
Investigating emission - background
> A Leslie cube is a hollow, watertight, metal cube made of e.g. aluminium, 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 radiation emitted by different surfaces.
Investigating emissions - practical
- Place an empty Leslie cube on a heat-proof mat.
- Boil water in a kettle and fill the Leslie cube with boiling water.
- Wit for the cube to warm up, then hold a thermometer against each of the four vertical faces of the cube. You should find that all 4 faces are the same temp.
- Hold an infrared detector a set distance away e.g. 10cm 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 one, and more from the matt surfaces than the shiny ones.
- As always, you should do the experiment more than once, to make sure your results are repeatable.
- It’s important to be careful when doing this experiment. Don’t try to move the cube when it’s full of boiling water - you might burn your hands. And take care if you’re carrying a full kettle too.
What is a perfect black body?
> A perfect black body is an object that absorbs all of the radiation incident on it.
A black body does not reflect or transmit any radiation.
Since a good absorber is also a good emitter, a perfect black body would be the best possible emitter.
Radiation emissions from objects
> The intensity and distribution of the wavelengths emitted by an object depend on the object’s temp.
As the temperature of an object increases, the intensity of every emitted wavelength increases.
However, the intensity increases more rapidly for shorter wavelengths. This causes the peak wavelength to decrease.
The curves on the graph in the book show how intensity and wavelength distribution of a black body depends on it temp.
Intensity - definition
Intensity is the power per unit area, i.e. how much energy is transferred to a given area in a certain amount of time.
Peak wavelength - definition
> The wavelength with the highest intensity.
>As an object gets hotter, the peak wavelength decreases.
Earth and radiation
> The overall temp of the Earth depends on the amount of IR radiation it reflects, absorbs and emits.
Radiation of Earth - times of day
- During the day, lots of radiation (like light) is transferred to the Earth from the Sun and absorbed. This causes an increase in local temp.
- At night, less radiation is absorbed than is being emitted, causing a decrease in local temperature.
- Overall, the temp of the Earth stays fairly constant.
- Changes to the atmosphere can cause a change to the Earth’s overall temp. If the atmosphere starts to absorb more radiation without emitting the same amount, the overall temp will rise until absorption and emission are equal again (global warming).
Sound waves
> Sound waves are caused by vibrating objects. These vibrations are passed through the surrounding medium as a series of compressions and rarefactions.
Type of longitudinal wave.
Sound generally travels faster in solids than in liquids, and faster in liquids than in gas.
When a sound wave travels through a solid it does so by causing the particles in the solid to vibrate.
Sound can’t travel in space, because it’s mostly a vacuum. NO particles to move and vibrate.
Sometimes the sound wave will eventually travel to into someone’s ear and reach their ear drum at which point they might hear the sound.
Lludspeaker
> Paper diaphragm in a speaker vibrates back ad forth, which causes the surrounding air to vibrate, causing compressions and rarefactions. A sound wave is creates.
The sound wave travels through the air as a series of compressions and rarefactions.
When the sound wave hits a solid object, the air articles hitting the object causes the particles in the solid to move back and forth (vibrate).
These particles hit the next particles in line and so on- passing the sound wave through the object as a series of vibrations.
How do we hear?
- Sound waves that reach your ear drum can cause it to vibrate.
- These vibrations are passed on to tiny bones in your ear called 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/hear the sound.
- Different materials can convert different frequencies of sound waves into vibrations. E.g. humans can hear sound in the range of 20HZ-20kHz. Microphones can pick up sound waves outside of this range, but if you tried to listen to this sound, you probably wouldn’t hear anything.
- Human hearing is limited by the size and shape of our ear drum as well as the structure of all the parts within the ear that vibrate to transfer the energy from the sound wave.
Reflection of soundwaves
> Sound waves will be reflected by hard flat surfaces.
>Echoes are just reflected sound waves.
Refraction of sound waves
> Sound waves will also refract as they enter different media.
As they enter denser material, they speed up.
This is because when a wave travels into a different medium, its wavelength changes but its frequency remains the same so its speed must also change.
However, since sound waves are always spreading out so much, the change in direction is hard to spot under normal circumstances.
Ultrasound
> Ultrasound is sound with frequencies higher than 20,000Hz.
Electrical devices can be made which produce electrical oscillations of any frequency.
These can easily be converted into mechanical vibrations to produce sound waves beyond the range of human hearing.
Reflection of ultrasound
> 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). This is partial reflection.
What this means is that 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.
Uses of ultrasound
> Ultrasound has medical and industrial uses.
>It’s also used in underwater exploration.
Medical uses of ultrasound
> Ultrasound waves can pass through the body, but whenever they reach a boundary between 2 different media (like fluid in the womb and skin of foetus) some of the wave is 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.
No one knows for sure if ultrasound is safe in all cases but X-rays would definitely be dangerous.
Industrial uses of ultrasound
> Ultrasound can also be used to find flaws in objects such as popes 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’s a flaw such as a crack inside the object, the wave will be reflected sooner.
Ultrasound uses in underwater exploration
> Echo sounding uses high frequency sound waves (including ultrasound).
It’s used by boats and submarines to find out the depth of the water they are in or to locate objects in deep water.
Waves - detect and explore
> Waves have different properties (e.g. speed) depending on the material they are travelling through.
When a wave arrives at a boundary between materials, a number of things can happen.
It can be completely reflected or partially reflected like in ultrasound imaging. The wave may continue travelling in the same direction but at a different speed or it may be refracted or absorbed.
Studying the properties and paths of waves through structures can give you clues to some of the properties of the structure that you can’t see by eye. You can do this with lots of different waves - ultrasound and seismic waves are 2 good, well-known examples.
Earthquakes
> Earthquakes and explosions cause seismic waves.
When there’s an earthquake somewhere, it produces seismic waves which travel out through the Earth. We detect these waves all over the surface of the planet using seismometers.
Seismologists work out the time it takes for the shock waves to reach each seismometer. They also note which parts of the Earth don’t receive the shock waves at all.
When seismic waves reach a boundary between different layers of material (which all have different properties, like density) inside the Earth, some waves will be absorbed and some will be 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.
Seismic Waves
> Seismic waves provide evidence for the Earth’s structure.
By observing how seismic waves are absorbed and refracted, scientists have been able to work out where the properties of the Earth change dramatically.
Our current understanding of the internal structure of the Earth and the size of the Earth’s core is based on these observation.
There are 2 different types of seismic waves you need to know about - P waves and S waves.
P-waves
> P-waves can travel through the Earth’s core.
P-waves are longitudinal.
They travel through solids and liquids.
They travel faster than S-waves.
If earthquake P-waves can pass through the core and are detected other side.
Refract as density changes.
S-waves
> S-waves can’t travel through the Earth’s core.
S-waves are transverse.
They can’t travel through liquids (or gases).
They’re slower than P-waves.
