Flashcards
Topic 6 - Waves
Cards in this deck (60)
- 1What direction do waves transfer energy?Waves transfer energy in the direction they are travelling. 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.
- 2What is the amplitude of a wave?The amplitude of a wave is the maximum displacement of a point on the wave from its undisturbed position.
- 3What is wavelength?Wavelength is the distance between the same point on two adjacent waves.
- 4What is wave frequency?Frequency is the number of complete waves passing a certain point per second. Frequency is measured in hertz (Hz). 1Hz=1 wave/sec.
- 5What is the equation for the period of a wave?T = 1/f
- 6What are the key points of transverse waves?In transverse waves, the oscillations are perpendicular to the direction of energy transfer (spring wiggled side to side). Including; all electromagnetic waves (light), ripples and waves in water, a wave on a string.
- 7What are the key points of longitudinal waves?In longitudinal waves, the oscillations are parallel to the direction of energy transfer (pushing spring). Including; sound waves in air (ultrasound), shock waves (some seismic waves).
- 8How do you measure the speed of sound?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 aligned on the display but have moved exactly one wavelength apart.
3. measure the distance between the microphones to find the wavelength. You can use the formula to find the speed of the sound waves, the frequency is just whatever you set the signal generator to. - 9What three things can happen to waves at a boundary between two different materials?1. the waves are absorbed by the material it is trying to cross into - this transfers energy to the materials energy stores.
2. the waves are transmitted - the waves carry on travelling through the new material, often leads to refraction.
3. the waves are reflected.
What happens depends on the wavelength of the wave and the properties of the materials involved. - 10What is the main rule for reflection?Angle of incidence=Angle of reflection
Angle of incidence is the angle between the incoming wave and the normal.
Angle of reflection is the angle between the reflected wave and the normal.
The normal is an imaginary line perpendicular to the surface at the point of incidence (dotted line). - 11Explain refraction?1. when a wave crosses a boundary between materials at an angle, it changes direction (refracted).
2. how much its refracted by depends on how much the wave speeds up or slows down, which usually depends on the density of the two materials (higher density, slower it travels).
3. If a wave crosses a boundary and slows down, it will bend towards the normal. If it speeds up it will bend away from the normal.
4. The wavelength of a wave changes when refracted but frequency stays the same. If the wave is travelling along the normal it will change speed but it's not refracted.
5. the optical density of a material is a measure of how quickly light can travel through it. - 12Explain how you can use transparent materials to investigate refraction?1. place a transparent rectangular block on a piece of paper and trace around it. Use a ray box to shine a ray 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 the incidence ray and the emerging point to show the path of the refracted ray through the block.
3. draw the normal perpendicularly at the point where the light ray entered the block. Measure the angle of incidence and the angle of refraction.
4. repeat this experiment using rectangular blocks made of different materials, keeping the incident angle the same throughout. - 13How do you measure the speed of water ripples using a lamp?1. dim the lights and turn on the lamp-wave patterns made by shadows of the wave crests on the screen below the tank.
2. 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.
3. use the formula to find wave speed. This set-up is suitable for investigating waves, because it allows you to measure the wavelength without disturbing the waves. - 14How do you use the wave equation for waves on strings?1. turn on the signal generator and vibration transducer, the string will start to vibrate.
2. you can adjust the frequency setting on the generator to change the length of the wave created. You should keep adjusting the frequency until there appears to be a clear wave on the string. This happens when a whole number of half-wavelengths fit exactly on the string. The frequency you will need will depend on the length of the string between the pulley and the transducer, and the masses you've used.
3. measure the wavelength by measuring all of the half wavelengths then dividing this by the total number of half wavelengths to get an average half wavelength. Then double this value to get a full wavelength.
4. The frequency of the wave is whatever the generator is set to, use the formula to find the speed of the wave. - 15Name some properties of electromagnetic waves?1. all EM waves are transverse.
2. all EM waves travel at the same speed through air or a vacuum.
3. EM waves form a continuous spectrum over a range of frequencies, they are grouped into seven basic types (based on wavelength and frequency). - 16Describe the patterns in the EM spectrum?Frequency increases as you go up the spectrum.
Wavelength decreases as you go up the spectrum. - 17Briefly explain how radio waves are made?1. EM waves are made up of oscillating electric and magnetic fields e.g. alternating currents are made of oscillating charges, as they oscillate, they produce oscillating electric and magnetic fields (EM waves).
2. the frequency of the waves produced is equal to the frequency of the alternating current.
