Section 3 - Waves Flashcards
Wave def
The oscillation of particles of fields
Progressive wave def
Moving wave - carries energy from one place to another without transferring any material
Evidence of how waves carry energy
- EM waves cause things to heat up
- X rays and gamma rays cause ionisation
- Sounds cause vibrations
- Wave power can be used to generate electricity
- Since waves carry energy away, the source loses energy
Wave parts diagram
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Wave Cycle
One complete vibration of the wave
Wave Displacement
X in metres - how far from a point a wave has moved from its undisturbed position
Wave Amplitude
A in metres, the maximum magnitude of displacement
Wave Wavelength
In metres - the length of one whole wave cycle, from crest to crest or trough to trough
Wave Period
T in seconds, the time taken for a whole cycle to complete or to pass given point
Wave Frequency
f in hertz, the number of cycles per second
Wave phase
A measurement of the position of a certain point along the wave cycle
Wave phase difference
The amount one wave lags behind another
Phase or Phase difference are measured as [2]
- Angles
- Fractions of a cycle
Wave reflection def
The wave is bounced back when it hits a boundary
Wave refraction def
The wave changes direction as it enters a different medium
Frequency formula
f = 1/T
Where T = time period
Wave speed formulae
Wave speed = Distance travelled / Time taken
c = d/t
Speed, Wavelength, Ferquency
C = wavelength*freq
Speed of light in a vacuum
3*10^8
Transverse Waves def
Waves that oscillate at right angles to the direction of energy transfer
Examples of transverse waves [3]
- All EM waves
- Ripples
- Waves on a string
How can waves be drawn [2]
Displacement against distance
Displacement against time
Longitudinal waves def
A wave in which the direction of oscillation in parallel to the direction of energy transfer
A longitudinal wave consists of ___&___
Compressions and rarefactions
Examples of longitudinal waves
Sound waves, shock waves
Polarised Wave def
A wave that only oscillates in one direction
Polarisation can only happen for ______
Transverse Waves
How do polarising filters work [4]
- Light waves are a mixture of different directions of oscillation
- Waves can be polarised when passed through a polarising filter
- This causes them to oscillate in one direction only
- Two polarising filters at right angles will cause no light to pass through
How can light be partially polarised
When light is reflected off some surfaces, it can become partially polarised
How do Polaroid sunglasses work
If you view reflected partially polarised light through a polarising filter at the correct angle, you can block out unwanted glare
Polarisation of TV and radio signals
- TV and radio signals are polarised
- The rods of the transmitting and receiving Ariel must be aligned to receive a strong signal
The principle of superposition
When two or more waves cross, the resultant displacement equals the vector sum of the individual displacements
Interference can be ___ or ____
Constructive or destructive
Constructive interference
Two crests result in a bigger crest
Destructive interference
A crest and a trough of equal magnitude result in nothing since the two displacements cancel each other out
If two points are in phase, they interfere _____
Constructively
When are two points exactly in phase
When they are both at the same point in the wave cycle, have the same displacement and velocity
Phase difference of a multiple of 360 degrees
When are points exactly out of phase
When they have a phase difference of an odd multiple of 180 degrees
To get an interference pattern, the two sources ____ __ ________
must be coherent
What makes two sources coherent
Two sources are coherent if they have the same wavelength and frequency, thus a fixed phase difference between them
Constructive or destructive interference depends on __________
The path difference
Constructive interference occurs when
Path difference = n*wavelength
Destructive interference occurs when
Path difference = (n+0.5)Wavelengths
Stationary wave def
A stationary wave is the superposition of two progressive waves with the same frequency and wavelength, moving in opposite directions
Key difference between progressive waves and stationary waves
Progressive waves transfer energy, stationary waves do not
demonstrating stationary waves with a string
Set up a driving oscillator at one end of a stretched string with the other end fixed
Wave Node Def
Where the amplitude of the oscillation is zero
Wave antinode def
Points of maximum amplitude
Resonant frequency
At resonant frequencies, an exact number of half wavelength fit on the string
First harmonic
1/2 wavelengths
Second Harmonic
1 wavelength
Third Harmonic
3/2 wavelengths
Demonstrating stationary waves with microwaves
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Demonstrating stationary waves in a column of air
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Investigating factors affecting the resonant frequencies of a string
Factors affecting resonant frequencies on a string
Length
Weight
Tension
Frequency of fist harmonic
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Diffraction def
The way that waves spread out as they come through a narrow gap or go round obstacles
What does the amount of diffraction depend on
The wavelength of the wave compared to the size of the gap
Diffraction if the gap is a lot bigger than the wavelength
Diffraction is unnoticeable
Diffraction - If the gap is several wavelengths wide
Noticeable diffraction
Diffraction - the gap is the same size as the wavelength
Most diffraction
Diffraction - Gap is smaller than the wavelength
Waves are mostly reflected back
Conditions to observe a clear diffraction pattern
A monochromatic, coherent light source
Demonstrating light diffraction patterns with a laser
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How can light diffraction be demonstrated using a laser
Passing a wave through a narrow slit and projecting on a slit.
