TOPIC 6 - WAVES Flashcards
What way do waves transfer energy?
The direction they are travelling
What happens when waves travel through a medium?
The particles of the medium oscillate and transfer energy between each other. But overall, the particles stay in the same place - only energy is transferred.
What is the amplitude?
The maximum displacement of a point on the wave from its undisturbed position.
What is the wavelength?
The distance between the same point on two adjacent waves. E.g. through the trough of one wave and the trough of the wave next to it.
What is the frequency?
The number of complete waves passing a certain point per second. Frequency is measured in hertz (Hz). 1 Hz is one wave per second.
What equation links period and frequency?
period = 1/frequency
What are transverse waves?
The oscillations are perpendicular to the direction of energy transfer.
Give examples of transverse waves
all electromagnetic waves
ripples and waves in water
a wave on a string
What are longitudinal waves?
The oscillations are parallel to the direction of energy transfer.
Give examples of longitudinal waves
Sound waves eg ultrasound
Shock waves eg some seismic waves
What is the equation for wave speed?
Wave speed = frequency x wavelength.
What is used to measure the speed of sound?
Oscilloscope.
Set up an oscilloscope so the detected waves at each microphone are shown as separate waves.
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.
Measure the distance between the microphones to find one wavelength.
How can you measure the speed of water ripples?
Using a lamp
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.
The distance between each shadow line is equal to one wavelength.
Measure the distance between the shadow lines that are 10 wavelengths apart, then divide the distance by 10 to find the average wavelength.
If it is difficult to measure the distance, then a photo can be taken.
What do you use for waves on strings?
Vibration transducer.
What happens when a wave arrives at a boundary?
The waves are absorbed by the material - this transfers energy to the material’s energy stores.
The waves are transmitted - the waves carry on travelling through the new material, this often leads to refraction.
The waves are reflected.
What happens when a wave is reflected?
the angle of incidence = angle of reflection
What is the angle of incidence?
The angle between the incoming wave and the normal.
What is the angle of reflection?
The angle between the reflected wave and the normal.
What is the normal?
An imaginary line that is perpendicular to the surface at the point of incidence. (the point where the wave hits the boundary).
The normal is usually drawn as a dotted line.
What is 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.
What is diffuse reflection?
When a wave is reflected by a rough surface eg a piece of paper, and the reflected waves 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 of angle of incidence = angle of reflection still applies.
What happens when light is reflected by a rough surface?
The surface appears matte and you don’t get a clear reflection of objects.
What type of waves are electromagnetic waves?
Transverse. They transfer energy from a source to an absorber.
All EM waves travel at the same speed through air or a vacuum (space).
EM waves form a continuous spectrum over a range of frequencies.
List the EM spectrum, in order of increasing frequency/decreasing wavelenth.
Radio waves Micro waves Infra red Visible light Ultra violet X-rays Gamma rays
How are EM waves generated?
By a variety of changes in atoms and their nuclei. Eg changes in the nucleus of an atom creates gamma rays.
What is refraction?
Waves changing direction at a boundary.
What does the amount of refraction depend on?
How much the wave speeds up or slows down, which usually depends on the density of the two materials. (the higher the density of the material, the slower the wave will travel through it).
What happens when a wave crosses a boundary and slows down?
It will bend towards the normal.
What happens when a wave crosses a material and speeds up?
It will bend away from the normal.
What is optical density?
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 travel through it.
How are radio waves made?
Oscillating charges. EM waves are made up of oscillating electric and magnetic fields. As the charges in an alternating current 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.
You can produce radio waves using an alternating current in an electrical circuit.
What is a transmitter?
The object in which charges (electrons) oscillate to create radio waves. When transmitted radio waves reach a receiver, the radio waves are absorbed.
What is a receiver?
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 waves that generated it.
What are radio waves used for?
Mainly used for communication.
Long wave radio diffract (bend) around curved surfaces of the earth. They can also diffract around hills and into tunnels.
Short wave radio signals can be received at long distances from the transmitter. That is because they are reflected from the ionsphere - an electrically charged layer in the Earth’s upper atmosphere. (doesn’t bend - needs direct sight of the transmitter)
Bluetooth, TV.
What are microwaves used for?
Satellites - 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 is picked up by the satellite receiver dish orbiting above the earth. The satellite transmits the signal back to Earth in a different direction. Then, it is 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 is the use of infra red radiation?
Can be used to increase or monitor temperature.
Infra red radiation is given out by all hot objects.
Infra red cameras can be used to detect infra red 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.
Food can be cooked using IR radiation.
Electric heaters
What are fibre optic cables?
Use visible light to transmit data.
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. 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.
What are flourescent lights?
They generate UV radiation, which is absorbed and re-admitted as visible light by a layer of phosphorus on the inside of the bulb. They are energy-efficient.
What are the uses of x-rays and gamma rays?
Used in medicine.
Explain x-rays
Pass easily through flesh, but not so easily through denser materials like bones or metal. So, it is the amount of radiation that is absorbed (or not absorbed) that gives you an x-ray image.
Explain gamma rays
Radiotherapy, to treat people with cancer. This is because high dose of these rays kill all living cells.
Also 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. It can pass through the body to be detected.
How do radiographers protect themselves?
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 a minimum.
What waves are more damaging to human tissue?
High frequency waves, with lots of energy, like UV, X-rays and gamma rays.
X-rays and gamma rays are types of ionising radiation. This means that they carry enough energy to knock electrons off atoms. This can cause gene mutation or cell destruction, and cancer.
What is radiation dose measured in?
Siervets. It is a measure of the risk of harm from the body being exposed to radiation. It is not a measure of the total amount of radiation that has been absorbed.
How do lenses form images?
By refracting light.
