Waves Flashcards
Transverse waves
Oscillations of particles are perpendicular to the direction of energy transfer
Longitudinal waves
Oscillations of particles are parallel to the direction of energy transfer
Examples of transverse waves
Electromagnetic waves
Water waves
Earthquake S waves
Examples of longitudinal waves
Sound waves
Earthquake P waves
What type of wave can be polarised?
Transverse
Applications of polarising waves
Signals from TV transmitters are sent polarised
Sunglasses
Why are signals from TV transmitters sent polarised?
So aerials only have to be aligned in one plane
How does a sound wave transmit energy through the area
-Energy is transferred by air molecules colliding
-Oscillations are parallel to the direction of energy transfer
What causes particles to move in a longitudinal wave?
There is a compression region of increased pressure which causes particles to move
Why can’t longitudinal waves be polarised?
-Transverse: perpendicular
-Longitudinal: parallel
-Polarisation is the restriction of displacement vector to one plane
Define the amplitude of a wave
The maximum displacement of the wave from the equilibrium position
What happens when a wave is polarised?
-The particles are only oscillating in one plane
How do polarising filters work?
There are small openings that can be orientated horizontally/vertically.
Vertical will only let waves oscillating vertically through
Horizontally will only let horizontal through
How to measure wave speed using 2 microphones
-set up 2 microphones a fixed distance apart
-Both connected to a computer
-The computer records when the first microphone heard the sound and then the second
-This gives a time measurement
Use c=f λ to find speed
How to measure wave speed by timing an echo
-Someone makes a loud noise opposite a flat wall
-You measure time it takes from the sound being made to you hearing the echo
-Then measure distance to the wall
-Remember to double the distance as the sound travels there and back
How to measure wave speed by timing a gunshot
-Someone shoots a starter pistol over a large distance
-Start timing when you see gun flash
-Stop timing when you hear the gunshot
-Measure distance
Conditions for creating a stationary wave
1) Superposition of 2 progressive waves travelling in opposite directions
2)With the same frequency/wavelength
3)And similar magnitude
What is a node?
Points of no displacement on stationary waves
Total destructive interference always occurs here.
What is an anti node?
Points of maximum displacement on stationary waves.
Constructive interference occurs here.
What is the distance between 2 nodes?
λ/2
What is the distance between 2 antinodes?
λ/2
Examples of stationary waves
-Strings on instruments eg guitars
-Insides of wind instruments
-Microwave ovens
When are points in phase in progressive waves?
-If the phase difference is 0, 360, 720 etc
When are points in anti-phase in progressive waves?
-If the phase difference is 180, 540 etc
Phase difference in stationary waves
2 points can only be in phase (360)
Or anti phase (180)
All points between 2 adjacent nodes are in phase
Progressive waves
Transfer energy
Differences between stationary and progressive waves
Stationary waves have nodes and antonodes, progressive waves don’t.
Progressive waves transfer energy, stationary waves don’t.
In practice, is the mass per unit length likely to change when tension is significantly increased?
-Stretching will cause the diameter to reduce
-This reduces the mass per unit length
How does increasing tension affect fundamental frequency?
It increases
How does increasing mass per unit length affect fundamental frequency ?
Decreases
How does increasing length of string affect fundamental frequency?
Decreases
How do you get maximum diffraction?
If the gap is the same as the wavelength
What happens if the gap is a bit larger than the wavelength?
There will be some diffraction
What happens if the gap is significantly larger than the wavelength?
No diffraction
What happens if the gap is the same as the wavelength?
Maximum diffraction
What type of interference causes dark fringes?
Destructive interference
What type of interference causes bright fringes?
Constructive interference
What is constructive interference?
Waves from across the width of the slit arrive at the screen in phase.
What is destructive interference?
Waves from across the width of the slit arrive at the screen in anti-phase
What is the brightest part of the intensity graph called?
Central maximum
What is the central maximum?
The brightest part of the intensity graph
Monochromatic
The light is all the same wavelength so the amount of diffraction will be the same.
Coherent
The light has the same frequency and a fixed phase difference
How to calculate distance between slits
d=1/N
N= Number of lines per metre.
d= Distance between slits
Maximum order equation
Nmax=d/λ
Always round answer down to nearest integer
Maximum order equation
dsinθ=nλ
Where does the light bend going from a lower refractive index to a higher refractive index
Low to high
Light bends towards normal
Where does the light bend going from a higher refractive index to a lower refractive index
High to low
Light bends away from normal
What happens when a wave passes into a denser material
It slows down and wavelength decreases
How to calculate refractive index using wave speeds
n=c/cs
cs= speed of wave in the material
How does frequency change in refraction?
Stays the same
Snells law
n1sinθ1=n2sinθ2
How to calculate relative refractive index. (Refractive index going from material 1 to material 2)
1n2=n2/n1
Refractive index of air
1
Rearrangement of Snell’s law that includes wave speed.
n1/n2=c2/c1
When does total internal reflection happen?
angle of incidence > critical angle
Wave is travelling from a higher to lower refractive index
Critical angle equation
Sinθc=n2/n1
How to fibre optic cables work?
They use total internal reflection to send messages at the speed of light.
The core has a high refractive index.
Cladding has a low refractive index.
Light hits and an angle greater than critical so TIR always happens
Advantages of fibre optic cables
Higher frequency so more info
Light doesn’t heat fibre so no energy lost as heat
No electrical interference
Cheaper to produce
Signal can travel a long way without signal loss
Modal dispersion
Signal can get wider due to dispersion of light.
One part of the signal travels further than another part so they don’t reach the end at the correct time
Material dispersion
Dispersion due to different wavelengths being diffracted by different amounts.
Solution of material dispersion
Use monochromatic light so amount of refraction is the same
Solution of modal dispersion
Use signal boosters
Explain how path difference can cause a minimum
-Path difference is (n+1/2) λ
-So waves arrive in antiphase
-So destructive interference occurs
How to calculate the wavelength of a stationary wave
Multiple distance by 2
Divide by the number of loops
Suggest a light source that would emit a continuous spectrum
-Sun
-Filament lamp
Blue vs red wavelength
Blue had a shorter wavelength
Diffracting white light
White central maxima
Dark fringe
Blue
Red
Double slit equation
W=lambda D / s
Lambda= wavelength
D= distance from slits to screen
S= Slit separation (distance between centres of slits)
