Waves Flashcards
What is a progressive wave
Transfers energy without transferring material, made of particles in a medium oscillating
Example of a progressive wave
Water waves - particles move up and down
What is the amplitude of a wave
Max displacement from equilbrium position - measured in meters
What is the frequency of a wave
Number of complete oscillations passing through a point per second - measured in Hz
What is the wavelength of a wave
Length of a whole oscillation - measure in meters
What is the speed of a wave
Distance travelled by wave per second - measured in ms^-1
What is the phase of a wave
Position of a certain point on a wave cycle - measured in radians, degrees or fractions of a cycle
What is the phase difference of a wave
Distance between two adjacent points in phase with each other - measured in radians, degrees or fractions of a cycle
What is the period of a wave
Time taken for one full oscillation - measured in seconds
When are two points completely out of phase
Odd integer of half cycles apart
Frequency, wavelength and speed formula
c = f x wavelength
Frequency, period formula
F = 1 / T
What are transverse waves
Oscillations of particles or fields at a right angle to the direction of energy transfer
Key features of electromagnetic waves
Transverse, travel at 3x10^8 ms^-1 in a vacuum
2 demonstrations for transverse waves
Shaking a slinky vertically, waves on a string attached to a signal generator
What are longitudinal waves
Oscillations of particles is parallel to the direction of energy transfer, compressions and rarefactions, can’t travel in a vacuum
Demonstration for longitudinal waves
Pushing a slinky horizontally
What sort of wave is sound
Longitudinal
Which sort of waves can be polarised
Only transverse
What are polarised waves
Waves that oscillates in only one plane (up and down or left and right)
What does polarisation provide evidence for
Nature of transverse waves, polarisation can only occur if a wave’s oscillations are perpendicular to its direction of travel (like in transverse waves)
What is proof of transverse waves
Polarisation - can only occur if a wave’s oscillations are perpendicular to its direction of travel
Application of polarisation
Polaroid sunglasses, TV and radio signals
How do polaroid sunglasses work
Reduce glare by blocking partially polarised light reflected from water and tarmac - only allow oscillations in the plane of the filter - making it easier to see
TV and radio signals as an application of polarisation
Plane-polarised by orientation of rods in transmitting ariel, receiving aerial must be aligned to the same plane of polarisation to receive signal at same strength
What is superposition
Displacements of 2 waves are combined as they pass each other, resultant displacement is the vector sum of each wave’s displacement
2 types of interference that occur during superposition
Constructive interference, destructive interference
What is constructive interference
When 2 waves has displacement in same direction
What is destructive interference
One has +ve displace, one has -ve displacement
What is total destructive interference
When 2 waves has equal but opposite displacements during superposition
How are stationary waves formed
Superposition of 2 progressive waves travelling in opposite directions in same plane, must have same frequency, wavelength and amplitude
Transferring of energy in a stationary wave
No energy is transferred
Formation of stationary waves - in phase
Antinodes are formed
Formation of stationary waves - out of phase
Nodes are formed
What are antinodes
Regions of max displacement
What are nodes
Regions of no displacement
Demonstration for stationary waves
String fixed at one end and attached to a driving oscillator at the other, wave fromed by oscillator, travels down string, reflected back, causes superposition of 2 waves
What is the lowest frequency at which a stationary wave can be formed
First harmonic
First harmonic
2 nodes, 1 antinode
Stationary waves, distance between adjacent nodes (or antinodes)
Half a wavelength
Frequency of a stationary wave on a string formula
F = (1 / 2L) (square root(T / U))
F = (1 / 2L) (square root(T / U)) what is F
Frequency
F = (1 / 2L) (square root(T / U)) what is L
Length
F = (1 / 2L) (square root(T / U)) what is T
Tension
F = (1 / 2L) (square root(T / U)) what is U
Mass per unit length
Relationship between first harmonic frequency and frequencies for other harmonies
Double first frequency to get second frequency (2 antinodes), triple first to get third (third antinodes) and so on
2 examples of stationary waves
Stationary microwaves, stationary sound waves
Stationary microwaves
Formed by reflecting a microwave beam at a metal plate, use a microwave probe to find nodes and antinodes
Stationary sound waves
Formed by placing a speaker at one end of closed glass tube, lay powder across bottom of tube, will shake at antinodes and settle at nodes, distance between nodes is half a wavelength, frequency of signal generator to speaker is known, c = f(wavelength), so speed of sound in air can be found
What is path difference
Difference in distance travelled by 2 waves
Features of a coherent light source
Same frequency and wavelength, fixed phase difference