3. to produce radio waves in a circuit using ac; the object in which charges (electrons) oscillate to create radio waves is called a transmitter. When transmitted radio waves reach a receiver, they are absorbed. The energy transferred by the waves to the electrons in the material of the receiver, causes the electrons to oscillate and if the receiver is part of a complete circuit, it generates alternating current (same frequency as the radio waves that generated it. - 18What are the uses of radio waves?Long-wave radio waves can be transmitted around the world because long wavelengths can diffract (bend) around the curved surface of Earth or over hills etc) so a direct line of sight is not needed.
Short-wave radio signals can be received at long distances but they are reflected from the ionosphere (charged layer in the atmosphere). And so can medium-wave but depends on atmospheric conditions and time of day.
TV and FM radio transmissions are very short wavelengths, need direct sight of the transmitter, doesn't travel far though buildings. - 19What are the uses of microwaves?For a satellite TV, the signal from a transmitter is transmitted to space ( need microwaves which pass easily through the watery atmosphere). Its picked up by the receiver dish and the satellite transmits the signal back to Earth in a different direction. Its received by a satellite dish on Earth (time delay as it has a long distance to travel).
In microwave ovens, the microwaves need to be absorbed by water molecules in food (different wavelength used). They penetrate a few cm into food before being absorbed and transferring the energy they are carrying to the water molecules in food, causing the water to heat up. The water molecules then transfer this energy to the rest of the molecules by heating, cooking the food. - 20What are the uses of infrared radiation?IR radiation is given out by all objects, the hotter the more it gives out.
Monitor temperature - 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 (hotter object, brighter picture).
Increase temperature - absorbing IR radiation causes objects to get hotter (used for cooking in a toaster). Electric heaters contain a long piece of wire that heats up when a current flows through it, eating lots of IR radiation (and some visible light-glowing). Energy transferred by the IR waves to the thermal energy stores of the objects causing the temp to increase. - 21What are the uses of visible light?Fibre optic cables - optical are thin glass or plastic fibres that can carry data (phones/computers) over long distances as pulses of visible light. 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 absorbed or scattered as it travels along a fibre.
- 22What are the uses of ultraviolet radiation?Fluorescent lights - fluorescence is a property of certain chemicals (layer of phosphorus in bulb), where UV radiation is absorbed and then visible light is emitted. They are energy efficient.
Security pens - mark property with your name (laptops), under UV light the ink will glow (fluoresce) but it's invisible otherwise.
UV beds - UV radiation is given out by sun causing suntans but under UV lams tanning can also happen, exposure to UV can be dangerous. - 23What are the uses of X-rays and gamma rays?X-ray photograph - X-rays pass easily through flesh but not so easily through denser material like bones or meta. So its the amount of radiation that's absorbed (or not ) that gives you an X-ray image.
Radiotherapy - Use X-rays and gamma rays to treat people with cancer, this is because high doses of these rays kill all living cells - so are directed towards cancer cells (avoids killing too many healthy cells).
Medical tracers - this is where a gamma-emitting source is injected into the patient, and its process is followed by the body, gamma can pass out of the body and be detected. - 24What precautions do radiographers do?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.
- 25Why is some EM radiation harmful to people?When EM radiation enters living tissue, it's often harmless but sometimes it causes harm depending on how much energy the wave transfers. Low-frequency waves (radio waves) don't transfer much energy and so mostly pass through soft tissue without being absorbed. High-frequency waves (UV, X-rays, gamma rays) all transfer lots of energy and so cause a lot of damage. UV damages surface cells causing sunburn and skin to age prematurely, blindness and increased risk of skin cancer. X-rays and gamma rays are types of ionising radiation ( enough energy to knock electrons off of atoms) causing gene mutation, cell destruction and cancer.
- 26How do you measure the risk of radiation?Radiation dose ( measured in sieverts - Sv) is a measure of the risk of harm from the body being exposed to radiation. The risk depends on the total amount of radiation absorbed and the type of radiation.
- 27What are all the different things on a ray diagram?1. Convex lens - curves outwards, causes rays of light parallel to the axis to converge at the principal focus.
Concave lens - curves inwards, causes rays of light parallel to the axis to spread out (diverge).
2. the axis of a lens is the line across the middle of the lens, called principle axis.
3. the principal focus of a convex lens is where rays hitting the lens parallel to the axis all meet.
4. the principal focus of a concave lens is the point where rays hitting the lens parallel to the axis appear to all come from, traced back to form a virtual image.
5. 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 focus length. - 28What are the three rules for refraction in a convex lens (arrows together)?1. an incident ray parallel to the axis refracts through the lens and passes through the principal focus on the other side.
2. an incident ray passing through the principal focus refracts through the lens and travels parallel to the axis.
