waves (p6) Flashcards
doesn't include ray diagrams or wave front diagrams
1
Q
what are the two categories all waves fall into?
A
transverse or longitudinal
2
Q
what are the two graphs used to show waves?
A
- distance (how far the wave has travelled from its starting point) against displacement (how far from the equilibrium point the wave has oscillated)
- time period (could show time taken for one complete oscillation) against displacement
3
Q
what is an oscillation/vibration?
A
waves moving up and down
4
Q
what are transverse waves?
A
the oscillations (vibrations) are perpendicular to the direction of energy transfer
- the oscillations are up and down, therefore perpendicular to the direction of energy transfer (to the side)
- not all transverse waves require a medium to travel through
5
Q
which waves are transverse?
A
- all electromagnetic waves (radio, light)
- ripples (waves in water)
- waves of strings (e.g. on a guitar)
6
Q
what are longitudinal waves?
A
- the oscillations are parallel to the direction of energy transfer (travel side to side)
- made up of compressions (where the air particles are close together) and rarefactions (where the air particles are spaced out)
- longitudinal waves require a medium to travel in (air, liquid, solid), as they move by by vibrating particles, and in a vacuum, there are no particles to vibrate
- e.g. sound waves through air, which travel as particles in the air that move side to side
7
Q
what do waves do?
A
transfer energy from one place to another
- they do this by vibrating/oscillating
- they DON’T transfer matter
- sometimes, we can interpret this energy (e.g. light/sound waves) as meaningful information
8
Q
in both transverse and longitudinal waves, is it the wave or the air/water particles that are moving?
A
TRANSVERSE
the wave is moving, and the particles do oscillate up and down, but don’t travel through the medium
LONGITUDINAL
the wave is moving, and the particles do oscillate from side to side, but don’t travel through the medium
9
Q
what is the amplitude of a wave?
A
the distance from the centre line (the undisturbed position) to the crest or the trough (the furthest point the wave vibrates from its undisturbed position)
10
Q
what is the wavelength of a transverse wave?
A
the distance from one point on a wave to the equivalent point on an adjacent wave (e.g. crest to crest)
11
Q
what is the wavelength on a longitudinal wave?
A
measure from one compression to the next, or one rarefaction to the next
12
Q
what is the symbol for wavelength?
A
the greek letter lambda ‘λ’
13
Q
what is the wave frequency?
A
the number of waves passing a point each second (number of complete oscillations per second)
- unit is Hertz (Hz)
- 1 Hz = 1 wave per second
14
Q
what is the relationship between frequency and wavelength?
A
the longer the wavelength, the lower the frequency
15
Q
what is a period, and how do you calculate it?
A
the time (in seconds) for one wave to pass a point/for one complete oscillation
- period = 1 / frequency
- frequency = 1 / period
16
Q
what is the wave speed, and how do you calculate it?
A
the speed at which the wave moves through the medium (ie the speed at which energy is transferred)
wave speed = frequency x wavelength
v = f x λ
v = m/s
f = Hz
λ = m
17
Q
what method would you use to measure the speed of sound waves in the air?
A
- two people separated by 500m
- Person A holding a pair of cymbals, Person B holding a timer
- Person B starts timing when they see Person A clash the cymbals together
- Person B stops timing once they hear the sound of the cymbals clashing
- calculate the speed of the sound waves by dividing the distance travelled by the time taken
18
Q
what problems and solutions are there with the method to measure the speed of waves in the air?
A
- every person has a different reaction time, so may take extra time to start/stop the timer
- have a large number of observers with timers, discard any anomalous results and take a mean value
- the time between seeing the cymbals clash and hearing the sound is very short, making it difficult to press the timer at the correct times
- increase the distance between the two people, so the time is longer, and makes it easier to start and stop the timer accurately
19
Q
describe the three things that can happen of waves when they encounter a boundary:
A
TRANSMISSION
transmitted through the material (e.g. passes from air to glass). hasn’t changed the wave, but sometimes leads to refraction
ABSORPTION
energy of the wave is absorbed by the material and its energy stores. the wave may not pass through the material at all
REFLECTION
reflected off the surface of the material, and never even enters it
20
Q
how do we know what will happen when a wave encounters a boundary?
