Waves Flashcards
what are progressive waves
A wave that transfers energy from one point to another without transferring the medium itself
frequency
Number of waves passing a certain point per second/per unit time. Measured in Hertz (Hz) or s-1
Amplitude
the maximum displacement of a particle in the wave from its equilibrium position
Wavelength
the distance between points on successive oscillations of the wave that are in phase
Displacement
the distance of a point on the wave from its equilibrium position. It is a vector quantity; it can be positive or negative
Period
the time taken for one complete oscillation or cycle of the wave
frequency =
1 / time period
wavespeed =
wavelength x frequency
what is phase difference between 2 waves
a measure of how much a point or a wave is in front or behind another
where can wave difference be found from
the relative position of the crests or troughs of two waves of the same FREQUENCY
what does it mean if a wave is in ‘phase’
when the crests or troughs are aligned
what does it mean if a wave is in ‘antiphase’
when the crest of one wave aligns with the trough of another
what is phase difference measured as
fractions of a cycle/wavelength, degrees or radians
how can the phase difference between 2 points be described as
in phase it is 360 degrees or 2 pi radians
in anti-phase it is 180 degrees or pi radians
what are the 2 types of waves
longitudinal and transverse
transverse
a wave in which the particles oscillate perpendicular to the direction of energy transfer of the wave
longitudinal
a wave in which the particles oscillate parallel/in the same direction to the direction of energy transfer and travel of the wave
what does a transverse wave look like
it shows areas of crests and troughs
what are examples of transverse waves
EM waves such as radio, visible and UV
vibrations on a guitar string, S waves
can transverse waves be polarised
yes
do transverse waves need a medium to travel in
no
what do longitudinal waves look like
they have areas of compressions and rarefactions
what are compressions
regions of increased pressure
what are rarefactions
regions of decreased pressure
what are examples of longitudinal waves
sound waves and ultrasound waves and on a slinky spring, P waves
can longitudinal waves be polarised
no
do longitudinal waves need a medium to travel in
yes
how is energy transmitted through a longitudinal wave
the particles in the medium are vibrating as they are given energy
the compressions cause the nearby particles to also vibrate with more energy
this produces a compression further along in the medium
what is the motion of particles as a transverse wave passes by
Up to maximum/crest
-Down to equilibrium position
-Down to minimum position/ trough
-Up to equilibrium position
what is the motion of particles as a longitudinal waves passes by
particles moves back and forth
what speed do all EM waves travel at in a vacuum
3 x 10^8 m/s
what is the movement of particles in a longitudinal wave
vibrate left and right but do not move
what is the movement of particles in a transverse wave
up and down
1 wave cycle =
360 degrees of 2 pi radians
what is phase difference
a measure of how much a point or a wave is in front or behind another.
what can phase difference be measured in
degrees, radians or fractions of a wave cycle
how do you know if two waves arent in phase
if one wave is ahead or behind the other wave
how do you know if 2 waves are in antiphase
if the trough of one wave aligns with the crest if another wave
what is polarisation
when particle oscillations occur in only one of the directions perpendicular to the direction of wave propogation
OR
The restriction of a wave so that it can only oscillate in a single plane.
why does polarisation occur only in transverse waves
because transverse waves oscillate in any plane perpendicular to the propogation direction
why does polarisation not occur in longitudinal waves
the particles in a longitudinal wave always oscillate parallel to the direction of energy transfer therefore you cannot isolate a particular direction of vibration from it
how can you polarise waves
through a polariser or polarising filter, as they only allow oscillations in a certain plane to be transmitted
how can light be polarised
through reflection, refraction and scattering
how does a polaroid filter work
it removes all the different planes of the wave except one
what speed do transverse waves travel at
all transverse waves travel at the same speed in a vacuum
what happens when two polarising filters perpendicular to each other are used
all the light is blocked out because the two filters filter out waves in different planes so completely block out all the wave.
