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chapter 11/12 - waves Flashcards

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1
Q

mechanical waves

A

energy is transferred by vibration of particles carrying the wave through a medium eg sound seismic water

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2
Q

longitudinal waves

A

oscillate parallel to the direction of energy transfer

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3
Q

transverse waves

A

oscillate perpendicular to the direction of energy transfer

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4
Q

amplitude

A

max displacement from undisturbed position

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5
Q

wavelength

A

distance between two adjacent points on the same cycle of a wave

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6
Q

frequency

A

number of complete oscillations / time

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7
Q

displacement position graphs

A

give a snapshot of what a wave looks like at a point in time
- to find which direction each part is moving draw the wave slightly later

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8
Q

speed

A

freq * wavelength

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9
Q

phase

A

is the fraction of a cycle it has traveled since the start of the cycle - always and angle in rad
1 cycle = 360 or 2𝞹

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10
Q

phase difference

A

difference in degrees/ rad / angle between points on the same wave or similar points on 2 waves
- fraction of the cycle that passes between 2 maximums

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11
Q

one complete cycle

A

2𝞹 = 360

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12
Q

in phase

A

two waves at the same point in the cycle
phase difference = 2pi or 360

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13
Q

antiphase

A

two waves doing opposite things at the same point
phase difference = pi or 180

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14
Q

coherent

A

waves with constant phase difference
must have the same freq to make phase diff constant

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15
Q

interference

A

the superposition of coherent waves

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16
Q

principle of superposition

A

when waves are at the same point there resultant displacement is = to the sum of individual displacements

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17
Q

CRO traces

A

amplitude - count squares * volts
time period - count squares * time base
frequency - 1/T

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18
Q

progressive wave

A

transfers energy from one point to another

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19
Q

phase difference equation

A

phase difference = 2pi d/ wavelength
phase diff/ 2 pi = d/ wavelength

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20
Q

intensity

A

radiant power per unit area
Power/ Area
- radiates equally in all directions I = P/ 4pi r^2
(inverse square law)

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21
Q

intensity is proportional to

A

KE
v^2
A^2
v is proportional to A

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22
Q

refraction diagram

A
  • incident ray
  • normal
  • angles
  • emergent ray
  • refracted ray
  • direction arrows
  • change of direction
  • title
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23
Q

reflection

A

wavelength is same because speed doenst change

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24
Q

refraction

A

wavelength decreases if speed decreases as f stays same

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25
Q

diffraction

A
  • no effect on wavelength or speed
  • waves going through a gap
  • larger aperture = minimal diffraction
  • max diffraction is when wavelength is closed to gap size
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26
Q

refractive index

A

tells us how much faster the wave travels in a vacuum than the medium

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27
Q

refractive index equations

A

n2/n1 = c1/ c2 = wavelength1/ wavelength2 = sin i / sin r

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28
Q

snells law

A

n1 sin i = n2 sin r

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29
Q

TIR

A
  • occurs if i> C
  • travelling into a less dense medium
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30
Q

critical angle

A

C = sin-1 (n2/n1)
in a vacuum/ air
C = sin-1 (1/n)

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31
Q

optical fibres (cladding)

A
  • cladding has lower n than core so light travels as it’s internally reflected along
  • if you just used air crossover would occur when fibres touch
  • cladding also products core and gives it strength
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32
Q

multi path dispersion

A
  • diff paths with diff lengths and times (eg straight through is fastest)
  • leads to pulse broadening which reduces max freq- can be prevented with narrower fibres
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33
Q

spectral dispersion

A
  • speed of light decreases with shorter wavelength - diff colours have diff λ so diff speeds
  • use monochromatic light to prevent pulse merging
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34
Q

images carried along optical fibres

A

formed from light and dark dots
- need a coherent bungle so image appears at the other end

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35
Q

analogue vs digital signals

A

when waves are sent noise and attenuation can occur
- when you use digital signals it’s easier to see the value of the wave (1 or 0)

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36
Q

hwo signals are sent

A

carrier wave and signal wave
- can be amplitude modulated - amplitude if the carrier wave shows the message wave
- or freq modulated - means the frequency of the wave shows the peaks and troughs do the message wave

37
Q

attenuation

A

further away the weaker the signal becomes

38
Q

hwo to fix attenuation

A

repeaters can regenerate digital signals to their full strength agains

39
Q

noise

A

signal becomes less sharp which can be prevented by an amplifier or repeater
- easier to fix on digital signals

40
Q

polarisation

A
  • limits vibrations to one plane
    (only transverse not longitudinal - as vibrations are perp to direction of travel)
41
Q

partial polarisation

A
  • when waves are reflected off transparent surfaces (glare)
42
Q

metal grille with polarised microwaves

A
  • put through polarising filter
  • grilled allows vertically polarised vibrations
  • when it’s turned 90° all waves are blocked (absorbed) as it would onyl let through horizontal polarisation
    as you tuyrn back to 180 /0 microwaves pass through (max at 180/0 as I = Icos²θ
43
Q

hwo to create microwaves

A
  • the transmitter has ac passed through it at the same freq as the intended wave
  • the electric field created oscillates vertically at the same freq as the current
44
Q

why does the grille block vertically polarised microwaves

A
  • free electrons in grille oscillate vertically inducing I
  • electrons emit an electric field- in all directions
  • do very little is transmitted in the direction of the receiver
45
Q

why does a horizontal grille not block vertical microwaves

A

-electrons in grille aren’t effected by vertical field
- so continues to transmit wave vertically as no current is induced

46
Q

grille at 45°

A

some are recieved as the field is resolved into parallel and perpendicular to the grille

47
Q

when reciever and emitter are differently orientated

A

if perp no signal is recieved
- if 180 = 0 so all is recieved

48
Q

inverse square law

A

I is proportional to 1/r^2
(also I is prop to a^2)

49
Q

malus law

A

intensity of polarised light
I = I0 cos^2(angle)
where I0 = initial intensity
angle = angle of the polariser

50
Q

polarise sunglasses

A
  • glare is partially polarised light in the horizontal plane by reflection
  • glasses only transmit vertically polarised light
  • glare isn’t let through so reduced
51
Q

2 filters

A

if they are perp no lights will pass through
strain also alters optical activity

52
Q

brewsters angle ???

