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What happens when waves travel through a medium?
Whats amplitude of wave?
Whats wavelength?
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 - only energy is transferred.
The amplitude of a wave is the maximum displacement of a point on the wave from its undisturbed position.
The wavelength is the distance between the same point on two adjacent waves (e.g. between the trough of one wave and the trough of the wave next to it).
Whats Frequency of a wave? What can you find from the frequency? What are all waves either?
Frequency is the number of complete waves passing a certain point per second. Frequency is measured in hertz (Hz). 1 Hz is 1 wave per second.
From the frequency, you can find the period of a wave using: T=1/f, T → period (s), f → frequency (Hz).
This is the amount of time it takes for a full cycle of the wave.
All waves are either transverse or longitudinal
What are oscillations like in tranverse waves? What waves are transverse?
In transverse waves, the oscillations (vibrations) are perpendicular (at 90°) to the direction of energy transfer.
Most waves are transverse, including:
1) All electromagnetic waves, e.g. light
2) Ripples and waves in water
3) A wave on a string.
What are oscillations like in longitudinal waves?Give 2 examples
In longitudinal waves, the oscillations are parallel to the direction of energy transfer.
Examples are:
1) Sound waves in air, ultrasound.
2) Shock waves, e.g. some seismic waves.
Whats wave speed? Whats the equation for wave speed?
The wave speed is the speed at which energy is being
transferred (or the speed the wave is moving at).
v = fλ, v → wave speed (m/s), f → frequency (Hz), λ -> wavelength (m)
How to use an Oscilloscope to measure the speed of sound? (method)
By attaching a signal generator to a speaker you can generate sounds with a specific frequency.
You can use two microphones and an oscilloscope to find the wavelength of the sound waves generated.
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 (λ).
4) You can then use the formula v = fλ to find the speed (v) of the sound waves passing through the air - the frequency (f) is whatever you set the signal generator to (around 1 kHz is sensible).
5) The speed of sound in air is around 330 m/s, so check your results roughly agree with this.
How can you measure the speed of water ripples using a lamp? (method)
Using a signal generator attached to the dipper of a ripple tank
you can create water waves at a set frequency.
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 as the waves.
The distance between each shadow line is equal to one wavelength. Measure the distance between shadow lines that are 10 wavelengths apart, then divide this distance by 10 to find the average wavelength. This is a good method for measuring small wavelengths (p.104).
If you're struggling to measure the distance, you could take a photo of the shadows and ruler, and find the wavelength from the photo instead.
Use v = fa to calculate the wave speed of the waves.
This set-up is suitable for investigating waves, because it allows you to measure the wavelength without disturbing the waves.
How can Use the Wave Equation for Waves on Strings? (method)
Set up the equipment shown on the right, then turn on the signal generator and vibration transducer. The string will start to vibrate.
Adjust the frequency of the signal generator until there's a clear wave on the string. 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 these waves. The best way to do this accurately is to measure the lengths of, say four or five half-wavelengths (as many as you can) in one go, then divide to get the mean half-wavelength. You can then double this mean to get a full wavelength.
The frequency of the wave is whatever the signal generator is set to.
You can find the speed of the wave using v = fλ.
What 3 things can happen when waves arrive at a boundary between 2 different materials? What does what actually happens depend on?
When waves arrive at a boundary between two different materials, three 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
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.
What’s the rule of angles of incidence/reflection? what’s the angle of incidence? what’s the angle of reflection?
Angle of incidence = angle of reflection
The angle of incidence is the angle between the incoming wave and the normal.
The angle of reflection is the angle between the reflected wave and the normal.
Whats the normal? How is the normal usually shown?
The normal is an imaginary line that's perpendicular (at right angles) to the surface at the point of incidence (the point where the wave hits the boundary).
The normal is usually shown as a dotted line.
what’s Specular reflection? What’s Diffuse reflection? Why does this happen? What happens when light is reflected by a rough surface?
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 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.