A
- depends on the wave’s wavelength
- depends on the properties of the two materials
21
Q
how do you construct a ray diagram to show where an image will appear in a mirror:
A
- draw the boundary and label the two materials (e.g. air and mirror). draw an incident ray from the object to the mirror
- draw the perpendicular normal line
- measure the angle of incidence, and using this, work out the angle of reflection and the reflected ray (angle of incidence = angle of reflection)
- draw another incident ray connecting to a different normal, and a different reflected ray
- extend the two reflected rays back into the mirror. where these lines meet tells us the position of the image
22
Q
what is the point of incidence?
A
where the incident ray comes into contact with the boundary
23
Q
describe how ears and microphones work:
A
MICROPHONE
- key part is a paper cone
- sound waves hitting the cone causes it to vibrate
- the microphone then converts this to electrical signals
HUMAN EAR
- sound waves in the air are funnelled into the ear, where they hit the ear drum, causing it to vibrate
- three small bones transmit this info to the cochlea, which produces electrical signals that are interpreted as sound by the brain
24
Q
describe why human hearing is limited:
A
- sound waves in the air trigger vibrations in solids
- only works over a limited range of frequencies
- normal human hearing has a frequency of 20Hz to 20,000Hz
- frequencies outside of that may not be able to make the eardrum vibrate
25
what happens when a wave passes from one medium to another?
- its speed can change
- wavespeed = frequency x wavelength, so as wavespeed changes (when passing from one medium to another), wavelength also changes, as frequency must remain constant. as wavespeed increases, wavelength also increases
- frequency doesn't change, as waves would have to be destroyed or created at the boundary, and that's not possible
26
why do we see lots of different colours when passing white light through a triangular prism?
- different wavelengths of light are refracted by different amounts
- passing white light (containing lots of colours and wavelengths of light) through a triangular prism means the different colours will bend to different degrees and separate
27
do sound waves travel faster in solids or gases?
- travel faster in solids
- the particles are closer together, meaning vibrations can pass more easily between them
28
describe the relationship between frequency and pitch:
high frequency = high pitch
low frequency = low pitch
29
describe the relationship between amplitude and sound:
small amplitude = quiet sound
large amplitude = loud sound
30
what is a reflected sound wave called?
an echo
31
how can we view the features of sound waves?
connect a microphone to a cathode ray oscilloscope. however, it represents them as if they were transverse, which is incorrect (they're longitudinal)
32
what is ultrasound?
soundwaves with a frequency higher than the upper limit of human hearing, so higher than 20,000Hz
33
how can ultrasound be used to image internal organs?
- ultrasound waves partially reflect, meaning some waves are transmitted, and some are reflected
- if we know the time taken for the ultrasound wave to leave the probe, bounce off the internal organ, and then be re-detected by the probe, we can calculate the distance between the probe and the internal organ
- works for any organ not surrounded by bone
34
how can ultrasound be used in industrial imaging?
- used to detect hidden defects or problems with a weld
35
describe the internal structure of the Earth:
- crust (solid, thin, around 50km)
- mantle (solid - upper mantle can flow slowly, but still classed as solid)
- outer core (liquid)
- inner core (solid)
36
how can earthquakes help us understand the structure of the interior of the Earth?
- earthquake happens
- causes seismic waves, carrying energy away from the earthquake
- pass through the Earth, can be detected by seismometers in different countries. the time taken for these waves to travel from the earthquake site to other areas of the Earth gives them clues about the structure of the Earth
- the patterns of these waves gives us info about the interior of the Earth, and what it's made of
37
why are waves gently refracted in the Earth, even though they haven't met a boundary?
the liquid outer core and the mantle aren't the same density throughout, causing gentle refraction and bending throughout
- this bending is more sudden as they pass from one layer to another
38
what are P waves (type of seismic wave)?
- longitudinal, can pass through both solids and liquids
- travel faster than S waves
- P-wave shadow zones (much less area than S-wave shadow zones) are due to the fact that P waves can travel through both solids and liquids
- P waves slow down as they enter the liquid outer core, causing them to refract. they also refract when they leave the outer core
- confirms that the outer core is liquid
39
what are S waves?