what happens when two polarising filters parallel to each other are used
the first filter will polarise the light in 1 axis/direction
All the polarised light will pass through the 2nd filter unaffected
Here, the maximum light intensity is transmitted
what is the intensity of light transmitted when two filters perpendicular to each other are used
minimum intensity of light is transmitted
what are the uses of polarisers
polarising sunglasses
TV and radio signals
cameras
what do polarising sunglasses do
reduce the glare of reflected light
what happens to light when it is reflected
it is partially polarised
why are the filters in polarising sunglasses oriented
so they cut out light reflecting from horizontal surfaces such as water, snow and tarmac
why do the polarising sunglasses not allow any horizontal light though
the polarising filters have vertically oriented transmission axis
why do TV and radio aerials need to be correctly aligned
to get the best reception and signal
how can you reduce interference between nearby transmitters
if one of the two transmitters is vertically aligned and other is horizontally aligned
how would a graph showing light intensity at different angles look like
at o degrees - max light intensity
at 90 - minimum
at 180 - maximum
at 270 - minimum
at 360 - maximum
describe what an observer would see as the 2nd polarising filter is rotated through 360 degrees (2)
they would see a variation in intensity between the max and minimum. There will be two maxima in a 360 degrees rotation
explain why it is important to correctly align the aerial of a TV in order to receive the strongest signal
transmitted radio waves are often polarised
Aerial rods must be aligned in the same plane of the wave
when does refraction occur
when light passes a boundary between two different transparent mediums/media
why does refraction occur
one side of the wavefront crosses the boundary first, changing its speed hence causing the wavefront to change directions
what happens to the light ray when it goes from a MORE DENSE to a LESS DENSE medium
the light ray speeds up and bends away from the normal
what happens when the light ray goes from a less dense to more dense medium
the light ray slows down and bend towards the normal
what changes during refraction
the speed and wavelength but NOT the frequency
what is refractive index
a property of a material that measures how much light slows down when passing through it
equation for refractive index
speed of light in a vacuum / speed of light in a substance
what does the speed that the light travels at depend on
the refractive index of the substance
higher refractive index means that …..
it is optically dense
what is the refractive index of air
1
why is the refractive index of air 1
because light doesn’t slow down significantly when travelling through air
equation for snell’s law
n1 sin θ1 = n2 sin θ2
what is n1
the refractive index of material 1
what is n2
the refractive index of material 2
what is theta 1
angle of incidence of the ray in material 1
what is theta 2
angle of refraction of the ray in material 2
at what angle of refraction is light reflected across the boundary
90 degrees and the angle of incidence here is the critical angle
sin theta c =
n2 / n 1 , where theta c is the critical angle
when does TIR occur
when the angle of incidence is greater than the critical angle and the incident refractive index, n1, is greater than the refractive index of the material at the boundary n2 (more dense to less dense)
what are the conditions for TIR
angle of incidence has to be greater than the critical angle
the refractive index n1 is greater than refractive index n2
what are fibre optics used to do
they use TIR to send high speed light signals over large distances
what are the uses of fibre optics
communications ( telephone and internet transmission)
Medical imaging ( endoscopes)
what are the 3 main components of fibre optics
1 . Optically dense core such as glass
- Lower optical dense cladding surrounding the core
- An outer sheath
What happens if n of cladding is greater than n of core
TIR cannot occur
what is the purpose of the outer sheath
prevents physical damage to the fibre
strengthens the fibre
protects the fibre from scratches on outside
what is the purpose of cladding
protects core from damage
keeps signals secure
prevents scratching of the core
Provides fibre with strength ( preventing breakage)
when does material dispersion occur
when white light is used instead of monochromatic light
why does material dispersion occur when white light is used
different wavelengths of light travel at different speeds e.g. blue light is slower than red due to greater refractive index
what is pulse broadening
the elongation of a signal passed down an optical fibre, commonly due to modal or material dispersion
how do you prevent pulse broadening
By using monochromatic light source so speed of pulse is constant
Make core narrow as possible to reduce possible differences in path length of the signal
Optic fibre repeaters so pulse is regenerated before significant pulse broadening takes place
when does modal dispersion occur
occurs when rays inside an optical fibre take slightly different paths. Rays taking longer paths take longer to travel through the fibre, so the duration of the pulse increases and the pulse broadens.