Features of lasers
Coherent, monochromatic
Uses of lasers
Often used as a source of light in diffraction experiments as they form clear interference patterns
What does Young’s double slit experiment demonstrate
Interference of light from 2 sources
Young’s double slit experiment light source
Use 2 coherent sources of light or 1 source and a double slit, can use a filter to make light monochromatic and a single slit before the double to make a fixed path difference
Young’s double slit experiment method
Shine coherent light source through 2 slits about same size as wavelength so it diffracts - each slit acts as a coherent point source
Young’s double slit experiment results
Makes a pattern of light and dark fringes
Young’s double slit experiment light fringes
Light meets in phase and interferences constructively (occurs when path difference is a whole number of wavelengths)
Young’s double slit experiment dark fringes
Light meets completely out of phase and interferes destructively (occurs when path difference is out by half a wavelength {[n+1/2]xWavelength})
Formula associated with double slit experiment
w = (wavelength)D / s
w = (wavelength)D / s what is w
Fringe spacing
w = (wavelength)D / s what is D
Distance between screen and slits
w = (wavelength)D / s what is S
Slit seperation
Double slit experiment, using white light instead of monochromatic
Wider maxima, less intense diffraction pattern, central white fringe, alternating spectra colours, violet closest to centre and red furthest out
Dangers of using lasers
Can permanently damage eyesight
Safety precautions for working with lasers
Laser safetly goggles, don’t shine it at reflective surfaces, warning sign, never shine at a person
Double slit experiment with sound waves
Instead of double slit, use 2 speakers connected to same signal generator, intesity of waves can be measured using a microphone to find the maxima (equivalent to light fringes) and minima (equivalent to dark fringes)
Evidence for wave nature of light
Young’s double slit experiment - diffraction and interference are wave properties, so EM radiation must act as a wave (at least partly)
Understanding of light before Young’s double slit experiment
Light was formed from tiny particles
Change of understanding due to scientific discoveries statement
Knowledge and understanding of any scientific concept changes over time in accordance to the experimental evidence gathered by the scientific community
What is diffraction
Spreading out of waves as they through or around a gap
When does the greatest diffraction occur
When gap is same size as the wavelength
Diffraction - what happens when gap is smaller than a wavelength
Most waves are reflected
Diffraction - what happens when gap is bigger than a wavelength
Less noticeable diffraction
What happens when a wave meets an obstacle
Diffraction around the edges, wider the obstacle compared to wavelength means less diffraction occurs
What happens when monochromatic light is diffracted through a single slit onto a screen
Interference pattern of light and dark fringes, bright central fringe is double the width of all other fringes, alternating dark and bright fringes on either side
What happens when monochromatic light is diffracted through a single slit onto a screen - bright fringes explanation
Constructive interference - waves meet in phase
What happens when monochromatic light is diffracted through a single slit onto a screen - dark fringes explanation
Destructive interference - waves meet out of phase
What happens when monochromatic light is diffracted through a single slit onto a screen - intensity of fringe
Decreases as it moves away from central fringe
What happens when white light is diffracted through a single slit onto a screen
Central white maxima, alternating bright fringes which are spectra, violet closest to central maxima, red furthest away
White light diffracted through single slit explanation
White light is made of all colours so different wavelengths of visible light, different wavelengths are all diffracted by different amounts so you get a spectrum of colour in the diffraction pattern
Ways to vary width of central maximum in single slit experiment
Slit width, wavelength
How varying the slit width changes the width of the central maxima in a single slit experiment
Increasing slit width decreases amount of diffraction so central maxima becomes narrower and intensity increases
How varying the wavelength changes the width of the central maxima in a single slit experiment
Increasing light wavelength increases diffraction as slit is closer in size to light’s wavelength, so central maxima becomes wider and it’s intensity decreases
What is a diffraction grating
Slide containing many equally spaced slits very close together
What happens when monochromatic light passes through a diffraction grating
Interference patten is much sharper and brighter than it would have been if it had passed through a double slits like in Young’s double slit experiment
Why do diffraction gratings give more clear interference patterns than double slits
More rays of light reinforcing the patten
What do you call the ray of light passing through the centre of a diffraction grating
Zero order line