What are all electromagnetic waves (EM)? All EM waves travel at? EM waves form a?
All EM waves are transverse waves (p.73) that transfer energy from a source to an absorber. E.g. a hot object transfers energy by emitting infrared radiation, which is absorbed by the surrounding air.
All EM waves travel at the same speed through air or a vacuum (space)
Electromagnetic waves form a continuous spectrum over a range of frequencies.
They are grouped into seven basic types, based on their wavelength and frequency.
what are the wavelengths of EM waves (7 basic types)? What is the pattern of frequency and wavelength?
…
Why is there a large range of frequencies (EM waves)? Because of their different properties…
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, different EM waves are used for different purposes.
Whats refracted (w wave)? What does how much its refracted depend on? what does the wavelength and frequency do?
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 two materials (usually the higher the density of a material, the slower a wave travels through it). 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…, Whats the optical density of a material?
If the wave is travelling along the normal it will change speed, but it's NOT refracted.
The optical density of a material is a measure of how quickly light can travel through it - the higher the optical density, the slower light waves travels through it.
How can you construct a ray diagram for a refracted light ray? (method)
First, start by drawing the boundary between your two materials and the normal (a line that is at 90° to the boundary).
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're given this angle, make sure to draw it carefully with a protractor.)
Now draw the refracted ray on the other side of the boundary. If the second material is optically denser than first, the refracted ray bends towards the normal (like on the right). The angle between the refracted ray and the normal (the angle of refraction) is smaller than the angle of incidence. If the second material is less optically dense, the angle of refraction is larger than the angle of incidence.
How can you use transparent materials to investigate refraction? (method)
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.
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.
Trace the incident ray and mark where the light ray 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.
Draw the normal at the point where the light ray entered the block. Use a protractor to measure the angle between the incident ray and the normal (the angle of incidence, I) and the angle between the refracted ray and the normal (the angle of refraction, R).
Repeat this experiment using rectangular blocks made from different materials, keeping the incident angle the same throughout.
You should find that the angle of refraction changes for different materials - this difference is due to their different optical densities
how can different materials reflect light by different amounts? (method)
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.
What are EM waves made up of? What are alternating currents made up of? the frequency of the waves produced…
EM waves are made up of oscillating electric and magnetic fields.
Alternating currents (ac) (p.31) 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.
How can you produce radio waves & what’s a transmitter? when transmitted radio waves reach a receiver…, the energy carried by the waves is…, this energy causes the…, this current has the same…
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 carried by the waves is transferred to the electrons in the material of the receiver.
This energy causes the electrons to oscillate and, if the receiver is part of a complete electrical circuit, it generates an alternating current.
This current has the same frequency as the radio wave that generated it.
What are Radio waves? Long-wave radio can be transmitted… that’s because… they can also…? This makes it possible for radio signals to be…?
Radio waves are EM radiation with wavelengths longer than about 10 cm.
Long-wave radio (wavelengths of 1 - 10 km) can be transmitted from London, say, and received halfway round the world. That's because long wavelengths diffract (bend) around the curved surface of the Earth. Long-wave radio wavelengths can also diffract around hills, into tunnels and all sorts.
This makes it possible for radio signals to be received even if the receiver isn't in line of the sight of the transmitter.
Short-wave radio signals can be received…?, Bluetooth uses what to send data? Medium-wave signals can also…? radio waves used for TV and FM radio transmission have…?
Short-wave radio signals (wavelengths of about 10 m - 100 m) can, like long-wave, be received at long distances from the transmitter. That's because they are reflected from the ionospheres - an electrically charged layer in the Earth's upper atmosphere.
Bluetooth® uses short-wave radio waves to send data over short distances between devices without wires (e.g. wireless headsets so you can use your phone while driving a car).
Medium-wave signals (well, the shorter ones) 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 very short wavelengths. To get reception, you must be in direct sight of the transmitter - the signal doesn't bend or travel far through buildings.