why is modal dispersion more common in wider cores
the light travelling along the axis of the core travels a shorter distance than light undergoing TIR at the core/cladding boundaries
OR
these fibres are broad enough to allow rays to take different paths
how do you prevent MODAL dispersion
Use narrow cores
What are the advantages of a narrow core
less light is lost by refraction
less overlapping pulse so modal dispersion decreases
Quality of signal improves and is less distorted
Signal is transferred quicker
Angle of incidence is less likely to be smaller than the critical angle
what is absorption of signal
part of the signals energy is absorbed by the fibre and some wavelengths are absorbed and so the signal becomes weaker
what does absorption do
reduces the amplitude of signal so there is a loss of information and signal strength falls
what does pulse broadening cause
different pulses to merge which leads to completely distorted final pulse
how can you reduce absorption
use an extremely transparent core
Use optical fibre repeaters
why does optical fibre repeaters reduce absorption
so the pulse is regenerated before the signal absorption occurs
how do you reduce pulse broadening
core should be as narrow as possible
use monochromatic light source
use optical fibre repeaters
why does using a narrow core reduce pulse broadening
it reduces the possible differences in path length of the signal and light is very nearly confined to one single path along the axis of the cable
why does using a monochromatic source reduce pulse broadening
the speed of the pulse will be constant
What is superposition
2 waves of the same type meet at the same point and overlap.
The resultant displacement is the vector sum of the displacements of each wave
What is superposition used for
Reducing unwanted noise
How are stationary / standing waves produced
by the superposition of 2 waves with the SAME frequency and amplitude travelling in opposite directions
When are stationary waves often produced
when reflections of a progressive wave superpose with original wave
what happens in constructive superposition
the 2 displacements add up
what happens in deconstructive superposition
the 2 displacements cancel out
when does constructive superposition occur
when the 2 waves are in phase
when does deconstructive superposition occur
when the 2 waves are out of phase
what are the differences in amplitude between progressive and stationary waves
progressive - all points (in turn) have same amplitude
stationary - each point has different amplitude depending on amount of superposition
what are the differences in phase between progressive and stationary waves
progressive - points exactly a wavelength apart are in phase
stationary - points between nodes are in phase. Points either side of a node are out of phase
what are the differences in energy between progressive and stationary waves
progressive - energy is transferred
stationary - energy is stored
what are the differences in speed between progressive and stationary waves
progressive - speed is the speed through the medium
stationary - each point oscillates as a different speed. The overall wave doesnt move
Nodes
Points with 0 displacement on a stationary wave. They do not move at all
Antinodes
Points of maximum displacement on a stationary wave. They only move vertically
Why do particles not oscillate at a node
The amplitudes of the 2 waves moving in opposite directions always cancel out so particles dont oscillate. Destructive interference
What are the points of deconstructive interference called
nodes
what are the points of constructive interference called
antinodes
How much are 2 nodes / 2 antinodes seperated by
half a wavelength
conditions for superposition
waves must be travelling in opposite directions
same frequency , amplitude and wavelength
How is sound in guitars for example, produced
vibrations caused by stationary waves produce sound
How are stationary waves produced by microwaves
Place a microwave source in line with a reflector with a detector in the middle
Reflector can be moved to vary the stationary wave pattern
When you move the detector, it picks up the nodes and antinodes
How are sound produced in instruments such as organs
Produced as a result of the formation of sound waves inside an air column
How are stationary waves produced by sound waves
Place fine powder in an air column and a loudspeaker at the open end
At certain frequencies, the powder is evenly spread to show places of 0 disturbance due to the nodes
There must be a node at one end and an antinode at the end with the loudspeaker, to produce the stationary wave
Harmonics
Different modes of vibration
1st Harmonic
Mode of vibration with the longest wavelength
2nd Harmonic
Mode of vibration with the 2nd longest wavelength
frequency of 1st harmonic =
1/2 length of string x root (tension in string / mass per unit length of string )
speed of wave travelling across 2 fixed ends =
root ( tension in string / mass per unit length of string )
Describe the structure of a step index optical fibre, outlining the purpose of the core and the cladding (3)
Core is a transmission medium for EM waves to progress by total internal reflection
