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aqa a level physics > waves > Flashcards

waves Flashcards

1
Q

state a property of microwaves [1]

A

microwaves are polarised

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2
Q

Distinguish between longitudinal and transverse waves [2]

A

For transverse waves oscillations are at right angles to direction of propagation while in
longitudinal waves they are in the same direction

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3
Q

Explain how a stationary wave is produced when a stretched string is plucked. [3]

A

Waves travel to the boundaries and are reflected
two waves, with the same frequency and amplitude, travelling in opposite directions interfere/superpose
Fixed boundaries (cannot move so) are nodes where destructive interference occurs

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4
Q

Suggest how stationary waves are formed in a microwave oven [2]

A

waves are reflected off of the oven wall
they superpose with wave travelling in opposite direction

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5
Q

Explain what is meant by a progressive wave. [2]

A

A wave which transfers energy from one point to another without transferring material

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6
Q

Explain how light from the diffraction grating forms a maximum on the screen [3]

A

Light from slits diffract
Path difference is a whole number of wavelengths so they arrive at the screen in phase
they superimpose (interfere constructively) to form maxima

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7
Q

Explain what is meant by modal dispersion in an optical fibre [2]

A

Spreading of pulse / parts of a pulse take different times to travel through the
fibre
Due to different paths through the optical fibre / due to entering the optical fibre
at different angles

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8
Q

Explain what would make two loudspeakers coherent sources of sound waves. [2]

A

they have the same frequency/wavelength AND
constant phase difference
And that this achieved by both speakers being connected to same signal (generator).

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9
Q

Discuss the properties of a step-index optical fibre [6]
. the names of parts
* a description of the functions of said parts
* a discussion of the problems caused by material dispersion and modal dispersion and how these problems can be overcome.

A

inner part is the core
outer part is cladding

the core propagates the wave
by total internal reflection
with low attenuation (loss of energy)
refractive index of the core > cladding

cladding protects core from damage

both cause pulse broadening - limited bandwidth
material - different wavelengths have different speeds - use monochromatic source
modal - different paths take different amounts of time - use a narrow core

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10
Q

Suggest why the amount of reflected light changes as the fibre bends [2]

A

Angle of incidence may become less than critical angle
Light may encounter impurities at different positions/angles

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11
Q

apart from wearing eye protection
Describe one other safety measure to minimise the risk of eye damage when using a laser in the laboratory. [1]

A

switch laser off when not in use
or
don’t look directly into the laser (wow, geniuses taking a-level physics huh)

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12
Q

define frequency

A

number of complete waves passing a point per second

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13
Q

frequency {}
waves

A

v / λ
Hz (hertz)

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14
Q

define wavelength

A

distance between two adjacent crests on a wave

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15
Q

define amplitude

A

maximum displacement of a wave from it’s rest(equilibrium) position

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16
Q

time period {}

A

1 / f
seconds (s)

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17
Q

what is a longitudinal wave

A

a wave in which the oscillation of the particles is parallel to energy transfer
e.g sound

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18
Q

rarefactions and compressions

A

areas of low and high pressure
in a longitudinal wave

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19
Q

what is a transverse wave

A

a wave in which the oscillation of the particles is perpendicular to energy transfer
e.g EM waves

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20
Q

speed of Electromagnetic (EM) waves in a vacuum

A

3.00 * 10^8 ms^-1

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21
Q

what is an electromagnetic wave

A

vibrating electric and magnetic fields perpendicular to the direction of energy transfer

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22
Q

what does a polarising filter do

A

only allows oscillations in one plane
(plane polarises a wave)

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23
Q

what is evidence for the nature of transverse waves

A

polarisation
Transverse waves usually have multiple planes of oscillation perpendicular to motion

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24
Q

how is polarisation used in antennas

A

Tv and radio signals are usually plane polarised by the orientation of the transmitting aerials
so receiving aerials must be aligned in the same plane so signal can be received at full strength