What does communication to & from satellites use? For a satellite TV the signal from a…? Where its picked up by the…? Where it’s received by a…
Communication to and from satellites (including satellite TV signals and satellite phones) uses microwaves. It's best 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 is a slight time delay between the signal being sent and received because of the long distance the signal has to travel.
What’s absorbed by what in microwave ovens? How much do they penetrate… before being…? What do the water molecules then do?
In microwave ovens, the microwaves are absorbed by water molecules in food.
The microwaves penetrate up to a few centimetres 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.
Infrared radiation (IR) is given out by…? Infrared cameras can be used to detect…? Absorbing IR radiation causes objects to…? Electric heaters heat a room in…?
Infrared (IR) radiation is given out by all hot objects - and the hotter the object, the more IR radiation it gives out.
Infrared cameras can be used to detect infrared radiation and monitor temperature. The camera detects the IR radiation and turns it into an electrical signal, which is displayed on a screen as a picture. The hotter an object is, the brighter it appears. E.g. energy transfer from a house's thermal energy store can be detected using infrared cameras.
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). The 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.
What are optical fibres? How do they work? What is used in optical fibres? Light is not easily…?
Optical fibres are thin glass or plastic fibres that can carry data (e.g. from telephones or computers) over long distances as pulses of visible light.
They work because of reflection (p.75). The light rays are bounced back and forth until they reach the end of the fibre.
Visible light is used in optical fibres.
Light is hot easily absorbed or scattered as it travels along a fibre.
What is fluorescence? What do Fluorescent lights generate? What can security pens be used for?
Fluorescence is a property of certain chemicals, where ultra-violet (UV) radiation is absorbed and then visible light is emitted. That's why fluorescent colours look so bright - they actually emit light.
Fluorescent lights generate UV radiation, which is absorbed and re-emitted as visible light by a layer of a compound called a phosphor on the inside of the bulb. They're energy-efficient so they're good to use when light is heeded for long periods (like in your classroom).
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.
What’s Ultraviolet radiation (UV) produced by? what do people do when it’s not sunny?
Ultraviolet radiation (UV) is produced by the Sun, and exposure to it is what gives people a suntan.
When it's hot sunny, some people go to tanning salons where UV lamps are used to give them an artificial suntan. However, overexposure to UV radiation can be dangerous (fluorescent lights emit very little UV - they're totally safe).
Radiographers in hospitals take…? What do X-rays pass easily through? Radiographers use X-rays and what to treat people…?
Radiographers in hospitals take X-ray 'photographs' of people to see if they have any broken bones.
X-rays pass easily through flesh but hot so easily through denser material like bones or metal. So it's the amount of radiation that's absorbed (or hot 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 are carefully directed towards cancer cells, to avoid killing too many normal, healthy cells.
Gamma radiation can also be used as a…? Both X-rays and gamma rays can be…?
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.
When EM radiation enters living tissue it’s often…? Low frequency waves, like radio waves, don't…? High frequency waves like…?
When EM radiation enters living tissue - like you - 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, X-rays and gamma rays all transfer lots of energy and so can cause lots of damage.
UV radiation damages… which can lead to…? X-rays and gamma rays are types of…?
UV radiation damages surface cells, which can lead to sunburn and cause skin to age prematurely. Some more serious effects are blindness and an increased risk of skin cancer.
X-rays and gamma rays are types of ionising radiation. (They carry enough energy to knock electrons off of atoms.) This can cause gene mutation or cell destruction, and cancer.
Whilst UV radiation, X-rays and gamma rays can all be harmful, they are also…? For example…? What’s radiation dose? This is not…?
Whilst UV radiation, X-rays and gamma rays can all be harmful, they are also very useful. Before any of these types of EM radiation are used, people look at whether the benefits outweigh the health risks.
For example, the risk of a person involved in a car accident developing cancer from having an X-ray photograph taken is much smaller than the potential health risk of not finding and treating their injuries.
