Topic 6 - EM waves, basics, seismic waves Flashcards
(58 cards)
1
Q
What do waves do?
A
waves transfer energy from one place to another without transferring matter
2
Q
What is the AMPLITUDE of a wave?
A
the maximum displacement of a point on the wave from its undisturbed position
3
Q
What is the WAVELENGTH of a wave?
A
distance between the same point of two adjacent waves
4
Q
What is the PERIOD of a wave?
A
amount of time it takes for a full cycle of the wave
5
Q
What is the FREQUENCY of a wave?
A
- number of complete waves passing a certain point per second
- measured in hertz (Hz)
6
Q
Describe transverse waves?
A
oscillations are perpendicular (90 degrees) to the direction of energy transfer
7
Q
Types of transverse wave?
A
- electromagnetic waves
- ripples and waves in water
- wave on a string
8
Q
Describe longitudinal waves?
A
the oscillations are parallel to the direction of energy transfer
9
Q
Types of longitudinal wave?
A
- sound waves in air, ultrasound
- shock waves, e.g. some seismic waves
10
Q
What are sound waves?
A
- sound waves are caused by vibrating objects - vibrations passed through the surrounding medium as a series of compressions and rarefactions
- sound is a type of longitudinal wave
- when a sound wave travels through a solid it does so by causing the particles in the solid to vibrate
11
Q
Can sound travel through space?
A
sound cannot travel through space because it is mostly a vacuum (there are no particles to vibrate)
12
Q
How do you hear sound?
A
- you hear sound when your eardrum vibrates - sound waves cause your eardrum to vibrate
- vibrations are passed on to tiny bones in your ear called ossicles - through the semicircular canals and to the cochlea
- cochlea turns these vibrations into electrical signals - get sent to your brain and allow you to sense the sound
13
Q
What is echosounding?
A
- 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
- 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
14
Q
What happens to ultrasound at boundaries?
A
- ultrasound waves get partially reflected at boundaries - when a wave passes from one medium into another -some of the wave is reflected off boundary between two media - some is transmitted (and refracted) - this is partial reflection
- wherever there are boundaries between one substance and another - some of the ultrasound gets reflected back
- time it takes for reflections to reach a detector can be used to measure how far away the boundary is
15
Q
What are uses of ultrasound?
A
- medical imaging - ultrasound waves can pass through body - whenever they reach 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
- exact timing and distribution of 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 usually reflected by far side of the material - if there is a flaw such as a crack inside the object - wave will be reflected sooner
16
Q
What are seismic waves?
A
energy transferred from an earthquake produces shock waves called siesmic waves
17
Q
What provides information about the structure of the earth?
A
study of seismic waves
18
Q
Where are earthquakes generated?
A
earths crust - solid layer of rock about 50km thick that surrounds a much thicker layer of molten rock called the mantle
19
Q
Where is the point that an earthquake originates from?
A
the focus
20
Q
What are primary waves (P-waves)?
A
- cause the initial tremors lasting about one minute
- are longitudinal
- can travel through liquids and solids
- travel faster than S-waves
21
Q
What are secondary waves (S-waves)?
A
- cause more tremors a few minutes later
- are transverse
- can’t travel through liquids or gases - can’t travel through the liquid outer core
- slower than P-waves
22
Q
What happens to P and S wave as they travel through the mantle?
A
they bend as they travel through the mantle - their speed changes gradually with depth - so their direction changes - resulting in curved path
23
Q
What happens to P-waves when they reach the mantle and outer core boundary?
A
they refract because their speed changes abruptly at the boundary
24
Q
What happens to S-waves when they reach the mantle and outer core boundary?
A
s-waves are transverse - they cannot travel through the liquid outer core
25
What are shadow zones?
areas on the earth's surface where seismic waves are not detected after an earthquake
26
Where are shadow zones?
usually from around 105 degrees to 142 degrees from the earthquakes epicenter
27
Why can S-waves not reach the shadow zone?
- s-waves cannot travel through the other core - they are transverse waves and can't travel through liquid
- s-waves are completely blocked by the outer-core
28
Why can P-waves not reach the shadow zone?
- p-waves refract at 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
29
What type of wave are electromagnetic waves?
transverse
30
How does frequency change from left to right of the EM spectrum?
frequency increases from left to right
31
How does wavelength change from left to right of the EM spectrum?
wavelength decreases from left to right
32
What are the waves from left to right on the EM spectrum?