Cladding provides lower refractive index so total internal reflection can occur
Cladding also offers protection of boundary from scratching which could lead to light leaving the core
How to reduce effect of pulse broadening caused by material dispersion (2)
Use a monochromatic light source so the speed of the pulse is constant
Use optic fibre repeaters to regenerate the signal before significant pulse broadening occurs
what happens inside microwaves when cooking food
Microwaves are generated by a magnetron
The waves reflect off the metallic inner surfaces to ensure the food cooks evenly
What happens to the food at the nodes and antinodes in a microwave
nodes - undercooked
Antinodes - overcooked
This is why microwaves have rotating turntables
Explain how a receiver, vertical metal rod and transmitter are used to demonstrate that the waves from the transmitter are vertically polarised (3)
- Rotate the aerial in the vertical plane
- When the aerial is vertical, the signal is at max
- When the aerial is horizontal, the signal is at min
Explain what happens to the signal detected by the receiver as the metal plate is moved slowly towards a point X (4)
- Received signal goes through max and min
- Reflected and direct microwaves interfere
- Path difference increases as the plate is moved
- Max is when aerial is aligned with the plane of polarisation of the microwave
What properties must 2 waves have to form superposed waves
Same frequency and amplitude and speed
Must be travelling in opposite directions
how many wavelengths is 2L
1
Area of a string =
pi x diameter^2 / 4
mass per unit length of a string using density =
pi x diameter^2 x density / 4
OR :
Pi x radius^2 x density
What is the aim of the stationary waves RP
To measure how the frequency of 1st harmonic is affected by changing :
Length of string
Tension
Different masses per unit length
What is the independant variable in the stationary waves RP
Length/tension/mass per unit length
What is dependant variable in the stationary waves RP
Frequency of the 1st harmonic
What is the control variable for the stationary RP if the length is varied
Same masses attached (tension) and same string (mass per unit length)
What is the control variable for the stationary waves RP if the tension is varied
Same length of the string and same string(mass per unit length)
What is the control variable for the stationary waves RP if the mass per unit length is varied
Same masses attached (tension) and same length of string
Method for the stationary waves RP
Attached 1 end of the string to vibration generator and pass the other end over the pulley and attached mass hangar.
Adjust the position of the bridge so L is measured from vibration generator to the bridge using a meter rules
Turn on the signal generator to set the string oscillating
Increase the frequency of the v. generator till 1st harmonic observed and read at which frequency this occurs
Repeat using different lengths
Repeat frequency readings 3 times and take an average
Measure tension in the string using T = mg
Calculate mass per unit length
Draw a table showing L, F1-3 and mean F
Draw a graph of F against 1/L
what is the y axis on the stationary waves Rp
frequency
What is the x axis on the stationary waves RP
1/L
What is the gradient on the stationary waves RP
V/2
What are the systematic errors in the stationary waves RP
Use an oscilloscope to verify signal gen readings
Leave signal generator for 20 mins to stabilise
Use as large as possible length to get greater resolution
What are the random errors in the stationary waves RP
The sharpness of resonance when the 1st harmonic is achieved
how to avoid the random error of the sharpness of resonance
Dont look at the amplitude as the wave is too fast
Adjust the frequency whilst looking closely at a node
Safety procedures in stationary waves RP
Use rubber strings instead of metal in case it snaps under tension
Wear goggles to protect eyes
Stand away from masses in case they fall
Coherence
If 2 waves have the same wavelength and frequency and there is a constant phase difference between the 2 waves
When does interference occur
When waves overlap and the resultant displacement is the sum of displacements of each wave
What is path difference measured in
Multiples of wavelength
Path difference
difference in distance travelled by 2 waves from their sources to the point where they meet
What happens to sound when coherent waves are in phase
Sound is louder due to constructive interference
What is the path difference for constructive interference
Path difference of n wavelengths
What is the path difference for deconstructive interference
Path difference of (n + 0.5)wavelengths
What is the safety issues with lasers
Lasers produce very high energy beam of light which can cause blindness or permanent eye damage
Safety precautions with lasers
never look directly at a laser or its reflections
Do not shine the laser towards a person
Wear safety goggles
Stand behind the laser
What are the conditions for light for two source interference
Wave sources must be coherent and monochromatic
Describe what happens when monochromatic lights is shined through a single and double slit
Monochromatic light is shone behind a single slit.