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.
What does risk of radiation dose depend on? A sievert is…?
The risk depends on the total amount of radiation absorbed and how harmful the type of radiation is.
A sievert is pretty big, so you'll often see doses in millisieverts (mSv), where 1000 mSv = 1 Sv.
What does a CT scan use to build up a picture? What does the table show? If a patient has a CT scan oh their chest…?
A CT scan uses X-rays and a computer to build up a picture of the inside of a patient's body.
The table shows the radiation dose received by two different parts of a patient's body when having CT scans.
If a patient has a CT scan oh their chest, they are four times more likely to suffer damage to their genes (and their added risk of harm is four times higher) than if they had a head scan.
Lenses form images by…? What does a convex lens do & cause? What does a concave lens do & cause?
Lenses form images by refracting light and changing its direction. There are two main types of lens - convex and concave. They have different shapes and have opposite effects on light rays.
A convex lens bulges outwards. It causes rays of light parallel to the axis to be brought together (converge) at the principal focus.
A concave lens caves inwards. It causes parallel rays of light to spread out (diverge).
The axis of a lens is…? The principal focus of a convex lens is…? The principal focus of a concave lens is…? There is a principal focus on…?
The axis of a lens is a line passing through the middle of the lens.
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 axis appear to all come from - you can trace them back until they all appear to meet up at a point behind the lens.
There is a principal focus on each side of the lens. The distance from the centre of the lens to the principal focus is called the focal length.
What are the 3 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 3 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 (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.
What’s a real image? What’s a virtual image? When you look in a mirror you see? You can get a virtual image when looking at?
A real image is where the light from an object comes together to form an image on a 'screen' - like the image formed on an eye's retina (the 'screen' at the back of an eye).
A virtual image is when the rays are diverging, so the light from 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 the object (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.
What 3 things do you need to say when describing an image properly?
How big it is compared to the object;
Whether it's upright or inverted (upside down) relative to the object;
Whether it's real or virtual.
How do you draw a ray diagram from an image through a convex lens?
Pick a point on the top of the object. Draw a ray going from the object to the lens parallel to the axis of the lens.
Draw another ray from the top of the object going right through the middle of the lens.
The incident ray that's parallel to the axis is refracted through the principal focus (F) on the other side of the lens. Draw a refracted ray passing through the principal focus.
The ray passing through the middle of the lens doesn't bend.
Mark where the rays meet. That's the top of the image.
Repeat the process for a point on the bottom of the object. When the bottom of the object is on the axis, the bottom of the image is also on the axis.
An object at 2F will produce…? Between F and 2F it'll make a…? An object hearer than F will
make a…?
An object at 2F will produce a real, inverted image the same size as the object, and at 2F.
Between F and 2F it'll make a real, inverted image bigger than the object, and beyond 2F.
An object hearer than F will make a virtual image the right way up, bigger than the object, on the same side of the lens.
How do you draw a ray diagram for an image through a concave lens?
Pick a point on the top of the object. Draw a ray going from the object to the lens parallel to the axis of the lens.
Draw another ray from the top of the object going right through the middle of the lens.
The incident ray that's parallel to the axis is refracted so it appears to have come from the principal focus. Draw a ray from the principal focus. Make it dotted before it reaches the lens.
The ray passing through the middle of the lens doesn't bend.
Mark where the refracted rays meet. That's the top of the image.
Repeat the process for a point on the bottom of the object. When the bottom of the object is on the axis, the bottom of the image is also on the axis.
What will a concave lens always create?
Unlike a convex lens, a concave lens always produces a virtual image. The image is the right way up, smaller than the object and on the same side of the lens as the object - no matter where the object is.
How do Magnifying glasses work? The object being magnified must? Since the image produced is a..? What do you need to remember if the ask about virtual images? What’s the magnification formula?
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.
Magnification = image height/ object height (You can also find the magnification by dividing the distance between the image and the lens by the distance between the object and the lens.)
