Topic 6 - Waves Flashcards
What do waves do?
- Waves transfer energy from one place to another wihtout transferring any matter.
What is the AMPLITUDE of a wave?
- The maximum displacement of a point on the wave from its undisturbed position.
What is the WAVELENGTH of a wave?
- The distance between the same point of two adjacent waves.
What is the FREQUENCY of a wave?
- The number of complete waves passing a certain point per second.
- It is measured in hertz (Hz).
What is the PERIOD of a wave?
- The amount of time it takes for a full cycle of the wave.
How to calculate frequency?
Frequency = 1 / Period
What are the two types of wave?
- Transverse and longitudinal
What kind of vibrations do transverse waves have?
- Sideways vibrations
Describe transverse waves?
- The oscillations are perpendicular (90 degrees) to the direction of energy transfer.
Types of transverse wave?
- Electromagnetic waves
- Ripples and waves in water
- A wave on a string
What kind of vibrations do longitudinal waves have?
- Parallel vibrations
Describe longitudinal waves?
- The oscillations are parallel to the direction of energy transfer.
Types of longitudinal wave?
- Sound waves in air, ultrasound.
- Shock waves, e.g. some seismic waves.
What is the formula for wavespeed?
- Wave speed = frequency x wavelength.
What are sound waves?
- 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.
Can sound travel through space?
- No. Sound cannot travel through space because it is mostly a vacuum (there are no particles to vibrate).
How do you hear sound?
- You hear sound when your eardrum vibrates. Sound waves cause your eardrum 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 the sound.
What is the range of sound for humans?
- 20Hz - 20kHz
What is echosounding?
- Echo sounding uses pulses of high-frequency sound waves to detect objects in deep water and to measure water depth.
- An echo is the reflection of soundwaves from a smooth surface.
- The time between a pulse of sound being transmitted and detected and the speed of sound in water can be used to calculate the distance of the reflecting surface or object.
How to calculate depth of water?
- 0.5 x vt (1/2 x the wave speed x the time)
What is ultrasound?
- Ultrasound is sound with frequencies higher than 20,000 Hz
What happens to ultrasound at boundaries?
- Ultrasound waves get partially reflected at boundaries. 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.
- This means that 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.
What are uses of ultrasound?
- Medical imaging. Ultrasound waves can pass through the body, but whenever they reach a boundary between two different media (like the 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.
- Industrial imaging. Ultrasound can also be used to find flaws in objects such as pipes or materials like wood and 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.
What are seismic waves?
- The energy transferred from an earthquake produces shock waves called siesmic waves.
What provides information about the structure of the earth?
- The study of seismic waves.
Where are earthquakes generated?
- The earths crust. This is a solid layer of rock about 50km thick that surrounds a much thicker layer of molten rock called the mantle.
Where is the point that an earthquake originates from?
- The focus
What are primary waves (P-waves)?
- Primary waves cause the initial tremors lasting about one minute.
- P-waves are longitudinal.
- They can travel through liquids and solids.
- They travel faster than S-waves.
What are secondary waves (S-waves)?
- S-waves cause more tremors a few minutes later.
- S-waves are transverse.
- They can’t travel through liquids or gases, so they can’t travel through the liquid outer core.
- They are slower than P-waves.
What happens to P and S wave as they travel through the mantle?
- They bend as they travel through the mantle. This is because their speed changes gradually with depth, and so their direction changes, resulting in a curved path.
What happens to P-waves when they reach the mantle and outer core boundary?
- They refract because their speed changes abruptly at the boundary.
What happens to S-waves when they reach the mantle and outer core boundary?
- S-waves are transverse - they cannot travel through the liquid outer core.
What are shadow zones?
- Areas on the earth’s surface where seismic waves are not detected after an earthquake.
Where are shadow zones?
- Usually from around 105 degrees to 142 degrees from the earthquakes epicenter.
Why can P-waves not reach the shadow zone?
- P-waves are at the boundary between the mantle and the outer core when the waves enter the core and leave the core.
- This refraction bends the waves away from the shadow zone.
Why can S-waves not reach the shadow zone?
- S-waves cannot travel through the other core because they are transverse waves and can’t travel through a liquid.
- The S-waves are completely blocked by the outer-core.
What type of wave are electromagnetic waves?
Transverse
How does frequency change from left to right of the EM spectrum?
Frequency increases from left to right
How does wavelength change from left to right of the EM spectrum?
Wavelength decreases from left to right
What are the waves from left to right on the EM spectrum?
- Radio waves
- Microwaves
- Infrared
- Visible light
- Ultraviolet
- X-waves
- Gamma rays
What speed to EM waves travel at?