3. an incident ray passing through the centre of the lens carries on in the same direction. - 29What are the three rules for refraction in a concave lens (arrows away)?1. an incident ray parallel to the axis refracts through the lens, and travels through the principal focus (so it appears to have come from the principal focus).
2. an incident ray passing through the lens towards the principal focus refracts through the lens and travels parallel to the axis.
3. an incident ray passing through the centre of the lens carries on in the same direction. - 30How do lenses produce real/virtual images?A real image is where light from an object comes together to form an image on a 'screen' - like the image formed on an eye's retina.
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 image seems to be behind the mirror.
To describe an image; how big it is (magnified/diminished), whether it's upright/inverted relative to the object and whether it's real or virtual. - 31How do you draw a ray diagram through a convex lens?1. draw a line from the top of the object to the lens parallel to the axis, then from this point draw a line through the principal focus and continue it for as long as can fit.
2. draw a line from the top of the object, through the middle of the lens and carry on as far as fits.
3. mark where the rays meet and draw the top of the image from the axis to this point if the bottom of the object is touching the axis, then so will the image. - 32How does distance from the lens affect the image?1. an object at 2F will produce a real, inverted image the same size as the object, and at 2F.
2. between F and 2F it'll make a real, inverted image bigger than the object, and beyond 2F.
3. an object nearer than F will make a virtual image that's upright, bigger than the object and on the same side of the lens. - 33How do you draw a ray diagram for an image through the concave lens?1. draw a line, parallel to the axis, from the top of the object to the middle of the lens. Then from this point draw a dotted-line to the principal focus, then extend it in full.
2. draw another line from the top of the object through the middle of the lens and extend it.
3. Mark where the line and the dotted line meet and then draw the image from the top at this point, if the bottom of the image is touching the axis then so will the bottom of the image.
Concave lenses always produce a virtual image. It is always upright, smaller than the object and on the same side of the lens as the object, no matter where it is. - 34How does a magnifying glass work?Uses convex lens to create a magnified virtual image. The object must be within the focal length. You can't project a virtual image onto a screen.
Magnification = image height/object height. - 35What is visible light?EM waves have a very large spectrum, we can only see a small part of the spectrum - visible light spectrum. Each colour has its own range of wavelengths (and frequencies) ranging from violets down at 400nm up to reds at 700nm. Different mixtures of these colours allow us to see even more shades, e.g. red and blue wavelengths seen together appear as purple. All of this put together is white light.
- 36How do we see the colour of an opaque object?1. different objects absorb, transmit and reflect different wavelengths of light in different ways. Opaque, transparent,translucent. 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 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.
- 37How do we see white and black objects?White objects reflect all of the wavelengths of visible light equally. Black absorbs all wavelengths of visible light.
- 38How do we see transparent or translucent objects?Transparent/translucent objects transmit light, e.g. 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/translucent objects. A transparent/translucent object's colour is related to the wavelengths of light transmitted and reflected by it.
- 39How do colour filters work?1. colour filters are used to filter out different wavelengths of light so only certain colours (wavelengths) are transmitted, the rest are absorbed. A primary colour filter will only transmit that colour, the rest will be absorbed. If you look at a blue object through a blue filter, it will still look blue as blue light is reflected from the object's surface and is transmitted by the filter. 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 would be absorbed by the filter. Filters that arent 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.
- 40When is infrared radiation (IR) emitted?All objects are continually emitting and absorbing infrared radiation. It's emitted from an object's surface. The hotter an object is, the more IR radiation it radiates at 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 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 they are absorbing it. Some colours and surfaces absorb and emit infrared radiation better than others.
- 41How do you investigate absorption with the melting wax trick?1. set up two ball bearings stuck to each side of the metal plate with solid pieces of candle wax and a bunsen in the middle. The other sides faced towards the flame (plates are placed the same distance away from the flame.
2. the sides of the plates that are facing the flame each have a different surface colour - one matt black and one silver.
3. the ball bearing on the black plate should fall first as the black surface absorbs more IR radiation-transferring more energy to the thermal energy store of the wax. This means the wax on the black plate melts first meaning the ball will drop first. - 42What is a Leslie cube?A Leslie cube is hollow, watertight, metal cube made of e.g. aluminium, whose four vertical faces have different surfaces (e.g one matt black, matt white, shiny metal, dull metal). Used for investigating IR radiation emitted by different surfaces.
- 43How do you investigate IR emission with a Leslie cube?1. place an empty Leslie cube on a heat-proof mat.
2. boil water in a kettle and fill leslie cube.
3. wait for cube to warm up, then hold a thermometer against each of the four vertical faces of the cube. All side should be same temp.
4. hold an IR detector a set distance (10cm) away from one of the cubes verticle faces, and record the amount of IR radiation it detects.
5. 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.