What’s the visible light spectrum? What does each colour have? What can colours also do? What’s created when all these colours mix?
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 wavelengths that we perceive as different colours.
Each colour has its own narrow range of wavelengths (and frequencies) ranging from violets down at 400 nm up to reds at 700 nm.
Colours can also mix together to make other colours. The only colours you can't make by mixing are the primary colours: pure red, green and blue.
When all of these different colours are put together, it creates white light.
Different objects absorb, transmit and reflect…? What’s opaque objects? The colour of an opaque object depends on…?
Different objects absorb, transmit and reflect different wavelengths of light in different ways
Opaque objects are objects that do hot 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. E.g. a red apple appears to be red because the wavelengths corresponding to the red part of the visible spectrum are most strongly reflected. The other wavelengths of light are absorbed.
For opaque objects that aren't a primary colour, they may be…? What do white objects do? What do black objects do?
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. So a banana may look yellow because it's reflecting yellow light OR because it's reflecting both red and green light.
White objects reflect all of the wavelengths of visible light equally.
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).
What do transparent & translucent objects do? some wavelengths of light may…?
Transparent (see-through) and translucent (partially see-through) 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 transparent or translucent object's colour is related to the wavelengths of light transmitted and reflected by it.
What are colour filters used for? What does a primary colour filter only transmit & example? what happens when you look at a blue object through a blue colour filter?
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 only transmits that colour, e.g. if 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.
What happens when you look at a red object through a blue colour filter? Filters that aren’t for primary colours let through?
However, if the object was e.g. red (or any 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.
What do all objects continually emit & absorb? The hotter an object is…? An object that’s hotter than its surroundings emits…?
All objects are continually emitting and absorbing infrared (IR) radiation. Infrared radiation is emitted from the surface of an object.
The hotter an object is, 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 (e.g. a cup of tea left on a table). And an object that's cooler than its surroundings absorbs more IR radiation than it emits as it warms up (e.g. a cold glass of water on a sunny day).
Objects at a constant temperature emit..? Some colours & surfaces absorb & emit…?
Objects at a constant temperature emit infrared radiation at the same rate that they are absorbing it.
Some colours and surfaces absorb and emit radiation better than others. For example, 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.
How can you investigate Emission with a Leslie cube?
A Leslie cube is a hollow, watertight, metal cube made of e.g. aluminium, whose four vertical faces have different surfaces (for example, matt black paint, matt white paint, shiny metal and dull metal). You can use them to investigate IR emission by different surfaces:
Place an empty Leslie cube on a heat-proof mat.
Boil water in a kettle and fill the Leslie cube with boiling water.
Wait a while for the cube to warm up, then hold a thermometer against each of the four vertical faces of the cube. You should find that all four faces are the same temperature.
Hold an infrared detector a set distance (e.g. 10 cm) away from one of the cube's vertical faces, and record the amount of IR radiation it detects.
Repeat this measurement for each of the cube's vertical faces. Make sure you position the detector at the same distance from the cube each time.
You should find that you detect more infrared radiation from the black surface than the white 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 you're doing this experiment. Don't try to move the cube when it's full of boiling water - you might burn your hands. And be careful if you're carrying a full kettle - your mate won't thank you if you spill boiling water into their bag (or down their back).
What’s a perfect black body? What do all objects emit? What do the intensity & distribution of wavelengths emitted depend on?
A perfect black body is an object that absorbs all of the radiation that hits it. No radiation is reflected or transmitted.
All objects emit electromagnetic (EM) radiation due to the energy in their thermal energy stores. This radiation isn't just in the infrared part of the spectrum - it covers a range of wavelengths and frequencies. Perfect black bodies are the best possible emitters of radiation.
The intensity and distribution of the wavelengths emitted by an object depend on the object's temperature. Intensity is the power per unit area, i.e. how much energy is transferred to a given area in a certain amount of time.