Light is diffracted producing 2 light sources at the double slit
Light waves are coherent so create an observable interference pattern
Interference patter is made up of light and dark fringes
fringe width =
wavelength x distance from double slit to screen / distance between centres of slits
What happens at bright fringes
constructive interference
Why is the distance between fringes very small
Due to the short wavelength of visible light
What happens to the interference pattern when monochromatic light is replaced with white light
Different interference patter forms.
White lights has all colours of visible light.
Each wavelength of v. light produces its own interference pattern
Central fringe is white because the path difference for all the wavelengths is 0 so they are in phase
the 1st. maximum is when the phase difference is 1 wavelength
Each colour produces maximum in different positions so colours spread into sprectrum
What was the 1st significant discovery
Newton - Visible light is a stream of microscopic particles (corpuscles) but it couldn’t explain diffraction or refraction
What was the 2nd significant discovery
Huygens - Original wave theory of light (its a series of wavefronts) to explain diffraction and refraction
What was the 3rd significant discovery
Young - light is a wave that undergoes (de)constructive interference
What was the 4th significant discovery
James Maxwell - E + M fields obey wave equation and light is made of e + m fields travelling perpendicular
What was the 5th and last significant discovery
Light behaves as a particle.
Light is described in terms of photons
Light behaves as a wave and particle
What is the aim of the Young’s double slit RP
To determine the wavelength of light using Young’s double slit apparatus
Describe the method for Young’s double slit RP
Place the laser in a clamp a few centimeters from the double slit in it’s holder. Place the screen approximately 1 metre from the double slit.
Adjust the position of the laser until an interference pattern is visible on the screen.
Increase the distance ‘D’ between the double slit and the screen to increase the fringe spacing ‘w’. This makes the fringe spacing easier to measure but if D too large the fringes will not be clearly defined.
The fringe width (or fringe spacing), w, can be measured by measuring across a large number of visible fringes. (Take care when counting – counting from the first bright fringe to the tenth bright fringe would represent nine fringe widths!).
Measure across 5 fringes and record the value 5w in your table.
Use the tape measure to measure D and obtain 5 further values for D with the corresponding value of 5w.
Plot a graph of w against D, draw a line of best fit and calculate the gradient. Given that the gradient is
λ/s provide a value for the wavelength of the laser light.
What is the independant variable in the Young’s double slit RP
Distance between double slit and the screen
What is the the dependant variable in the Young’s double slit RP
The distance between maxima
What are the control variables in the Young’s double slit RP
Slit seperation, distance between laser and screen, wavelength of the laser
Diffraction
Spreading out of waves when they pass an obstruction (usually a narrow slit/arpeture)
What does the extent of diffraction depend on
High wavelength = more diffraction
Smaller gap = more diffraction
Diffraction is most prominent when the width of the slit is = to the wavelength of the waves
What is the only property of a wave which changes when a wave is diffracted
Its amplitude because some energy is dissipated when a wave is diffracted through a gap
NOT WAVELENGTH OR WAVESPEED OR FREQUENCY
Features of the single slit separation pattern
Central maximum with high intensity
Subsidiary maxima equally spaced, successively smaller in intensity and half the width of the central maximum
What would happen if the laser was replaced with a non-laser source emitting white light
Central maximum would be white
Shortest wavelength would appear nearest to the central maximum
Longest wavelength would appear furthest from central maximum
Fringe spacing smaller and maxima wider
What happens if the blue laser was replaced with a red laser
Wavelength of red light is longer so light diffracts more
Intensity fringes wider
What happens to the intensity and fringe spacing if the slit was made narrower
Intensity = decreases as less light goes through
Fringe spacing = Wider
equation for angles at which maxima form
spacing between adjacent slits x sin0 = order of maxima x wavelength of source
Spacing between adjacent slits =
1 / number of lines per metre on grating
number of maxima =
d / wavelength
path difference at 0th order
0
path difference at 1st order
1 wavelength therefore there is constructive interference
equation for the first order maxima
sin0 = wavelength of light x slit separation
Uses of diffraction gratings
Separating lights of different wavelengths with high resolution
Used in spectrometers
X-ray crystallography
How are diffraction grating used in spectrometers
To analyse light from stars
Chemical analysis
Measure red shift
Measure frequency or wavelength of light from a star