- transverse, can only travel through solids
- completely avoids outer core (can't pass through liquid), so forms an area where they can and can't be detected. where they can't be detected is the S-wave shadow zone
- tells us that the Earth must have a liquid core
40
what are electromagnetic waves?
- transverse waves
- transfer energy from the source of the waves to an absorber (e.g. microwave, transfers energy from microwave to food)
41
describe the electromagnetic spectrum:
- the red end has a lower frequency and a longer wavelength. it's the opposite at the violet end
- goes in the order of the colours of the rainbow from left to right
- continous spectrum: the cut-off between one type of wave and the next is unclear
- from the left: radio waves, microwaves, infrared, visible light, ultraviolet light, x-rays, and gamma rays
42
can electromagnetic waves travel in a vacuum?
- transverse, therefore don't require particles to vibrate
- can travel in a vacuum
- all electromagnetic waves travel at the same speed in a vacuum: 300,000,000 m/s
- in different mediums, however, they travel at different speeds, leading to refraction
43
describe microwave and light ray transmission, absorption, and reflection patterns:
- microwaves are absorbed by foods containing water molecules
- microwaves are reflected from metals
- light waves are absorbed by black surfaces
- light waves are reflected by shiny, metallic surfaces
44
describe refraction:
where waves change direction when they change speed, when moving from one medium to another.
- for example, when light passes from air into glass, the velocity decreases, causing the direction of the waves to change. when passing out, it's once again refracted, making the image seem as if it's shifted position
45
why do waves change speed in different mediums?
- different mediums have different densities. the higher the density, the slower the wave will travel through it
- if the wave is travelling perpendicular to the boundary, it'll simply be transmitted
- if the wave enters at an angle of incidence, it'll be refracted
46
describe the acronym FAST:
- when waves get FASTER, they move AWAY from the normal
- when waves get SLOWER, they move TOWARDS the normal
47
what is the emergent ray?
the ray that is leaving a medium after having been refracted (incident ray -> refracted ray -> emergent ray)
48
what is a wavefront?
- imagine a group of identical waves travelling together, creating a 3D shape
- the wavefront is an imaginary line that connects all the same points in a set of waves
- makes it easier to visualise lots of waves moving together
49
describe refraction using wavefronts:
- light waves moving from air into glass
- when the first wavefronts move into the glass, those parts slow down, therefore their wavelength decreases (get closer together)
- causes them to change direction towards the normal - refraction
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- wave fronts approaching the glass along the normal
- they slow, and their wavelength decreases, but the whole wavefront slows down at the same time, meaning it doesn't change direction
50
what are the two ways electromagnetic waves can be emitted from atoms?
- heating atoms (supplying it with EM waves) causes electrons to move up an energy level
- when it returns to its original energy level, it emits an EM wave, e.g. light
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- gamma rays can be emitted from the nucleus of radioactive atoms
- once it's been emitted, the nucleus has less energy than at the start
51
what are the hazards of ultraviolet, x and gamma rays?
- potentially hazardous to the human body
ULTRAVIOLET increases the risk of skin cancer, causes it to age prematurely
X-RAYS + GAMMA RAYS are ionising radiation (knock electrons off atoms when absorbed). can cause the mutations of genes and potentially cancer
52
describe the transmission and absorption of radio waves:
- radio waves produced when electrons oscillate in electrical circuits
- these radio waves can be absorbed, e.g. by an electrical circuit in an aerial
- this causes electrons in this circuit to oscillate, creating an alternating current with the same frequency as the radio waves
53
describe the uses of radio waves:
- transmit radio and terrestrial TV (received using an aerial) signals
- can travel long distances before being absorbed (e.g. by buildings/trees)
- can also spread out between hills (diffraction)
- can reflect off a layer of charged particles in the atmosphere
- these all allow us to send radio waves and transfer information very long distances around the Earth
54
describe long radio waves:
- can be transmitted huge distances (e.g. london to singapore)
- doesn't have to interact with anything along the way
-diffract along the curved surface of the Earth
55
describe short radio waves:
- can also travel long distances
- cannot diffract around the Earth
- are reflected off the ionosphere (electrically charged layer of the upper atmosphere)
- bounce back and forth along this, and so can cover long distances through this
- can also send data over short distances (e.g. bluetooth)
56
describe very short radio waves:
- used for TV and FM radio
- must travel directly from transmitter to receiver
- don't always get a good radio signal when in a car, as surrounding structures (e.g. hills/tunnels) can interfere with that direct transmission)
57
how do we generate and detect radio waves?
