Waves Flashcards
What is a progressive wave
A moving wave that transfers energy from one point to another without moving matter
What is the displacement of a wave
The distance a point on a wave has moved from equilibrium position
What is amplitude
The maximum magnitude of displacement of a point on a wave from equilibrium position
What is wavelength
The distance between 2 adjacent points on a wave that are moving in phase
What is a time period
The time taken for a complete wave cycle to pass
What is frequency
The number of cycles per second
How are freuency and time period linked
f = 1/T
How do you calculate wavespeed
c = fλ
Transverse waves features
Oscillations of the particles/ field is perpendicular to the direction of wave travel/ energy transfer
e.g. electromagnetic waves, water waves, seismic S waves
Longitudinal waves featrues
Oscillations of the particles/ field is parallel to the direction of wave travel/ energy transfer
e.g. sound waves (ultrasound, infrasound), seismic P waves
What is an unpolarised wave
A wave that has oscillations in all planes that are perpendicular to the direction of wave travel
What is a polarised wave
A wave that has oscillations in only one plane that is perpendicular to the direction of wave travel
How can waves become polarised
Pass the waves through a polarising filter
Only oscillations in a certain plane (the transmission axis) are transmitted
Oscillations in other planes are absorbed
Intensity is reduced
What is partial plane polarisation
It happens when waves are reflected from a reflective surface
e.g. if the surface is horizontal, a proportion of the reflected light will oscillate more in the horizontal plane than the vertical plane
What are some uses of polarisers
Polaroid sunglasses and photography - to reduce glare
Radio and microwave signals - aerial orientation
What is the principle of superposition
When 2 (or more) waves arrive at a point, they interfere and the resultant displacement is the vector sum of the displacements of each wave.
What are coherent waves
Waves with the same frequency (or wavelength)
Waves with a fixed phase difference between them
What occurs when 2 coherent waves are in phase
Constructive interference
What occurs when 2 coherent waves are in anti-phase
Destructive interference
What are stationary waves
Waves that store energy
How do you form stationary waves
The superposition of 2 progessive waves travelling along the same line
The waves must have the same speed, frequency (or wavelength), similar amplitude and be travelling in opposite directions
Stationary wave features
Nodes have zero amplitude, anti-nodes have maximum amplitude
The wavelength is twice the distance between 2 adjacent nodes
Between 2 nodes: the points are in phase and in anti-phase with the points bewteen the next set of nodes
What occurs at resonant frequencies
An exact number of half wavelengths fit into the string
Features of the nth harmonic (string of length L, with wavespeed v)
n + 1 nodes and n anti-nodes
Number of wavelengths is n/2
λ = 2L/n
fn = vn/2L = nf1
How does the lenght of the string affect the resonant frequency
f = v/2L, f ∝ 1/L
As length increases, frequency decreases
How does mass per unit length affect the resonant frequency
μ = m/L
As μ increases, a heavier string causes waves to travel slower, as f = c/λ, f decreases as c decreases
f ∝ 1/√μ
How does the tension in the string affect the resonant frequency
T = mg
As T increases, waves travel faster
As f = c/λ, f increases
f ∝ √T
What is the equation for the resonant frequency of the first harmonic
f1 = 1/2L x √T/√μ
What is path difference
The difference in the distance travelled by 2 waves from their sources to the point where they meet
What does a path difference of nλ mean
Constructive interference occurs
The waves are in phase
What does a path difference of (n + 1/2)λ mean
Destructive interference occurs
The waves are in anti-phase
What happens during Young’s double slit experiment
Light diffracts through 2 slits
Light superposes and interferes
Produces pattern on a screen
How are bright fringes formed in Young’s double slit
The maxima are formed by constuctive interference
The path difference is nλ (waves are in phase)
How are dark fringes formed in Young’s double slit
Minima are formed by destructive interference
Path differernce is (n + 1/2)λ (waves are in anti-phase)
Features of the pattern on the screen in Young’s double slit
Bright and dark fringes with even spacing parallel to the slits
The intensity of bright fringes decreases as you move away from the centre
Define fringe spacing
The distance between the centre of 2 adjacent bright (or dark) fringes
What is Young’s double slit equation
w = λD/s
w is fringe spacing, D is distance from the slit to the screen, s is slit seperation
What does the Young’s double slit screen pattern look like for white light
White central bright fringe
Outer bright fringes are a spectrum (violet closest to centre and red furthest out)
Outer bright fringes are wider (may merge)
Outer dark fringes become harder to see
Ways to be safe around lasers
Never look directly at laser
Don’t shine laser at people
Wear safety goggles
Stand behind the lases
