Waves Flashcards
Equilibrium Position
Position of the wave when no energy is being transferred through it
Displacement
Distance + direction of a vibrating particle from its equilibrium position
Amplitude
Maximum displacement of a vibrating particle
Wavelength
The least distance between two particles with the same displacement and velocity
Time period
The time for one complete wave to pass a fixed point
Frequency
The number of complete waves passing a point per second
Equation for frequency
f(Hz) = 1/T(s)
Wave speed
Speed at which the wave propagates
Equation for wave speed
c = frequency x wavelength
Speed of light in a vacuum
3 x 10^8
Phase Difference
The fraction of a cycle between the vibrations of two particles
What is phase difference measured in
Radians
2pi = one complete cycle
Phase of a vibrating particle
Fraction of its cycle it has completed since the start
In phase
when two points on a wave oscillate with the same displacement and velocity at the same time
Phase difference = 2pi raidans
Antiphase
Two points on a wave oscillate with equal but opposite displacement and velocity at the same time
Phase difference = pi radians
Path difference
The difference in length between two paths in terms of wavelengths
Progressive Waves
Transfer energy but no net movement of the medium that carried the wave
Particles of the medium oscillate around equilibrium position
Transverse Waves
The oscillations of the wave are perpendicular to the direction of energy transfer
Can be polarised
Example of transverse waves
Water, electromagnetic, seismic and waves on a string
Longitudinal waves
Oscillations of the wave are parallel to the direction of energy transfer
Compression
Region of relatively high density and pressure in a longitudinal wave
Rarefaction
Region of relatively low density and pressure in a longitudinal wave
Polarisation
Perpendicular oscillations to wave propagation van happen in different planes
EM waves consist of?
Perpendicular oscillating electric and magnetic fields
Also perpendicular to wave propagation
Diagrams typically only show electric field
Polaroid filters
Used to polarise visible light
Absorbs polarisation components of the light which are perpendicular to the axis of the filter
Transmit polarisation components of light which are parallel to the axis of the filter
What is a transmission axis?
Axis of light that will be transmitted by the filter
What happens when visible light is sent through a pair of polaroid filters?
First filter will be vertically polarised and the same for the second
Transmitted light will be vertically polarised and intensity is at a maximum
If the second filter is horizontal plane then the vertically polarised light by the first filter will be polarised perp to the direction
Light will be completely absorbed
Application of Polarisation
Light reflected of horizontal surfaces like water are partially polarised horizontally
Polaroid sunglasses are designed to absorb this polarisation of light
Microwaves an be polarised using metal grids due to very long wavelengths
Radio Aerials
Radio waves and microwaves produced by an antenna typically polarised - if antenna is vertical then EM waves are vertically polarised
Polarisation allows two antennas to send out signs of same frequency without interference due to different polarisation
Superposition
Two waves traveling through each other in the same region
Principle of superposition
Two waves travel through the same region, total displacement is equal to vector sum of individual displacements
Constructive interference
Waves are in phase
Crests meet and lead to a resultant wave with a larger amplitude
Destructive interference
Waves are completely out of phase
Crest of one wave meets trough of another
Reducing amplitude to remaining resultant value
Reflection from a barrier
When a wave reaches a barrier it reflects thus creating a pulse in opposite direction
Has equal and opposite displacement so there is always destructive interference
Stationary waves
Patterns of oscillations but crest of the wave does not appear to travel - do no transfer energy
How is a stationary wave formed?
Superposition of two travelling waves in opposite directions
Equal speed, frequency and wavelength
Nodes
Points of stationary waves with zero displacements
Antinodes
Form halfway between nodes
Points of oscillations with maximum amplitude
Difference between stationary points and progressive points?
Stationary Points
Stationary points
- Same frequency
- Same amplitude
- Points which lie between adjacent nodes have same phase
- Points on either side are 180 degrees out of phase
Difference between stationary points and progressive waves?
Progressive waves
Same frequency
Varying amplitude
Neighbouring points have a different phase
Points separated by one wavelength are in phase
What is a harmonic?
Specific wavelengths which form stationary waves
Most simple stationary waves has?
First harmonic
2 nodes and one anti nodes
L = 1/2 wavelength
What is the first harmonic?
Lowest possible frequency of a stationary wave on a string
Equation for the first harmonic
f = 1/2L x Square root of T/μ
f = frequency
L = Length of string
T = Tension
μ = mass per unit length
Stationary waves in closed pipes
One sealed end and on open
Oscillations at closed end have 0 amplitude as particles have nowhere to go
Open end has antinode as pressure drops to equilibrium atmospheric pressure
First harmonic in a closed pipe
n = 1
Node at closed part of the pipe
Antinode at open part of the pipe
L = 1/4 wavelength
Open pipe stationary sound waves
Open at both ends
Antinodes on both ends
First harmonic fits half a wave
Stationary Soundwaves
Continuous soundwaves reflect from a hard surface at a normal incidence
Stationary produced as the hard surface acts like the end of a pipe
Stationary Microwaves
Reflect a metal sheet
Microwaves directed at a metal sheet at 90 using a microwave transmitter which are reflected by a metal sheet leading to a stationary wave
Why do microwave appliances commonly have a spinning table?
Receiver placed between transmitter and sheet and is then moved backwards and forwards, there is a strong microwave signal at antinodes and no signal at nodes
Diffraction
Wave passes through a narrow gap of almost 0 width
Spread out on the other side of the gap
Wave emerges as semi-circular and equal amplitude
What happens if a wave diffracts from more than one gap?
Waves from the different gas superpose
Waves interfere giving waves with lower or higher amplitude
Coherent Wave sources
Needed for a stable interference pattern
Same frequency and constant phase difference
How to get a constant phase difference?
Use light from a laser
Or
Light from a bulb which has first been passed through a single slit
Example of incoherent light
Bulbs, LED’s, flames and the sun
Laser safety procedures
Do not point the laser at anybody else
Do not look directly along the beam
Make sure laser cannot be reflected into your eyes
Path length
Distance the waves travel along their paths
What determines the type of interference which occurs?
The phase dfference which depends on the path difference
What is path difference?
AP - BP