- connect a transmitter to an oscilloscope. the oscilloscope allows us to see the frequency of the alternating current, telling us the frequency of of the wave produced
- the generated radio wave can be detected by a receiver, which absorbs the energy, and generates another exactly the same alternating current, displayed on another oscilloscope
58
describe the uses of microwaves:
WAVES THAT ABSORB WATER MOLECULES:
- heat food - most foods contain water molecules, which absorb the energy of microwaves, as the water molecules vibrate more, affecting other molecules and causing the temperature of the food to increase through conduction/convection
WAVES THAT AREN'T ABSORBED BY WATER MOLECULES:
- used to communicate with satellites in space (can pass through the Earth's atmosphere without being reflected/refracted as they're not absorbed by water molecules)
- received by satellites, then transmitted back to Earth and detected by a satellite dish for satellite TV
59
describe the uses of infrared waves:
- used in ovens/grills. the metal is heated and emits lots of IR radiation. this heats our food by transferring heat energy. however, unlike microwaves, it doesn't penetrate the food
- electric heaters. electrical energy heats the metal, and it emits IR radiation to the surroundings
- also used in infrared cameras (e.g. to check buildings for heat losses, to spot living organisms, to see in the dark). warm animals/humans will appear bright, as they emit lots of infrared radiation
60
where is infrared radiation specifically emitted from?
all objects that have thermal energy. the amount that is emitted depends on the object's temperature
- the hotter the object, the more IR radiation it emits
61
describe the uses of visible light:
- communication using fibre optics
- optical fibres are very thin strands of glass/plastic, and we can transmit pulses of light down these (light is reflected and constantly bounces back and forth on the internal surface) over long distances to carry information
- carry telephone and cable TV signals
- visible light has a short wavelength, so optical fibres can carry a lot of info
62
what are the positives and negatives of fibre optics?
- by encoding info into these light pulses, we can transmit data very quickly
- however, we must ensure that the material used only reflects the light and doesn't absorb any. must also ensure that only specular reflection occurs, so light isn't scattered
63
what are the advantages of optical fibres over copper wires and electricity?
- can transmit much more information
- the signal is less likely to be distorted during transmission
64
can humans see all of the waves on the EM spectrum?
- no, they can only see visible light
- the different wavelengths within this section are the different colours we can see (red is the longest, violet is the shortest)
65
why are white and black not on the colour spectrum?
white is all of the colours combined. black is the absence of light, so there is no light/colour
66
what determines the colour an object appears?
- the wavelengths of light hitting it
- the properties of the object
- these determine which light rays are absorbed, reflected or transmitted, and so determines how the object appears
67
describe the uses of ultraviolet:
- energy efficient lightbulbs
- the ultraviolet light is created inside the bulb, then absorbed by the internal surface (a layer of phosphorus) of the bulb and converted to visible light (transfers a greater proportion of electrical energy into light energy than filament bulbs. save on energy bills, CO2 emissions)
- short wavelength, so it carries more energy than visible light
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- sun tanning (e.g. sun bed). also emitted by the sun, which can cause a sun tan/burn
68
what are some other uses of UV radiation?
(security, sterilisation)
- used in security pens (completely invisible ink until UV light is shone on them). UV light can also be used to verify passports and bank notes, making them harder to forge
- can also be used to sterilise water, as it destroys microorganisms
69
describe fluorescence:
- ultraviolet light is absorbed
- the energy is re-emitted as visible light
- this is why fluorescent objects appear so bright: they're actually emitting light, instead of simply reflecting it
70
describe the uses of X-rays:
- short wavelength, high frequency
- view the internal structures of objects: fires x-rays through a person's body and recording the ones that get through using a detector plate. x-rays are absorbed by dense materials (e.g. bones), but can pass through other things that are mainly air (e.g. lungs, intestines), only pass partially through fleshy parts (e.g. hearts). the monitor goes black if the detector receives radiation
- used to detect broken bones, other diseases
- emits a low dose of radiation that is hardly harmful for patient. quick and cheap test. danger is much higher for staff, so they wear lead aprons and leave the room during the process
71
how can gamma rays be used?