The wavelength is about the same size as the appeture
Observation for light diffraction using a laser [2]
A central bight fringe (central maximum)
Dark and bright fringes alternating on either side
How does the diffraction of white light create a spectra of colors [3]
- White light is a mixture of wavelengths
- When passed through the slit, all wavelengths are diffracted by a different amount
- This produces a spectra of colors
Light intensity [2]
The number of photons
Power per unit area
Light intensity of the central maximum
Highest Light intensity at the centre of the central maximum
Effect of increasing slit width on the width of the central maximum, light intensity [3]
Decreases amount of diffraction
Narrower central maximum
Higher light intensity
Effect of increasing wavelength on the width of the central maximum, light intensity [3]
Increases amount of diffraction
Wider Central Maximum
Lower light intensity
Demonstrating two source interference in water and sound
Two coherent sources with the same wavelength and frequency
Conditions for Youngs Double slit experiment
Two coherent monochromatic sources, the slit size is about the same as the wavelength
Youngs double slit setup
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Laser Safety [5]
1) Never shine the laser towards a person
2) Wear laser safety goggles
3) Avoid shining the laser beam at a reflective surface
4) Have a warning sign on display
5) Turn the laser off when it is not needed
Microwaves two source interference setup
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Youngs double slit fringe spacing formula
Fringe Spacing = (Wavelength*Distance)/slit spacing
How is the Youngs Experiment evidence for the Wave nature of EM radiation [2]
- Diffraction and interference are both uniquely wave properties
- The experiment shows that light can diffract and interfere
Interference patterns get ______ when you diffract through more slits
Sharper, the multiple beams reinforce the pattern
Why are sharper beams more useful
More accurate measurements
Observations for a monochromatic diffraction grating
1) All the Maxima are sharp lines
2) There is a line of maximum brightness at the centre called the zero order line
3) The following Paris are called the first order lines and so on
Diffraction grating formula
slit spacing* sin () = n wavelength
Diagram of diffraction gratings for monochromatic light
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Deriving the equation for diffraction gratings
- triangle, d sin a = wavelength
Diffraction gratings, bigger wavelength
Sin () is bigger
so () is bigger
The pattern is more spread out
Diffraction gratings, bigger slit spacing
Sin () is smaller
() is smaller
The pattern is less spread out
Diffraction gratings for values of sin() >1
Do not exist since sin() can not be greater than 1
What happens when diffraction gratings are used for non-monochromatic light
- White light is made of a mix of wavelengths
- Different wavelengths diffract different amounts
- Each order in the pattern becomes a spectrum
- Zero order stays white
Image, diffraction of white light through a grating
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Uses of diffraction gratings [4]
- More accurate
- Called X ray crystallography
- Used to measure the spacing between atoms
- Used to discover DNA
Refractive index def
A measure of how much light slows down in a material / how much light will defreact
Why diffraction happens
Light slows down in materials because it interacts with the particles in it. The more dense the material is, the more light slows down. The slowing down causes light to bend at a boundary
Formaula for absolute refractive index
n = Speed of light in a vacuum / speed of light in medium
Formula for relative refractive index
1n2 = c1 / c2
relative refractive index in terms of absolute refractive index
1n2 = n2 / n1
Absolute refractive index vs relative refractive index
the absolute refractive index is the property of a single material only while the relative refractive index is the property of the interface between two materials
Refractive index of air / vaccum
1
When light enters a denser medium
Refraction towards the normal
When light enters a less dense medium
Refraction away from the normal
Snells Law
n1 * sin()1 = n2 * sin()2
Critical Angle def
The maximum angle unto which refraction occurs. At () greater than the critical angle, all the light is reflected back into the material, total internal reflection
Critical angle formula
1n2 = sin ()c
How do optical fibres work [3]
1) Optical fibres are made of a material with a high refractive index which is surrounded by a material with a low refractive index
2) This means that light is totally internally reflected
3) light bounces from side to side at angles greater than the CIR, moving along the firbre
Why is the cladding needed on an optical fibre
Protects fibres from scratches which would cause light to escape
Allows TIR to happen
Absorption in optical fibres
Causes loss in amplitude
Energy is absorbed by the material
Modal Dispersion in optical fibres
Light rays enter at different angles and take different paths, some take a longer path
Preventing modal dispersion in optical fibres
Single mode fibre that only lets light take one path, preventing modal dispersion
Material dispersion in optical fibres
Light consists of different wavelengths so some wavelengths reach the end of the fibre faster than others
Preventing material dispersion in optical firbres
Using monochromatic light
Result of dispersion in optical fibres
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pulse broadens at each end