- radio waves
- microwaves
- infrared
- visible light
- ultraviolet
- x-waves
- gamma rays
33
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
34
How do satellites use microwaves?
- microwaves are used by satellites
- communication to and from satellites uses microwaves - they pass easily through earths watery atmosphere
- for satellite TV - signal from a transmitter is transmitted into space
- picked up by satellite receiver dish orbiting earth - satellite transmits signal back to earth in different direction
- received by satellite dish on ground - slight time delay between signal being sent and received due to the long distance the signal has to travel
35
How do ovens use microwaves?
- in microwave ovens - microwaves are absorbed by water molecules in food
- 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
- water molecules transfer energy to rest of the molecules in the food by heating - quickly cooks the food
36
How does micro and radio wave properties change with wavelegth?
- shorter the wavelength - more information they can carry
- shorter the wavelength - shorter their range (due to increasing absorption by the atmoshphere)
- shorter the wavelength - less they spread out
37
How are radio waves made?
- alternating currents are made up of oscillating charges
- as the charges oscillate they produce electromagnetic waves
- frrequency of waves produced are equal to frequency of the AC
- object in which charges oscillate to create radio waves is called a transmitter
38
What happens when transmitted radio waves reach a receiver?
- when transmitted radio waves reach receiver - radio waves are absorbed
- energy carried by waves transferred to electrons in material of the receiver
- this energy causes the electrons to oscillate - if receiver is part of complete electrical circuit - it generates alternating current
- current has same frequency as radio wave that generated it
39
How are radio waves used?
mainly used for communication
40
How can long-wave radio be received?
- long wavelengths diffract around curved surface of the earth
- makes it possible for radio signals to be received even if receiver isn't in line of sight of transmitter
41
How can short-wave radio signals be received?
- can be received at long distances
- are reflected from ionosphere - electrically charged layer in earths upper atmosphere
42
How does bluetooth work?
b uses short-wave radio waves to send data over short distances between devices without wires
43
What type of radio wave is used for TV and FM radio?
radio waves with very short wavelengths
44
How do infrared cameras work?
- IR cameras used to detect IR radiation and monitor temperature
- camera detects IR radiation - turns it into an electrical signal - displayed on a screen as picture
- hotter object is - brighter it appears
45
What is infrared radiation?
- IR radiation given out by all hot objects
- hotter the object - more IR radiation it gives out
46
How can IR radiation be used to cook food?
- absorbing IR radiation causes objects to get hotter
- food can be cooked using IR radiation - temperature of food increases when it absorbs IR radiation
47
How do electric heaters work?
- electric heaters contain a long piece of wire that heats up when a current flows through it
- wire emits lots of IR radiation
- emitted IR radiation absorbed by objects and air in the room - energy transferred by the IR waves to thermal energy stores of the objects - causing their temperature to increase
48
What are optical fibres?
thin glass or plastic fibres that can carry data over long distances as pulses of visible light
49
How to optical fibres work?
- work because of reflection
- light waves are bounced back and forth until they reach end of the fibre
- visible light is used in optical fibres
- light is not easily absorbed or scattered as it travels along a fibre
50
What is fluorescence?
fluorescence is a property of certain chemical where UV radiation is absorbed and then visible light is emitted
51
How do fluorescent lights work?
- they 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
52
What are security pens used for?
- can be used to mark property with your name - under UV light the ink will glow (fluoresce), but it is invisible otherwise
53
How are X rays and gamma rays used?
in medicine
54
How are X-rays used to give you an X-ray image?
- x-rays pass easily through flesh but not easily though denser materials likes bones or metal
- it is the amount of radiation absorbed, or not absorbed, that gives you the X-ray image
55
How are X-rays and gamma rays used in medicine?
- radiographers use X-rays and gamma rays to treat people with cancer (radiotherapy)
- high doses of these rays kill all living cells - they are carefully directed towards cancer cells, to avoid killing too many normal, healthy cells
56
Why are high-frequency waves dangerous?
they all transfer lots of energy and so can cause lots of damage
57
Why can gamma rays and X-rays be dangerous?
- they are ionising radiation
- can cause gene mutation or cell destruction and cancer
58
Why are UV rays dangerous?
- damages surfaces cells - can lead to sunburn + cause skin to age prematurely
- more serious effects are blindness and increased risk of skin cancer