A

n = tan(angle)
angle of incidence at which light with particular polarisation is perfectly transmitted

53
Q

arials

A
  • mesh reflects signal back to the front
  • circular rod picks up the signal ( 1/2 wavelength tall)
  • other rods boost signal (1/2 wavelength apart)
    adjacent emitters alternate between horizontal and vertical to avoid interference if they overlap signal areas
54
Q

superposition

A

at the point waves overlap we can find the wave position using the principle of superposition

55
Q

constructive interference

A

waves arriving at a point in phase causes reinforcement
crest + crest = supercrest

56
Q

destructive interference

A

waves in anti phase cause cancellation
crest + trough = nothing

57
Q

nodes

A

form where waves are in antiphase (destructive interference)

58
Q

antinodes

A

where waves are in phase - constructive interference

59
Q

path difference

A

difference in the distance from 2 sources
if an odd multiple of 1/2 wavelengths it’s antiphase
if it’s a whole no. is in phase

60
Q

change in phase difference

A

2 pi path difference / wavelength

61
Q

lines of maxima

A

connect points where path difference is a whole no. wavelengths
(where displacement is a maximum)

62
Q

lines of minima

A

connect points where the path difference is an odd multiple of 1/2 wavelength
(where displacement is zero)

63
Q

nth order maxima

A

path difference if n wavelengths
phase difference of 2n pi
central maxima is where path+ phase diff = 0

64
Q

measuring the wavelength of microwaves

A
  • probe is moved over paper from the central maxima along
  • positions of maxima marked and path diff is determined
  • each maxima can be used to calc wavelength and then you average
65
Q

young’s double slit experiment

A
  • pass light through a colour filter (monochromatic light) then two slits
  • creates fringes of interference
66
Q

fringes

A

uniformly spaced near the centre of the interference pattern - bright and dark patches

67
Q

how to find wavelength from youngs double split

A

wavelength = ax/ D
a = slit separation
x = finge separation
D = distance to screen

68
Q

S2P-S1P (distance from far slit to maxima - distance from close slit to maxima)

A

is the path difference = n * xa/D =n λ
for first order maxima path difference = 1λ

69
Q

youngs double slit risks

A
  • don’t look at laser
  • remove reflections
  • do in dark room
  • warning sign of laser
  • take into account laser orientation
70
Q

diffraction grating equation

A

more slits = narrower bands of brightness
d sin(θ) = n λ
n(max) = d/ λ (as angle can’t exceed 90 (sin(90) = 1))
if x lines per mm
slit serrations = 1 / x*1000

71
Q

ripple tank

A

transparent tray with water which is lit up from above - waves can be seen with crests as bright and troughs as dark
if you use a strobe light the waves will appear to nto move

72
Q

plane waves

A

travel the same direction at the same speed
equally spaced wavefronts
energy perp to wavefronts

73
Q

circular waves

A

wavefronts travel in all directions outwards at the same speed

74
Q

circular wave reflections at plane barriers

A

image of the source is formed on the other side of the reflector as waves appear to originate from there

75
Q

reflection at a concave barrier

A

circular waves produce plane waves
plane waves form a circular wave

76
Q

diffraction

A

plane incident on a gap will produce a circular wave pattern - bend through the gap
the larger the gap the less the diffraction
gap = wavelength means max diffraction

77
Q

EM waves

A
  • transverse
  • travel in vacuum
  • at speed of light
  • electrics field travels perp to magnetic field
78
Q

progressive waves

A

transfer energy

79
Q

stationary waves

A

when two waves moving in opossite directions with the same freq, v and a are supperposed
where a = zero = node
where a = max - antinode

80
Q

hwo stationary waves are formed

A

waves are transmitted
waves refelct of the wall
waves meed transmitted waves and superpose - the two waves are moving in opossite directions with the same freq, v and a so when supperposed form a stationary wave

81
Q

stationary waves - antinodes / nodes

A

where a = zero = node
where a = max - antinode
the distance between two nodes/ antinodes is λ/2

82
Q

fundamental frequency - streched string

A

when the freuqency is = to the first harmonic - there is one antinode and 2 nodes
L = λ/2

83
Q

nth harmoic - on a string

A

there are n antinodes and n+1 nodes
L = nλ/2
f = nc/2L = nf₀

84
Q

pipe open both ends

A

f₀- antinode at both ends and node in middle
fₙ - n+1 antinodes and n nodes
L = nλ/2
f = nc/2L = nf₀

85
Q

pipe closed a both ends

A

f₀- node at both ends antinode in middle
fₙ - n+1 nodes and n antinodes
L = nλ/2
f = nc/2L = nf₀

86
Q

pipe - open one end closed other

A

f₀ - one node one antinode
fₙ - n nodes n andtinodes
L = (2n+1)λ/4
f = (2n+1)c/4L = (2n+1)f₀

87
Q

open end

A

antinode

88
Q

closed end

A

node

a - physics (20 decks)

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