can treat cancer through radiotherapy, can be used in medical imaging
- exposing some medical equipment to high temperatures to kill the bacteria could damage them (e.g. plastics). gamma rays don't do any damage
- gamma rays kills all harmful microbes in food, without altering it in any way, keeping it fresh for longer (no microorganisms to break it down)
72
what is the trade off when using x-rays and gamma rays in medicine?
both are ionising radiation, which can damage our cells, and potentially lead to cancer. however, they also help us diagnose and treat diseases, so it's usually worth the risk
73
where do EM waves come from?
- gamma rays come from radioactive decay
- visible, ultraviolet, and x-rays are emitted when electrons drop down energy levels
- infra-red radiation comes from when the bonds holding molecules together vibrate
74
what are the different EM waves in order from left to right?
- RADIO
- MICRO
- INFRARED
- VISIBLE
- ULTRAVIOLET
- X-RAYS
- GAMMA
75
describe convex lens:
- have the bulgy shape (thicker at the centre than at the edges)
- the symbol is a line with an arrow at each end
- refract parallel rays of light inwards to a single point, the principle focus. they converge
76
where is the principle focus/focal point on a lens?
on both sides. always sits on the axis (a line passing through the centre of the lens)
- lenses are therefore symmetrical and can work both ways
77
what is the relationship between the focal length and power of the lens?
the shorter the focal length, the more powerful the lens. it will refract light more strongly
78
how do you make a lens more powerful?
- make it more curved
- use a different material that refracts light more strongly
- decrease focal length
79
what is the key feature of all lenses?
all refract light. when light enters, it bends towards the normal, and then away from the normal when it passes out
80
what is the principle axis on a convex lens diagram?
- the central ray passes through the lens without being refracted (passes directly along the normal)
81
what is the principle focus and the focal length on a convex lens diagram?
PRINCIPLE FOCUS
- all other rays refract, and are focused on a point after the lens
- called the principle focus (F)
FOCAL LENGTH
the distance from the centre of the lens to the principle focus
- different convex lenses have different focal lengths depending on the strength of the lens
82
how would you produce a ray diagram for a convex lens?
1. draw a ray passing from the top of the object through the centre of the lens without changing direction
2. draw another line from the top of the object, running parallel to the principle axis. when the line hits the lens, it's refracted through the principle focus
- where the two lines meets shows the top of the image
83
what are the three key features of an image that you must describe?
1. magnified or diminished (bigger or smaller)
2. inverted or upright (upside down or right side up)
3. real or virtual image (can it be projected onto a screen or not)
84
what are the properties of an object over 2 focal lengths away from the lens?
- diminished
- inverted
- real
85
what are the properties of an object 2 focal lengths away from the lens?
- same size
- inverted
- real
86
what are the properties of an object between 1 and 2 focal lengths away from the lens?
- magnified
- inverted
- real
87
what are the properties of an object less than 1 focal length away from the lens?
- magnified (this is how magnifying glasses work)
- upright
- virtual (the rays don't actually meet at a point, this is the only time a convex lens produces a virtual image. shown using dotted lines, which don't actually represent the path of the rays)
88
how do you calculate the magnification of a lens?
magnification = image height / object height
89
describe a concave lens:
- thicker at the edges than in the centre (caves inwards)
- represented with a line, and inverted arrows on either end
- refract parallel rays outwards, dispersing the light. make light rays diverge (spread out)
90
describe where the principle focus is on a concave lens ray diagram:
- light rays run parallel to each other until they hit the lens, when they diverge
- plotting the lines behind the lens means they come to a point, which is the principle focus
- however, the lens isn't actually focusing the light at the principle focus, they only appear to be coming from it
91
how would you draw a ray diagram for a concave lens?
