3. Waves Flashcards
What is a progressive wave?
a wave that transfers energy from one place to another without transferring material
What causes a wave?
something that makes particles or fields oscillate
What three things can happen to a wave?
reflection, refraction, diffraction
Define displacement
the distance and direction of a particle on a wave from its position of equilibrium
Define amplitude
the maximum magnitude of displacement from the position of equilibrium
Define period
the time taken for one complete cycle to pass a given point
Define frequency
the number of complete wave cycles per second passing a given point
Define wavelength
the length of one whole oscillation (the distance between successive peaks/troughs)
Define speed
the distance travelled by the wave per unit time
Define phase
the measurement of the position of a certain point along the wave cycle
Define phase difference
the amount by which one wave lags behind another wave
What are the units of phase and phase difference?
radians, degrees or fractions of a cycle
What is a transverse wave?
the oscillations of particles are perpendicular to the direction of energy transfer
What is a longitudinal wave?
the oscillations of particles are parallel to the direction of energy transfer
State three types of transverse waves
- electromagnetic (EM) waves
- water waves
- earthquake S-waves
State two types of longitudinal waves
- sound waves
- earthquake P-waves
Give one characteristic of longitudinal waves
areas of compression and rarefaction
At what speed do EM waves travel in a vacuum?
3 x 10^8 m/s
What does it mean if a wave is unpolarised?
the particles/field oscillate in more than one plane
What does it mean if a wave is polarised?
the particles/field oscillate in just one plane
What can be used to polarise unpolarised light?
a polarising filter
What does a polarising filter consist of?
very small openings which can be orientated horizontally or vertically
How can all light be blocked?
using a combination of vertical and horizontal filters
Are the signals sent from TV transmitters polarised or unpolarised?
polarised
Why must TV aerials be aligned correctly?
to receive a signal, the aerial must be aligned in the same plane of polarisation as the polarised wave
How do Polaroid sunglasses reduce glare?
- the reflected light is partially polarised
- Polaroid sunglasses have polarising filters which, when perpendicular to the plane of polarisation, can block out some of the polarised light
State three examples of surfaces which act as natural polarising filters
- water
- glass
- tarmac
How can some surfaces act as natural polarising filters?
when these surfaces are hit with unpolarised light, the light that reflects off is partially polarised, making it easier to block with a filter
What determines how much light is polarised by some surfaces?
the angle of the surface
State the equation linking wave speed, wavelength and time period
wave speed = (wavelength)/(time period)
How are frequency and period related?
frequency = 1/period
inversely proportional
State the equation linking wave speed, frequency and wavelength
wave speed = frequency x wavelength
What is superposition used to describe?
two or more waves combining together
What is the principle of superposition of waves?
when two waves meet, the total displacement at a point is equal to the sum of each individual displacement of each wave at that point
What can the superposition of two or more waves result in?
interference
What are the three main types of interference?
- constructive interference
- destructive interference
- total destructive interference
What is constructive interference?
when two waves meet, if their displacements are in the same direction, the displacements combine to give a bigger displacement
What is destructive interference?
if a wave with a positive displacement meets and wave with a negative displacement, they partially cancel each other out
What is total destructive interference?
if two waves with equal and opposite displacements (the same amplitude) meet, they cancel each other out completely
When are two points on a wave in phase?
if they are both at the same point in the wave cycle e.g. 0, 360, 720…degrees
What must two waves have to be in phase?
the same frequency and the same wavelength
What is a stationary wave?
the superposition of two progressive waves travelling in opposite directions, with the same frequency/wavelength and similar amplitude
How does a stationary wave differ from a progressive wave?
no energy is transferred
What happens when the two progressive waves forming a stationary wave overlap?
a larger stationary wave is created via constructive interference
What happens when the two progressive waves forming a stationary wave line up with peak to trough?
the stationary wave becomes flat via total destructive interference
What are the two points that will form on a stationary wave?
nodes and anti-nodes
What are the characteristics of a node?
- points of no displacement
- constantly remain stationary
- total destructive interference is always occurring here
What are the characteristics of an anti-node?