6. should be that you detect more IR radiation from the black surface than the white one, and more from the matt surfaces than the shiny ones.
7. repeat experiments more than once, to make results repeatable.
8. it's important to be careful when you're doing this experiment; burn hands. - 44What is a perfect black body?A perfect black body is an object that absorbs all of the radiation that hits it. No radiation is reflected or transmitted.
- 45How are perfect black bodies the best emitters of radiation?1. as good absorbers are also good emitters, perfect black bodies are the best possible emitters of radiation. (All objects emit electromagnetic 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.)
2. the intensity and distribution of wavelengths emitted by an object depend on the objects 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.
3. as the temperature of an object increases, the intensity of every emitted wavelength increases.
4. however, the intensity increases more rapidly for shorter wavelengths. This causes the peak wavelength (the wavelength with the highest intensity) to decrease.
5. the curves on the graph above show how the intensity and wavelength distribution of a black body depends on its temperature. - 46What is sound?Sound waves are caused by vibrating objects. These vibrations are passed through the surrounding medium as a series of compressions and rarefactions (longitudinal wave). Sound generally travels faster in solids than liquids, and faster in liquids than in gases. When sound waves travel through a solid it does so by causing the particles to vibrate (not in the vacuum).
- 47How do you hear sounds?Sound waves cause the eardrum to vibrate.
vibrations passed on to tiny bones called ossicles, through the semicircular canals and to the cochlea.
the cochlea turns the vibrations into electrical signals which are sent to the brain and allow you to hear the sound. - 48How is human hearing limited?Different materials can convert different frequencies of sound waves into vibrations, e.g. humans can hear sound in the range of 20Hz-20kHz. Human hearing is limited by the size and shape of our eardrum, as well as the structure of all the parts within the ear that vibrate to transfer energy from the sound wave.
- 49How are echoes formed?Sound waves will be reflected by hard flat surfaces (echoes). Sound waves will also refract as they enter different media. As they enter denser material, they speed up because its wavelength changes, but its frequency remains the same, so its speed must also change.
- 50What is the frequency of ultrasound?Ultrasound is sound with frequencies higher than 20,000Hz.
- 51What is partial reflection?When a wave passes from one medium to another, some of the waves are reflected off the boundary between the two media, and some are transmitted (and refracted). This happens for ultrasound waves. This means that you can point a pulse of ultrasound at an object, and wherever there are boundaries between one substance and another, some 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.
- 52Briefly explain the uses of ultrasound?1. US waves can pass through the body, but whenever they reach a boundary between two different media some is reflected back and detected, the timing and distribution of these echoes are processed into an image of the foetus.
2. Used to find flaws in objects like pipes or materials, usually reflected from far side but if there is a crack it will be reflected sooner.
3. by boats and submarines to find the depth of water. - 53How are seismic waves formed?When there is an earthquake or explosion, it produces seismic waves which travel through the earth. Seismologists use seismometers to detect. When these waves reach a boundary between different layers of material inside the earth, some waves are absorbed and some will be refracted. Usually, if the waves are refracted they change speed gradually, resulting in a curved path. But when the properties change suddenly, the speed changes suddenly and there's a kink. By observing the way these waves are affected we have been able to work out where the properties of the earth change dramatically.
- 54What are the two types of wave?P-wave - can travel through earth's core, longitudinal, travel through solids and liquids, faster than S-waves.
S-wave - cannot travel through the earth's core, transverse, can't travel through liquids or gases, slower than P-waves. - 55What is a 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.
- 56What is 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 the angle of incidence is different for each ray. The rule still applies. When light is reflected by a rough surface, the surface appears matte and you don't get a clear reflection of objects.
- 57How would you investigate how different surfaces reflect light?1. draw straight line across paper and place object so one of its sides lines up with this line.
2. shine a ray of light at the object's surface and trace the incoming/reflected light beams.
3. draw a normal at the point where the ray hits the object. Measure and record the angle of incidence and reflection. Also, make note of the width and brightness of the reflected light ray. Repeat this experiment for a range of objects. Smooth surface = clear reflections, rough surfaces = diffuse reflection (wider and dimmer). - 58How does radiation affect the Earth's temperature?In the day, lots of radiation (e.g.light) is transferred to the earth from sun and absorbed causing an increase in local temp. At night, less is absorbed than is emitted causing a decrease in local temp. Changes to atmosphere an cause a change in 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).
- 59How can waves be used to detect and explore?When a wave arrives at a boundary between materials;
it can be completely/partially reflected, speed altered, refracted or absorbed. From this, we can tell some of the properties of the structure. - 60How do you convert GHz to Hz?1 GHz = 1 x 10^9 Hz