What is a convex lens?
Bulges outwards. It causes rays of light parallel to the axis to be brought together (converge) at the principle focus.
What is a concave lens?
Caves inwards. It causes parallel waves of light to spread out (diverge).
What is the axis of a lens?
A line passing through the middle of the lens.
What is the principle focus of a convex lens?
Where rays hitting the lens parallel to the axis all meet.
What is the principle focus of a concave lens?
The point where rays hitting the lens parallel to the axis appear to all come from. You can trace them back until they appear to meet up at a point behind the lens.
What is the focal length?
There is a principle focus on each side of the lens. The distance from the centre of the lens to the principle focus is called the focal length.
What are the rules for refraction in a convex lens?
An incident ray parallel to the axis refracts through the lens and passes through the principle focus on the other side.
An incident ray passing through the principle focus refracts through the lens and travels parallel to the axis.
An incident ray passing through the centre of the lens carried on in the same direction.
What are the rules for refraction in a concave lens?
An incident ray parallel to the axis refracts through the lens, and travels in line with the principle focus (so it appears to have come from the principle focus).
An incident ray passing through the lens towards the principle 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 is a real image?
A real image is where the light from an object comes together to form an image on a ‘screen’. Like the image formed on the eye’s retina.
What is a virtual image?
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 the mirror, you see a virtual image of your face, because the object appears to be behind the mirror.
You can get a virtual image when looking through a magnifying glass.
You can’t project a virtual image onto a screen.
What lenses create virtual images always?
Concave lenses. 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.
What lenses do magnifying glasses use?
Magnifying glasses use convex lenses. They create a magnified, virtual image.
What is the magnification formula?
magnification = image height / object height.
What are the primary colours?
red, green and blue.
What are opaque objects?
Objects that do not transmit light. When visible light waves hit them, they absorb some wavelengths of the light and reflect others. The colour of an opaque object depends which wavelengths of the light are most strongly reflected.
What do transparent and translucent objects do?
They transmit light - some passes through. A transparent or translucent object’s colour is related to the wavelength of light transmitted and reflected by it.
What are colour filters?
Colour filters only let through particular wavelengths. They 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. If a red object was viewed through a blue filter, it would appear black. This is because 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 surfaces are best at emitting and absorbing infra red radiation?
Black and matte.
What is a Leslie cup?
A hollow, watertight, metal cube made of, for example, aluminium, whose four vertical faces have different surfaces (e.g. matt black paint, matt white paint, shiny metal and dull metal).
What is a perfect black body?
A perfect black body is an object that absorbs all of the radiation that hits it. No radiated is reflected or transmitted.
They are also the best possible emitters of radiation.
What is intensity?
The power per unit area i.e. how much energy is transferred to a given area in a certain amount of time.
What happens as the temperature of an object increases?
Shorter wavelength and higher intensity.
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.
How does radiation affect the world’s temperature?
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 the local temperature.
At night, less radiation is being absorbed than is being emitted, causing a decrease in local temperature.
Overall, the temperature of the Earth stays fairly constant. (Global warming can effect the amount emitted).
How are sound waves caused?
Caused by vibrating objects. These vibrations are passed though the surrounding medium as a series of compressions and rarefractions.
Sound is a type of longitudinal wave.
When do sound waves travel fastest?
Faster in solids than liquids, and faster in liquids than gases.
How does a sound wave travel through a solid?
When a sound wave travels through a solid it does so by causing the particles in the solid to vibrate.
Why can’t sound travel through space?
It is a vacuum, there are no particles to move or vibrate.
How do you hear sound?
When your ear drum vibrates.
Sound waves that reach your ear drum can cause it to vibrate.
These vibrations are passed to tiny bones in you 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.
What frequency range can humans hear?
20 Hz - 20 kHz.
When are sound waves reflected?
Sound waves will be reflected by hard flat surfaces. Echoes are just reflected sound waves.
When will sound waves refract?
Sound waves will also refract as they enter different media. As they enter denser material, they speed up.
What happens when a wave travels into a different medium?
Its wavelength changes but its frequency stays the same, so its speed must also change.
What frequency are ultra sound waves?
Frequencies higher 20 000 Hz.
What are ultrasound waves?
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.
What happens to ultrasound waves at boundaries?
They get partially reflected. When a wave passes from one medium to another, some of the wave is reflected off the boundary between the two media, and some is transmitted (and refracted). This means 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.
How are ultrasounds used in medical imaging?
E.g. pre-natal scanning of a foetus.
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 f 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.
How is ultrasound used in industrial imaging?
E.g. finding flaws in materials.
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 far side of the material.
If there is a flaw such as a crack inside the object, the wave will be reflected sooner.
What is echo sounding?
Echo sounding uses high frequency sound waves (including ultrasound). It is used by boats and submarines to find out the depth of the water they are in or to locate objects in deep water.
What types of waves do earthquakes and explosions cause?
Seismic Waves.
How do you detect seismic waves?
Seisometers.
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.
What happens when a seismic wave reaches a boundary?
When a seismic reaches a boundary between different layers of material inside the Earth, some waves will be absorbed and some will be refracted.
Most of the time, if the wave 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 waves can travel through the Earth’s Core?
P-waves can travel through the Earth’s core, S-waves can’t.
What are the two types of seismic waves?
P-waves and S-waves.
What are P-waves?
Can travel through the Earth’s core.
Longitudinal.
They travel through solids and liquids.
They travel faster than S-waves.
What are S-waves?
Can’t travel through the Earth’s core.
They are transverse and can’t travel through liquids (or gases). They are slower than P-waves.
What does observing seismic waves allow scientists to do?
By observing how seismic waves are absorbed and refracted, scientists have been able to work out where properties of the Earth change dramatically.