- points of maximum displacement
- where amplitude reaches a maximum
- constructive interference is occurring here
What is the distance between two nodes or between two anti-nodes?
half a wavelength
What is the easiest way to make a stationary wave?
reflect a wave back on itself
What is the difference between the phase difference of a progressive wave and a stationary wave?
- phase difference can be any value between two points on a progressive wave
- two points on a stationary wave can only be in phase or in anti-phase
When can stationary waves form?
when a whole number of half wavelengths can fit
What are resonant frequencies/harmonics?
when an exact number of half wavelengths fit on the string
What is the lowest possible resonant frequency called?
the first harmonic/the fundamental frequency
In what three ways can the fundamental frequency of a string be changed?
- changing the length of the string
- changing the tension in the string
- changing the mass per unit length of the string
What happens to the fundamental frequency as wavelength decreases?
the fundamental frequency will be higher
How can you change the tension in the string?
changing the amount of mass hanging from the string
What happens to the speed of the waves when tension is increased?
speed increases
What happens to the fundamental frequency when the tension in the string is lower?
low tension = slower waves = lower frequency
What happens to the speed of waves when the string is heavier?
the waves will move slower
What happens to the fundamental frequency when the mass of the string increases?
heavier string = slower waves = lower frequency
How much diffraction do you get when the gap is significantly larger than the wavelength?
no diffraction
How much diffraction do you get when the gap is a bit larger than the wavelength?
some diffraction
How much diffraction do you get when the gap is the same size as the wavelength?
maximum diffraction
What happens when the gap is smaller than the wavelength?
the wave can’t pass through
What do you need to use to get light to diffract through a single slit?
coherent, monochromatic light
What is the meaning of the term monochromatic light?
light of a single wavelength
What sort of waves diffract?
all waves
What does the term coherent mean?
light has the same frequency and a fixed phase difference
What is the easiest way to achieve coherent, monochromatic light?
using a laser
What is a fringe pattern?
the diffraction pattern that is seen on the screen
What is a fringe pattern made up of?
a bright central fringe (central maximum) with alternating dark and bright fringes on either side
What causes bright fringes?
constructive interference, where waves from across the width of the slit arrive at the screen in phase
What causes dark fringes?
total destructive interference, where waves from across the width of the slit arrive at the screen in anti-phase
Where is the brightest part of the pattern? Why? What is this known as?
- the middle, where the light is most intense
- central maximum
What is intensity?
the power per unit area
How does the width of the central maximum compare to the other maxima?
the central maximum is twice the width of the other maxima
What is two source interference?
when the waves from two sources overlap with each other
How can two source interference be achieved?
- using two speakers producing sound waves
- two pebbles dropped into a pond
- a laser beam shone through two small slits
What do the waves need to be in order to achieve two source interference?
coherent
Where do you get a point of maximum displacement? What is this known as?
- where a peak/trough meets a peak/trough (where constructive interference takes place)
- a maxima
Where do you get a point of no displacement? What is this known as?
- where a peak meets a trough (where total destructive interference takes place)
- a minima
What would two source interference using sound waves result in?
loud and quiet spots
What is produced when laser light is shone through two small slits?
a fringe (interference pattern)
How does the pattern produced by two small slits differ from a single slit pattern?
- the pattern is much more intense and clearer
- all of the maxima are exactly the same width
What are maxima?
point of constructive interference
What are minima?
points of total destructive interference
What is path difference?
a measure of how much one wave has travelled further than another
What happens when the path difference is zero?
- waves arrive in phase
- constructive interference occurs
- bright spot forms
What happens when the path difference is half a wavelength?
- waves arrive in anti-phase
- total destructive interference occurs
- dark spot forms
What happens when the path difference is one wavelength?
- waves arrive in phase
- constructive interference occurs
- bright spot forms
What safety precautions must be made when using a laser?
- never shine towards a person
- wear laser safety goggles
- avoid shining at a reflective surface
- having a warning sign
- turn the laser off when not needed
What is produced when a laser is shone through one slit?