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25
what is a stationary wave
a wave which transfers no energy positions of max and min amplitude are constant
26
what is a node
a point on a stationary wave where displacement from equilibrium position is 0
27
what is an antinode
a point on a stationary wave where displacement from equilibrium position is at its maximum
28
conditions for a stationary wave to be produced
waves must be of same frequency, wavelength and amplitude must be travelling in opposite directions (often when a wave is reflected upon itself)
29
how are stationary waves produced
superposition of two progressive waves travelling in opposite directions with the same frequency, wavelength and amplitude where waves meet in phase - constructive interference - antinodes where waves meet completely out of phase - destructive interference - nodes
30
describe the first harmonic for a stationary wave with two closed ends
2 nodes 1 antinode in the middle L = 1/2 λ
31
describe the second harmonic for a stationary wave with one closed end and one open
2 nodes 1 at closed end 2 antinodes 1 at open end
32
define coherence
potential for two waves to interfere, fixed phase difference same frequency and wavelength
33
why is a laser often used to model diffraction and interference
laser uses monochromatic light more defined patterns produced
34
describe young's double slit experiment (bro these a-levels are making me want to do a double slit experiment too man(i'm joking))
single light source directed at a double slit acts as two coherent light sources diffracts through slit interferes constructively and destructively to form pattern
35
describe the interference pattern produced by doing young's double slit experiment with white light
bright white central maximum fringes of decreasing intensity with violet closest to order zero and red furthest
36
why is an interference pattern produced when white light is shone through a single slit
all the different wavelengths are diffracted by different amounts instead of clear fringes - spectra of colours
37
how does the size of the slit relate to the amount of diffraction
slit should be closer to wavelength for more diffraction
38
what does increasing the slit width do to the central maximum what does increasing wavelength do
increasing slit width decreases diffraction so central maximum becomes narrower and more intense more wavelength more diffraction central maximum wider and less intense
39
approximate refractive index of air
1
40
when light enter a more optically dense material does it bend towards or away from the normal
towards
41
when does total internal reflection occur
when light is at a boundary to a less optically dense medium angle of incidence > critical angle
42
purpose of cladding in step index optical fibre
protects core from scratches allows total internal reflection as it has a lower refractive index than the core
43
what is attenuation
part of the signal's energy is absorbed by the fibre so the amplitude is reduced
44
what is pulse broadening
received signal is wider than the original this can cause overlap of signals and loss of information
45
how does modal dispersion cause pulse broadening
light enters fibre at different angles (different angles of incidence) so takes different paths so rays take different times to travel down fibre
46
what is material dispersion
when light with different wavelengths is used different wavelengths travel at different speeds arrive at different times pulse broadening
47
how can you reduce modal dispersion
use a narrow core so possible difference in path lengths is smaller
48
how can you reduce material dispersion
use monochromatic light
49
how can both absorption and dispersion be reduced
use a optical fibre repeater to regenerate signal
50
why optical fibre cables > traditional copper cables
signals can carry more information as light has a high frequency no energy lost as heat cheaper very fast
51
what happens when angle of incidence = critical angle
goes along boundary angle of refraction is 90 degrees
52
you know the refractive indices of two materials, how do you work out the critical angle
sin Critical = n2 / n1 where n1 > n2
53
finding the angle or refraction {}
n1sini = n2sinr
54
what does a higher refractive index mean
more optically dense The higher the refractive index the slower the light travels More refraction
55
refractive index {}
n = c / v refractive index = speed of light / speed in substance
56
2 applications of diffraction gratings
splitting light up received from stars to form line absorption spectra to identify elements present x-ray crystallography crystal sheet acts as a diffraction grating x-rays pass through to find spacing between atoms
57
what is a first order maximum
path difference between two adjacent rays of light is 1λ
58
nλ = {}
nλ = dsinθ where n is the order λ is the wavelength d is the separation (in m) θ is the angle between the normal to the grating and the ray
59
when light passing through a diffraction grating is changed from blue to red what happens to the orders
red has a greater wavelength than blue light so will diffract more so orders will become more further apart
60
what is diffraction
the spreading out of waves when they pass through (or around) a gap
61
what are wave properties shows by young's double slit experiment
diffraction and interference
62
fringe separation {}
w = (λD) / s fringe spacing = ( wavelength * distance between source and screen) / slit separation
63
what is path difference
the difference in path travelled by 2 waves
64
frequency {} including tension