What is diffraction
The spreading out of waves as they pass through a gap or go around an obstacle
How does reducing the size of the gap affect the amount of diffraction
The amount of diffraction increases
How does decreasing the wavelength for the same sized gap affect the amount of diffraction
Diffraction amount increases
Features of the single slit diffraction pattern
Central bright fringe is 2x width of other fringes
Central bright fringe has musch higher intensity
Intensity of other fringes decreases as you go further out
How are the bright fringes formed in single slit diffraction formes
Constructive interference
Waves arrive in phase
Path difference is nλ
How are the dark fringes formed in single slit diffraction
Destructive interference
Waves arrive in anti-phase
Pathe difference is (n + 1/2)λ
What is the pattern formed by white light for single slit diffraction
Central bright fringe is white, has the highest intensity and 2x width of other fringes
Outer bright fringes are a spectrum (violect closest to centre and red furthest out)
Wider outer fringes than with monochromatic light
How does increasing the slit width affect the intensity of the central maximum
Amount of diffraction decreaases
Central maximum is narrower
Intensity of the central maximum is higher
As the same amount of energy is concentrated over a smaller area
How does increasing the wavelength have an effect on the intensity and width of the central maximum
The amount of diffraction increases
The central maximum is wider
The intensity of the central maximum is lower
As the same amount of energy is spread over a larger area
What is a diffraction grating
Plate with hundreds of parallel slits per mm
Explain the pattern created by using a diffraction grating on monochromatic light in terms of path difference
Central maximum is the 0 order where path difference = 0λ
Outer maxima are the nth order lines where path difference = nλ
Minima are formed by destructive interference where path difference = (n + 1/2)λ
How do you calculate the maximum number of orders
d/λ rounded down to nearest integer
What is the diffraction grating equation for monochromatic light
dsinθ = nλ
d is distance between slits
n is order number
θ is angle between the incident beam and the nth order
What happends to the angle of diffraction if the wavelength of light is increases
sinθ = nλ/d
sinθ increases, θ increases
Pattern is more spread out
What happens to the angle if the distance between the slits is increases
sinθ = nλ/d
sinθ decreases, θ decreases
Pattern is less spread out
Features of the diffraction grating pattern for white light
The zero order maximum is white
All other orders are a spectrum (violet closest to centre and red furthest out, as sinθ ∝ λ)
Applications of diffraction gratings
Identifying elements
X-ray crystallography (discovering structure of DNA)
What is refraction
When a wave crosses a boundry between 2 medium at an angle which makes is change direction
What happens when light enters a more optically dense material
Wave goes towards normal as its wavespeed decreases
Frequency does not change so wavelength also decreases
What is refractive index and how do you calculate it
Refractive index is a measure of optical density (how much light slows down when it enters a material)
n = c/cs
The refractive index of air ≈ 1
What is Snell’s law
n1sinθ1 = n2sinθ2
What happens when in Snell’s law, θ2 = 90°
sinθ2 = 1
Light is refracted along the boundary
This is when θ1 = θc (the critical angle)
Under what conditions does total internal reflection occur
If n1 > n2
If the angle of incidence is greater than the critical angle
Then all light is reflected back into the material
How do you calculate the critical angle
sinθc = n2/n1
(where n1 > n2)
Uses of optical fibres
Communications - high speed internet transmission
Medical imaging - endoscopy for digestive system problem diagnosis
Features of the core in step index optical fibres
Medium the light travels through (made of plastic/ glass)
Very narrow
Has a greater refractive index than the cladding
Features of the cladding in step index optical firbes
Lower refractive index than the core
Protects core from scratches or breakages
Prevents signal crossover of adjacent cores
What is signal degredation by absorption and how can you fix it
Material of the core absorbs signal’s energy
Amplitude of signal is reduces
Solution: make core out of a low absorption material
What is signal degredation by modal dispersion and how can you fix it
Light enters the core at different angles so they take different paths
Longer paths take more time which causes pulse broadening (signals overlap and mix up)
Solution: single-mode fibre (very narrow and has small difference between refractive indeces of core and cladding)
What is signal degradation by material dispersion and how can you fix it
When light of different wavelengths is used, some travel slower (violest slowest, red fastest)
Some light reaches the other end faster causing pulse broadening
Solution: use monochromatic light
What can solve all 3 types of signal degradation
Optical fibre repeater which boosts and regenerates the signal
This reduces signal degradation