- a very blurry interference pattern
- central maximum twice the size
What is produced when a laser is shone through two slits?
a more intense pattern of maxima and minima due to interference
What is produced when a laser is shone through a diffraction grating?
- light diffracts through multiple slits
- produces a very intense pattern
What does a diffraction grating contain?
lots of equally sized slits very close together (hundreds of slits per millimetre)
What are orders?
the bright spots formed by a diffraction pattern using a diffraction grating
What is the straight through position of the diffracted light called?
the zero order
What are the two either side of the zero order called?
the first order
How do you calculate the maximum number of orders?
(distance between slits)/wavelength
- rounded down
What happens when you diffract white light?
- central fringe will be white
- subsidiary maxima will each be composed of an entire spectrum
- red will be furthest from the centre and blue/violet will be closest
- there will be less fringe spacing with smaller dark fringes and wider maxima
Where does light travel fastest?
in a vacuum
What is diffraction?
when a wave bends as it passes through a gap or around an obstacle
What is refraction?
when a wave changes speed and direction as it enters a different medium
What is reflection?
when a wave is bounced back when it hits a boundary
What is the absolute refractive index of a material?
the ratio between the speed of light in a vacuum and the speed of light in that material
What is the refractive of index of air?
1
What happens when a wave passes into a denser material?
- the wave slows down
- the wavelength decreases
- the frequency stays the same
When a wave travels into a less dense material, does it bend towards or away from the normal? Why?
- bends away from the material
- the wave speeds up
What is the angle of incidence?
the angle that incoming light makes to the normal
What is the angle of refraction?
the angle the refracted ray makes with the normal
What happens to a small amount of light when travelling from a high to a low refractive index?
it gets reflected
What is the greatest value of the angle of refraction?
90 degrees
What is the critical angle?
the value of the angle of the incidence when the angle of refraction reaches 90 degrees and the light cannot escape material 1
What are the two criteria needed to achieve the critical angle?
- can only be achieved when going from a higher refractive index to a lower refractive index
- when the angle of refraction is 90 degrees
How can total internal reflection (TIR) be achieved?
- if the angle of incidence is higher than the critical angle
- when light is going from a higher to a lower refractive index
What is a fibre optic cable?
a very thin flexible tube of glass or plastic fibre that can carry light signals over long distances and round corners using total internal reflection
Does the core of a fibre optic cable have a low or a high refractive index?
the core has a very high refractive index
Does the cladding of a fibre optic cable have a low or a high refractive index?
the cladding has a very low refractive index
What are the advantages of using fibre optic cables to transmit data rather than copper cables?
- signal can carry more information as it has a higher frequency
- light doesn’t heat up fibre so no energy lost as heat
- no electrical interference
- much cheaper to produce
- signal can travel a long way quickly without much signal loss
State two functions of the cladding of fibre optic cables
- to protect the core from scratches
- to prevent crossover of signal/to ensure security of data/prevent loss of information
What are the three main problems with fibre optic cables?
- absorption
- modal dispersion
- material dispersion
How does absorption cause signal degradation?
- some of the signal’s energy is absorbed by the material due to imperfections in the fibre e.g. dust and bubbles
- this energy loss results in the amplitude of the signal being reduced
What do both modal and material dispersion cause?
pulse broadening
What is the effect of pulse broadening?
- the received signal is broader than the initial signal
- broadened pulses can overlap each other, leading to information loss
How does modal dispersion occur?
light rays enter the optical fibre at different angles, causing them to take different paths down the fibre
How can modal dispersion be reduced?
by using a single-mode fibre
What does a single-mode fibre do?
only allows light to follow a very narrow path
How does material dispersion occur?
- different wavelengths are refracted by different amounts
- different wavelengths slow down by different amounts in a material
- so different parts of the signal will arrive at different times
How can material dispersion be reduced?
by using monochromatic light
What can be used to help reduce the signal degradation from both absorption and dispersion?
optical fibre repeaters
What do optical fibre repeaters do?
they can be used to regenerate the signal every so often