- EM waves travel at 3 x 10^8 m/s
- All EM waves travel at the same speed through air or a vacuum
How do satellites use microwaves?
- Microwaves are used by satellites
- Communication to and from satellites uses microwaves. This is because microwaves pass easily through the earth’s watery atmosphere.
- For satellite TV, the signal from a transmitter is transmitted into space.
- Then it is picked up by the satellite receiver dish orbiting earth. The satellite transmits the signal back to earth in a different direction.
- Here 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.
How do ovens use microwaves?
- In microwave ovens, the microwaves are absorbed by water molecules in food
- The microwaves penetrate up to a few centimeters in 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 the energy to the rest of the molecules in the food by heating - this quickly cooks the food
How does micro and radio wave properties change with wavelegth?
- The shorter the wavelength, the more information they can carry
- The shorter the wavelength, the shorter their range (due to increasing absorption by the atmoshphere)
- The shorter the wavelength, the less they spread out
How are radio waves made?
- You can produce radio waves using an alternating current in an electrical circuit. The object in which charges 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
How are radio waves used?
Radio waves are mainly used for communication
How can long-wave radio be received?
- Long-wave radio (wavelengths of about 1-10km) can be transmitted and received halway around the world
- This is because long wavelengths diffract around the curved surface of the earth
- This makes it possible for radio signals to be received even inf the receiver isn’t in line of sight of the transmitter
How can short-wave radio signals be received?
- They can also be received at long distances like long-wave radio signals
- This is because they are reflected from the ionosphere
How does bluetooth work?
- Bluetooth uses short-wave radio waves to send data over short distances between devices without wires
What type of radio wave is used for TV and FM radio?
Radio waves with very short wavelengths
What is infrared radiation?
- IR radiation is given out by all hot objects
- The hotter the object, the more IR radiation it gives out
How do infrared cameras work?
- Infrared cameras can be used to detect IR radiation and monitor temperature
- The camera detects the IR radiation and turns it into an electrical signal, which is displayed on a screem as a picture
- The hotter an object is, the brighter it appears
How can IR radiation be used to cook food?
- 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
How do electric heaters work?
- Electric heaters contain a long piece of wire that heats up when a current flows through it
- This wire emits lots of IR radiation
- The emitted IR radiation is absorbed by objects and the air in the room and energy is transferred by the IR waves to the thermal energy stores of the objects, causing their temperature to increase.
What are optical fibres?
- Optical fibres are thin glass or plastic fibres that can carry data over long distances as pulses of visible light
How to optical fibres work?
- They work because of reflection
- The light waves are bounced back and forth until they reach the end of the fibre
- Visible light is used in optical fibres
- Light is not easily absorbed or scattered as it travels along a fibre
What is fluorescence?
Fluorescence is a property of certain chemicals, where UV radiation is absorbed and then visible light is emitted
How do fluorescent lights work?
- 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 are energy efficient so they are good to use when light is needed for long periods
What are security pens used for?
- Security pens can be used to mark property with your name. Under UV light the ink will glow (fluoresce), but it is invisible otherwise
What radiation gives people suntan?
UV radiation is produced by the sun and exposure to it is what gives people a suntan
How are X rays and gamma rays used?
In medicine
How are X-rays used to give you an X-ray image?
- X-rays pass easily through flesh but not so easily though denser materials likes bones or metal
- This means it is the amount of radiation absorbed, or not absorbed, that gives you the X-ray image
How are X-rays and gamma rays used in medicine?
- 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
Why are high-frequency waves dangerous?
They all transfer lots of energy and so can cause lots of damage
Why can gamma rays and X-rays be dangerous?
They are ionising radiation
What three things can happen when a wave reaches a boundary between two different materials?
- The waves are absorbed by the material the wave is trying to cross into - this transfers energy to the materials 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 is the law of reflection?
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?
The normal is an imaginary line that is perpendicular to the surface at the point of incidence
What is specular reflection?
Specular reflection happens when a wave is reflected in a single direction by a smooth surface
What is diffuse reflection?
- Diffuse reflection is when a wave is reflected by a rough surface and the reflected waves 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
- When light is reflected by a rough surface, the surface appears matte, and you dont get a clear reflection of objects
What happens when a wave crosses a boundary between materials at an angle?
It changes direction - it is refracted
What affects how much a wave is refracted?
- How much the wave speeds up or slows down
- This usually depends on the density of the two materials
How does density affect refraction?
- 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 a wave crosses a boundary and speeds up, it will bend away from the normal
How do wavelength and frequency change when a wave is refracted?
- The wavelength changes
- The frequency stays the same
What is optical density?
A measure of how quickly light can travel through a material - the higher the optical density, the slower light waves travel through it