1. draw a ray passing from the top of the object through the centre of the lens without changing direction
2. draw another parallel ray from the top of the object, this passes through the lens and is refracted outwards. it must appear to have come from the principle focus, so draw a dotted line connecting the two
3. where these lines meet shows the position of the top of the image (behind the lens)
4. repeat this to find the bottom of the image, but if the bottom of the object is on the axis, so is the image
92
what image will a concave lens always produce?
- virtual image (dotted line). only appear to cross there
- diminished
- upright
93
what is the difference between a real and virtual image?
- a real image is formed when the light rays actually do come together to form an image, and it could be captured on a screen (e.g. a retina)
- a virtual image is formed when the light rays don't come together where the image appears to be. we trace the points where the light rays hit the lens back, using virtual rays, to form an image. therefore it's virtual (e.g. a mirror)
94
what is specular reflection?
- on the surface of a mirror, all the light rays are reflecting in a single direction, as all of the normals are in the same direction
- this takes place on smooth, flat surfaces
- produces a clear image
95
what is diffuse/scattered reflection?
- light can reflect off a rough surface
- even though the incoming light rays are in the same direction, the normals will all be different as the surface isn't flat, therefore the light will be reflected in all different directions
- doesn't produce an image
96
describe coloured filters:
- work by absorbing specific wavelengths and transmitting other wavelengths
- red filter, for example. shine white light onto it, and it will absorb all the colours of visible light, apart from red, so only that is transmitted through
97
what is the difference between transparent and translucent objects?
- both transmit light through them
transparent: we can easily see through them. transmit nearly all of the light
translucent: scatter the light rays, so we can't see through them clearly. only transmit some of the light, and the proportion of light transmitted determines how well we can see through them
98
what does the colour of a translucent object depend on?
which colour/wavelength of light is being transmitted most
99
what determines the colour of an opaque object?
- opaque objects don't transmit any light. all wavelengths of light are absorbed/reflected. it's the reflected wavelengths that determine the colour
- white objects appear white because they reflect all of the wavelengths of visible light equally
- black objects appear black as they absorb all the wavelengths of visible light
- red objects absorb all of the colours of white light apart from red, which is reflected, giving it a red colour
100
describe colour filters:
filter out particular colours, so that only certain wavelengths can pass through - only transmit certain wavelengths of light, and absorb the rest
101
describe a primary colour filter:
- only allows one of the three primary colours to be transmitted (red, green, blue)
- passing white light through a green filter means only the green light will filter through. white paper through this will appear green
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- a blue object (reflecting only blue light) through a green filter will appear black
- the filter would block the blue light from passing through, so none would be reflected into your eye
102
describe non-primary colour filters:
e.g. violet, yellow
- allow wavelengths of the same colour as the filter through
- also allow wavelengths of the primary colours that can be added together to make that colour through (e.g. for yellow filters, would let through red and green)
103
what are the three primary colours of light?
red, blue, green
104
why do objects heat up as they absorb radiation?
radiation is pure energy
- similarly cools down when it emits radiation
- therefore the balance between absorbing and emitting radiation will affect the object's temperature
105
describe which objects emit and absorb infrared radiation:
- matte black surfaces are the best absorbers and emitters of infrared radiation
- all objects, no matter what temperature, emit and absorb infrared radiation, but hotter objects will emit more infrared radiation in a given time, compared to cooler objects
106
describe the relationship between temperature and wavelength and intensity of radiation:
- very hot objects emit shorter wavelength radiation than cooler objects, which is more intense
- this is why very hot objects produce visible light, as they're so intense (the hotter the object, the whiter it is, as it has a short wavelength and is closer to the violet end of the spectrum)
AS TEMPERATURE INCREASES, THE INTENSITY OF EVERY EMITTED WAVELENGTH INCREASES, AND THE WAVELENGTH DECREASES
107
what radiation do objects at room temperature emit?
infra-red radiation, which is why we can't see it
108
describe the Earth in terms of radiation absorption and emission?