As the temperature of an object increases, the intensity…? The intensity increases more rapidly for…? The curves on the right show how the…?
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 (the wavelength with the highest intensity) to decrease.
The curves on the right show how the intensity and wavelength distribution of a black body depends on its temperature.
The overall temperature of earth depends on…? During the day, lots of radiation…?At night…?
The overall temperature of the Earth depends on the amount of radiation it reflects, absorbs and emits.
During the day, lots of radiation (like light) is transferred to the Earth from the Sun and absorbed. This causes an increase in local temperature.
At hight, less radiation is being absorbed than is being emitted, causing a decrease in the local temperature.
Overall, the temperature of the earth stays…? Changes to the atmosphere can cause a change to…?
Overall, the temperature of the Earth stays fairly constant. You can show the flow of radiation for the Earth on a handy diagram.
Changes to the atmosphere can cause a change to the Earth's overall temperature. 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.
Sound waves are caused by…? Sound generally travels faster in…? When a sound wave travels through a solid…?
Sound waves are caused by vibrating objects. These vibrations are passed through the surrounding medium as a series of compressions and rarefactions (sound is a type of longitudinal wave)
Sound generally travels faster in solids than in liquids, and faster in liquids than in gases.
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…? Sometimes the sound wave will eventually travel into…?
Sound can't travel in space, because it's mostly a vacuum (there are no particles to move or vibrate).
Sometimes the sound wave will eventually travel into someone's ear and reach their ear drum at which point they might hear the sound
Why does your ear drum vibrate? These vibrations are passed on to? The cochlea turn these vibrations into? Different materials can convert different frequencies of… & example? Human hearing is limited by the…?
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 semi-circular canals and to the cochlea.
The cochlea turns these vibrations into electrical signals which get sent to your brain and allow you to sense (i.e. hear) the sound.
Different materials can convert different frequencies of sound waves into vibrations. For example, humans can hear sound in the range of 20 Hz - 20 kHz. 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.
Sound waves will be reflected by… & what are echoes? Sound waves will also refract as they enter…?
Sound waves will be reflected by hard flat surfaces. Echoes are just reflected 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.)
Electrical devices can be made which produce electrical oscillations over… & what’s ultrasound?
Electrical devices can be made which produce 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 (i.e. frequencies above 20 000 Hz). This is called ultrasound and it pops up all over the place.
what’s partial reflection? What this means is that you can point a pulse of ultrasound at an…? What can be used to measure how far away the boundary is?
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.
(Medical imaging, e.g. pre-natal scanning of a foetus) Ultrasound waves can pass through the body, but whenever they reach…? The exact timing & distribution of these echoes r preocessed by..? Is an ultrasound safe?
Ultrasound waves can pass through the body, but whenever they reach a boundary between two different media (like fluid in the womb and the skin of the 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 imaging, e.g. finding flaws in materials) What can ultrasounds be used to find? What will happen to ultrasound waves entering a material? When will a wave be reflected sooner? What’s Echo sounding?
Ultrasound can also be used to find flaws in objects such as 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 as a crack inside the object, the wave will be reflected sooner.
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 have different properties depending on..? When a wave arrives at a boundary between materials what number of things can happen? What does studying properties & paths of waves through structures give you?
Waves have different properties (e.g. speed) depending on the material they're travelling through.
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 (like S-waves).
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 two good, well-known examples.
what does a earthquake produce? What do Seismologists work out? When seismic waves reach a boundary…? What happens most of the time when waves are refracted?
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 the properties change suddenly, the wave speed changes abruptly, and the path has a kink.
What are 2 different types of seismic waves? What are scientists able to work out by observing seismic waves?
There are two different types of seismic waves - P waves and S waves
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 observations.
Describe 3 things about P-waves inside the Earth?
P-waves are longitudinal.
They travel through solids and liquids.
They travel faster than S-waves.
Describe S-waves inside the earth?
S-waves are transverse and can't travel through liquids (or gases). They're slower than P-waves.
