waves Flashcards
state a property of microwaves [1]
microwaves are polarised
Distinguish between longitudinal and transverse waves [2]
For transverse waves oscillations are at right angles to direction of propagation while in
longitudinal waves they are in the same direction
Explain how a stationary wave is produced when a stretched string is plucked. [3]
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
Suggest how stationary waves are formed in a microwave oven [2]
waves are reflected off of the oven wall
they superpose with wave travelling in opposite direction
Explain what is meant by a progressive wave. [2]
A wave which transfers energy from one point to another without transferring material
Explain how light from the diffraction grating forms a maximum on the screen [3]
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
Explain what is meant by modal dispersion in an optical fibre [2]
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
Explain what would make two loudspeakers coherent sources of sound waves. [2]
they have the same frequency/wavelength AND
constant phase difference
And that this achieved by both speakers being connected to same signal (generator).
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.
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
Suggest why the amount of reflected light changes as the fibre bends [2]
Angle of incidence may become less than critical angle
Light may encounter impurities at different positions/angles
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]
switch laser off when not in use
or
don’t look directly into the laser (wow, geniuses taking a-level physics huh)
define frequency
number of complete waves passing a point per second
frequency {}
waves
v / λ
Hz (hertz)
define wavelength
distance between two adjacent crests on a wave
define amplitude
maximum displacement of a wave from it’s rest(equilibrium) position
time period {}
1 / f
seconds (s)
what is a longitudinal wave
a wave in which the oscillation of the particles is parallel to energy transfer
e.g sound
rarefactions and compressions
areas of low and high pressure
in a longitudinal wave
what is a transverse wave
a wave in which the oscillation of the particles is perpendicular to energy transfer
e.g EM waves
speed of Electromagnetic (EM) waves in a vacuum
3.00 * 10^8 ms^-1
what is an electromagnetic wave
vibrating electric and magnetic fields perpendicular to the direction of energy transfer
what does a polarising filter do
only allows oscillations in one plane
(plane polarises a wave)
what is evidence for the nature of transverse waves
polarisation
Transverse waves usually have multiple planes of oscillation perpendicular to motion
how is polarisation used in antennas
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
what is a stationary wave
a wave which transfers no energy
positions of max and min amplitude are constant
what is a node
a point on a stationary wave where displacement from equilibrium position is 0
what is an antinode
a point on a stationary wave where displacement from equilibrium position is at its maximum
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)
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
describe the first harmonic for a stationary wave with two closed ends
2 nodes
1 antinode in the middle
L = 1/2 λ
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
define coherence
potential for two waves to interfere, fixed phase difference
same frequency and wavelength
why is a laser often used to model diffraction and interference
laser uses monochromatic light
more defined patterns produced
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
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
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
how does the size of the slit relate to the amount of diffraction
slit should be closer to wavelength for more diffraction
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
approximate refractive index of air
1
when light enter a more optically dense material does it bend towards or away from the normal
towards
when does total internal reflection occur
when light is at a boundary to a less optically dense medium
angle of incidence > critical angle
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
what is attenuation
part of the signal’s energy is absorbed by the fibre
so the amplitude is reduced
what is pulse broadening
received signal is wider than the original
this can cause overlap of signals and loss of information
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
what is material dispersion
when light with different wavelengths is used
different wavelengths travel at different speeds
arrive at different times
pulse broadening
how can you reduce modal dispersion
use a narrow core
so possible difference in path lengths is smaller
how can you reduce material dispersion
use monochromatic light
how can both absorption and dispersion be reduced
use a optical fibre repeater to regenerate signal
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
what happens when angle of incidence = critical angle
goes along boundary
angle of refraction is 90 degrees
you know the refractive indices of two materials, how do you work out the critical angle
sin Critical = n2 / n1
where n1 > n2
finding the angle or refraction {}
n1sini = n2sinr
what does a higher refractive index mean
more optically dense
The higher the refractive index the slower the light travels
More refraction
refractive index {}
n = c / v
refractive index = speed of light / speed in substance
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
what is a first order maximum
path difference between two adjacent rays of light is 1λ
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
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
what is diffraction
the spreading out of waves when they pass through (or around) a gap
what are wave properties shows by young’s double slit experiment
diffraction and interference
fringe separation {}
w = (λD) / s
fringe spacing = ( wavelength * distance between source and screen) / slit separation
what is path difference
the difference in path travelled by 2 waves
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)
wave speed {}
v = fλ
wave speed = frequency * wavelength
what is phase
the position of a wave point on a wave cycle
measured in
radians
fractions of a cycle nλ
degrees
what is reflection
when the wave is bounced back when it hits a boundary
what is refraction
when the wave changes direction as it enter a different medium
result of the wave slowing down or speeding up
what is time period
time taken for one whole wave cycle
what do you get with two polarising filters at right angles to one another
no light passes through
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
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.
describe constructive interference
when two waves pass through each other and their displacements combine to make a displacement with greater magnitude
total destructive interference
when two waves of equal and opposite displacements meet and cancel each other out completely
what do points in phase share in common
displacement and velocity
1 complete cycle of a wave in radians
2 π (360 degrees)
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
when are two points are in phase
phase difference of 0
multiple of 360
even multiple of half wavelength (π)
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
explain the first harmonic
a stationary wave vibrating at its lowest possible resonant frequency
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
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
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)
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 )
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
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
what is intensity
power per unit area
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°)
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
what is a diffraction grating
lots of equally spaced slits
creates a sharp interference pattern since there are many beam reinforcing the pattern
when do maxima occur (diffraction grating experiment)
nth order maxima occur when path difference = nλ
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
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)
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
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
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
total internal reflection
at angles greater than the critical angle refraction doesn’t occur
all the light is reflected back into the material
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