f = (1/2L) * √(T/u) where T = tension (N) u = unit per mass length (kgm^-1) L = length of harmonic (2L = λ)
65
wave speed {}
v = fλ wave speed = frequency * wavelength
66
what is phase
the position of a wave point on a wave cycle measured in radians fractions of a cycle nλ degrees
67
what is reflection
when the wave is bounced back when it hits a boundary
68
what is refraction
when the wave changes direction as it enter a different medium result of the wave slowing down or speeding up
69
what is time period
time taken for one whole wave cycle
70
what do you get with two polarising filters at right angles to one another
no light passes through
71
what are some real life applications of polaristion
glare reduction light reflected off of some surfaces is partially polarised when light is reflected by surfaces they can cause glare since it is partially polarised some of it can be filtered out using polarising filters improving TV and radio signals signals are polarised by orientation of the rods on the transmitting aerial receiving aerials should be lined up similarily so the signal can be received at full strength
72
what is superposition
When two or more waves cross at a point, the displacement at that point is equal to the sum of the displacements of the individual waves.
73
describe constructive interference
when two waves pass through each other and their displacements combine to make a displacement with greater magnitude
74
total destructive interference
when two waves of equal and opposite displacements meet and cancel each other out completely
75
what do points in phase share in common
displacement and velocity
76
1 complete cycle of a wave in radians
2 π (360 degrees)
77
when are two points on a wave completely out of phase
when their phase difference is an odd multiple of 180 degrees odd multiple of π odd multiple of 1/2 λ
78
when are two points are in phase
phase difference of 0 multiple of 360 even multiple of half wavelength (π)
79
what is resonant frequency
when an exact number of half wavelengths fit on the string Natural frequency at which the medium oscillates with highest amplitude
80
explain the first harmonic
a stationary wave vibrating at its lowest possible resonant frequency
81
how does frequency vary with the harmonics
second harmonic has twice the frequency of the first harmonic third harmonic, thrice the frequency of the first harmonic
82
investigating resonant frequency
signal generator string masses for tension vary tension and move prism along string to find the harmonic record length of the string
83
how is resonant frequency affected by length of the string
the longer the string the lower the resonant frequency (since half wavelength is longer, v = fλ, greater lamda -> f decreases for a fixed v)
84
how is resonant frequency affected by material of the string
heavier ( more unit per mass length ) strings give a lower resonant frequency waves travel more slowly down the string ( λ remains the same , v= fλ , if v decreases so must f )
85
how is resonant frequency affected by tension on the string
the greater the tension on the string the higher the resonant frequency this is because waves travel faster on a tight string
86
diffraction around an obstacle
diffraction about the edges leaves a "shadow" the greater the obstacle in comparison to the wavelength, the less diffraction so a longer shadow
87
what is intensity
power per unit area
88
where does constructive interference occur
path difference is a whole number of wavelengths in phase Constructive interference occurs when the phase difference between the waves is an even multiple of π (180°)
89
double slit experiment
two source interference use monochromatic light source - laser shine through slits of about the same size of the wavelength so diffraction occurs pattern of light and dark fringes w = (λD)/s fringe separation
90
what is a diffraction grating
lots of equally spaced slits creates a sharp interference pattern since there are many beam reinforcing the pattern
91
when do maxima occur (diffraction grating experiment)
nth order maxima occur when path difference = nλ
92
what are conclusions that can be drawn from the diffraction grating formula about order
values of sinθ greater than 1 are impossible so if for a certain n you get sinθ > 1 that order doesn't exist
93
what are conclusions that can be drawn from the diffraction grating formula about wavelength
if wavelength is bigger sinθ is bigger therefore θ is bigger larger the wavelength - the more spread out the pattern is (the more diffraction)
94
what are conclusions that can be drawn from the diffraction grating formula about d
if d is small ( number of slits per metre is large ) the greater the angle of diffraction more spread out pattern
95
what is critical angle
where angle of refraction is 90 light is refracted along the boundary when light is passing from a more optically dense material to a less optically dense one
96
how does total internal reflection occur in step-index optical fibres
light is shone in the fibre is so narrow that the light always hits the boundary between the fibre and the cladding at an angle greater than the critical angle so all light is totally internally reflected
97
total internal reflection
at angles greater than the critical angle refraction doesn't occur all the light is reflected back into the material
98
how do you calculate the highest order of maxima visible
n = d/λ nλ = dsinθ max angle to see orders of maxima is 90 sin 90 = 1 nλ = d

aqa a level physics (10 decks)

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