- constantly bombarded by EM radiation from sun, emits its own infra-red radiation, as it's a big, warm object. the atmosphere can also reflect, absorb, and emit radiation
- during the day, more energy is absorbed by the Earth and atmosphere than is emitted, increasing the local temperature
- at night, less energy is absorbed than emitted, meaning the local temperature decreases
- however, at all times some part of the Earth is in the sun, so the overall temperature stays constant
109
describe a perfect black body:
absorbs all of the radiation incident on it. none is reflected, none is transmitted
- an object that absorbs radiation well will also emit radiation well, therefore, it's also the best possible emitter of radiation
110
describe the relationship between an object's temperature and its absorption and emission of radiation:
- if an object is warmer than its surroundings, it will emit more radiation than it absorbs, and its temperature will decrease
- if an object is cooler than its surroundings, it will absorb more radiation than it emits, and its temperature will increase
- if an object is at a constant temperature, it's absorbing radiation at the same rate as it is emitting radiation
111
how does radiation affect the temperature of the Earth?
- absorbing or emitting radiation is the only way the Earth can gain or lose energy
- the sun emits short wavelength radiation, e.g. visible light/ultraviolet. this radiation travels to the Earth, and some is reflected by the clouds, and the remaining is then absorbed by the surface of the Earth, causing its temperature to increase
- the Earth now emits infrared radiation back into space, however, some of this energy is trapped by greenhouse gases in the atmosphere
- more heat energy is trapped in the atmosphere, and less is radiated into space
112
why do cloudy nights tend to be warmer than clear nights?
clouds can reflect infrared back to Earth and prevent it from being radiated into space
113
define intensity:
the power of radiation per unit area
- how much energy radiation transfers to a given area in a certain amount of time (more wavelength?)
114
are sound waves transverse or longitudinal?
sound waves are vibrations that pass through the molecules of a medium
- they're longitudinal, so they travel as a series of compressions and rarefactions
115
what happens when sound waves travel through a solid?
cause particles inside the solid to vibrate, and these particles will then collide with their neighbours and pass on the vibrations. this happens repeatedly, and the soundwaves are passed through the material
- sound waves need particles to be transmitted, so the more densely packed the particles are, the faster they travel
- can't travel in a vacuum, as there are no particles for the sound to vibrate through
116
what happens when sound changes speed?
when sound speeds up, the wavelength gets longer, as the frequency can't change. the wavelength will get shorter as it slows down in low density materials (e.g. air)
117
can sound be refracted, reflected and absorbed?
yes. hard, flat surfaces reflect the most sound (echoes)
- they can be refracted as sound changes speed through differently dense materials
118
how does human hearing work?
sound waves reach the ear. they travel along the ear canal and hit the ear drum, causing it to vibrate. these vibrations will be transmitted along a series of tiny bones (ossicles), through the semi-circular canals and into the cochlea.
- the cochlea then converts the vibrations into electrical signals, and these get sent along the auditory nerve to the brain
- the brain interprets these signals as sounds. higher frequencies are interpreted as higher pitches, and more intense signals are louder
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what is the human frequency hearing range?
20Hz to 20,000Hz
- as we get older, our hearing range decreases, mainly due to the wear and tear of the cochlea and auditory nerve
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what is ultrasound?
vibrates at frequencies above 20,000 Hz (above range of human hearing)
- humans produce it through electrical devices. produce electrical oscillations which are converted into ultrasound waves
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what do bats use ultrasound for?
naturally create it
- used for communication, echolocation
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how is ultrasound partial reflection useful?
- when it hits a boundary, some is reflected and some is transmitted and refracted
- if we know the speed of the wave, and the time it takes for it to be reflected, we can work out how far away that boundary is
- by doing this repeatedly, we can learn about the different boundaries and internal structures of an object
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- can be used to scan foetuses. ultrasound device transmits and receives waves on the belly
- every time the wave hits a boundary, some waves are reflected back to this device
- the timings and distributions of these echoes are processed to produce a live image of the foetus to check if it's healthy
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how can we use ultrasound to check the quality of products in industry?
fire ultrasound waves at an object. if it's completely solid, the waves should pass straight through, and only be partially reflected at the very beginning and end
- if there's a crack, the waves will be reflected back at the crack, so the company can tell there's a fault with the object
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what else can ultrasound waves be used for?
sonar in submarine ships. can work out distance of submarine from seabed, if we know the speed of the wave and how long it took to return to the ship
- ensure you halve the time, though, as this is the time to reach the seabed AND return. we only want the time to reach the seabed