What do you call the lines on either side of the zero order line
First order lines
Formula associated with diffraction gratings
d sin0 = n (wavelength)
d sin0 = n (wavelength) what is d
Distance between slits
d sin0 = n (wavelength) what is sin0
Half a wavelength
d sin0 = n (wavelength) what is n
The order
d sin0 = n (wavelength) effect of a changing wavelength
As wavelength increases, distance between the orders will increase because angle is larger due to increase in diffraction as the slit spacing is closer in size to the wavelength so pattern will spread out
d sin0 = n (wavelength) what angles are impossible
When sin0 > 1
How to derive d sin0 = n (wavelength)
Distance between slits is d, draw a line going from slit 1 and meeting line 2 perpendicularily (adjacent), angle between d and a is theta, distance between a and d on ray 2 is wavelength (opposite), sin(theta) = wavelength / d so d sin(theta) = wavelength x n (n because we used the first maxima)
Applications of diffraction gratings
Split up light from stars, X-ray crystallography
Splitting up light from stars as an application for diffraction gratings
Split up light from stars using a diffraction grating to get a line absorption spectra which can show which elements are present in stars
X-ray crystallography as an application for diffraction gratings
X-rays are directed at thin crystal sheet which acts as a diffraction grating to form a diffraction pattern, because wavelength of x-rays is similar in size to the gaps between the atoms, diffraction grating can be used to measure atomic spacing in certain materials
What is the refractive index
A property of a material which measures how much it slows down light passing through it - symbol is n
How to calculate refractive index
n = c / c_s (speed of light in vacuum / speed of light in that substance)
How else can you refer to a material with a higher refractive index
More optically dense
Why is the refractive index of air approximately 1
Light doesn’t slow down significantly when travelling through air compared to a vacuum
When does refraction occur
When a wave enters a different medium, causing it to change direction, either towards or away from normal depending on material’s refractive index
What is snell’s law used for
Calculations involving refraction of light
Snell’s law
n1sin(theta)1 = n2sin(theta)2
n1sin(theta)1 = n2sin(theta)2 what is n1
Refractive index of material 1
n1sin(theta)1 = n2sin(theta)2 what is (theta)1
Angle of incidence of the ray in material 1
n1sin(theta)1 = n2sin(theta)2 what is n2
Refractive index of material 1
n1sin(theta)1 = n2sin(theta)2 what is (theta)2
Angle of incidence of the ray in material 2
What is the angle of incidence
Angle between normal and wave entering the material
What changes as light moves across the boundary of 2 materials
Speed which causes direction to change
When does a ray of light slow down and bend towards the normal
When the material it is entering is more optically dense than the one it was already in
What happens when a ray of light enters a less optically dense material
Bends away from the normal
What increases with the angle of incidence
Angle of refraction
Limit for angle of refraction
90 degrees
What happens when the angle of refraction is exactly 90, light is refracted along the boundary, the angle of incidence has now reached the critical angle (theta)c
Formula to find the critical angle
sin(theta)c = n2 / n1 where n1 > n2
When can total internal reflection (TIR) occur
When angle of incidence is greater than critical angle, and n1 > n2 (when entering a more optically dense material)
Application of total internal reflection
Optical fibres
What are are optic fibres
Flexible, thin tubes of plastic or glass which carry information in the form of light signals
Optic fibres structure
Optically dense core, surrounded by cladding with a lower optical density allowing total internal reflection to occur
Secondary purposes of cladding on optic fibres
Protects core from damage, prevents signal degradation through light escaping from core (which can cause information to be lost)
What can signal degradation be caused by
Absorption or dispersion
Signal degredation - what is absorption
Part of signal’s energy is absorbed by fibre, reduces amplitude of signal, can lead to loss of information
Signal degredation - what is dispersion
Causes pulse broadening, received signal is broader than original transmitted signal, can overlap causing loss of information
2 types of dispersion
Modal or Material
What is modal dispersion
Caused by light rays entering fibre at different angles, so take different paths through fibres, so rays take different amounts of time to travel along the fibre, causing pulse broadening
What can reduce modal dispersion
Making core narrower, makes difference between different path lengths smaller
What is material dispersion
Cause by light consisting of different wavelengths, so light rays travel at different speeds along the fibre, leads to pulse broadening
What can reduce material dispersion
Using monochromatic light
What can reduce absorption and dispersion
Using an optical fibre repeater
What does an optical fibre repeater do
Regenerates signal during its travel to its destination
Refractive index of air
Approximately 1
