Section 3 - Waves Flashcards
What is a wave?
A periodic disturbance of particles or fields
What is a progressive wave?
A wave that carries energy from one place to another without transferring any material
What causes a wave to form?
Something making particles or fields oscillate at a source
Define amplitude
The maximum magnitude of displacement
Define a cycle of a wave
One complete vibration of a wave
Define wavelength
Length of one whole wave cycle, crest to crest or trough to trough
Define time period
Time taken for a whole cycle to pass a given point
Define frequency
No. of cycles passing a point per second
Define phase
A measurement of the position of a certain point along the wave
Define phase difference
The amount one wave lags behind another
Define reflection
The wave bouncing back when it hits a boundary
Define refraction
The wave changing direction as it enters a different medium
What is the speed of EM waves in a vacuum?
3x10^8 m/s
Define transverse waves
Oscillations that are perpendicular to the direction of energy transfer
What type of waves are all EM waves?
Transverse
Define longitudinal waves
Oscillations that are parallel to the direction of energy transfer
Which direction does a polarised wave oscillate in?
Only one direction
What type of waves can be polarised?
Only transverse waves
Give evidence that EM waves are transverse
They can be polarised
What do polarising filters do?
Only transmit vibrations in one direction
What happens if you place 2 polarising filters at right angles to eachother?
No light gets through
Describe the graph of the 2 polarising filters, as the second one is rotated from 0 to 360 degrees
0 - Full intensity 90 - 0 intensity 180 - full intensity 270 - no intensity 360 - full intensity
Define partially polarised
Some of the wave vibrates in the same direction
Give 2 applications of polarisation
Polarised sunglasses, television and radio signals
When does superposition occur?
When 2 or more waves pass through each other
Define the principle of superposition
When 2 or more waves cross, the resultant displacement equals the vector sum of the individual displacements
What are the 2 types of interference?
Constructive and destructive
When does constructive interference occur?
When a crest meets a crest, or trough meets a trough
When does destructive interference occur?
When a crest meets a trough
What happens when a crest and trough are not the same size?
The interference is not total. For it to be noticeable, the amplitudes should be nearly equal
When are 2 points on a wave in phase?
When they have the same displacement and velocity
Describe mathematically when 2 points on a wave are in phase
They have a phase difference of 0 or 360 deg
Describe mathematically when 2 points on a wave are out of phase
They have a phase difference of odd number multiples of 180 deg
How do you get clear interference patterns?
By making the 2 sources coherent
Define coherent sources
2 sources are coherent if they have the same wavelength and frequency, and a fixed phase difference between them
What does constructive and destructive interference depend on?
Path difference
Define path difference
The amount by which the path travelled by one wave is longer than the path travelled by the other wave
When do you get constructive interference?
- At any point an equal distance apart from 2 coherent and in phase sources
- When the path difference is a whole number of wavelengths
When do you get destructive interference?
When the path difference is n+1/2 lamba
State the constructive path difference equation
Path difference = n lambda
State the destructive path difference equation
Path difference = (n+1/2)lambda
What can be created when a progressive wave is reflected at a boundary?
A stationary wave
Define stationary wave
The superposition of 2 progressive waves with the same frequency and wavelength, and moving in opposite directions
How much energy is transmitted by a stationary wave?
0
What are nodes?
When the amplitude of the vibration is 0
What are antinodes?
When the amplitude of the vibration is the maximum
How many wavelengths are there for the first, second and third harmonic?
1st - half a wavelength
2nd - 1 wavelength
3rd - 1 1/2 wavelength
Describe the 1st harmonic
- Stationary wave with the lowest possible resonant frequency
- Node at each end with an anti node
- 1/2 lambda
Describe the 2nd harmonic
- Stationary wave with twice the frequency of the first harmonic
- Node at each end and middle. 2 anti nodes
- 1 lambda
Describe the 3rd harmonic
- Stationary wave with triple the frequency of the first harmonic
- 4 nodes and 3 antinodes
- 1 1/2 lambda
What experiment can you use to demonstrate stationary waves?
Microwaves and sounds
Describe how you can demonstrate stationary waves with microwaves
Order of equipment left to right:
- Metal plate, probe connected to loud speaker, microwave transmitter
- Can find nodes and antinodes by moving probe between transmitter and reflecting plate
Describe the experiment you would have to do in order to find the factors affecting the resonant frequency of a string
- Measure the mass, length, and find mu (M/L)
- Set up apparatus and record mu, measure length and work out tension using T=mg
- Turn on signal generator until you find first harmonic
- Explain how length, tension, MPL affect resonant frequency, keep 2 constant and change other (total 3 times)
What conclusions should be reached after you have completed the resonant frequency experiment?
- Longer string, lower frequency
- Heavier the string, lower the resonant frequency
- Looser the string the lower the resonant frequency
What does the amount of diffraction depend on?
The wavelength compared to the size of the gap
What is the diffraction like when the gap is much bigger than the wavelength?
No noticeable diffraction
What is the diffraction like when the gap several wavelengths bigger than the wavelength?
Noticeable diffraction
What is the diffraction like when the gap is the same size as the wavelength?
The most it could be
What is the diffraction like when the gap smaller than the wavelength?
The waves are mostly reflected back, not as much
What is monochromatic light?
A light with the same wavelength and frequency
How can you demonstrate light diffraction with a laser?
Have coherent, monochromatic light source and shine it thorough a slit and observe the pattern on the screen
What diffraction pattern can you observe in the experiment with a laser and a slit?
Central bright fringe, with dark and bright fringes alternating on either side
In the diffraction light experiment with a laser, what causes
Caused by destructive and constructive interference
What does diffracted white light create?
A spectra of colours
Why does a spectrum get created when white light is diffracted?
Because of the different wavelengths in the white light, and they diffract by different amounts
What does the intensity of light mean?
The number of photons
What is intensity?
Power per unit area
What does the width of the central maximum vary with?
Wavelength and slit size
What does increasing the slit size do to the diffraction, and what is the effect of this on the central maximum?
Decreases the amount of diffraction, leads to a narrower, but more intense central maximum
What does increasing the wavelength do to the diffraction, and what is the effect of this on the central maximum?
Increases the amount of diffraction, leads to a wider, but less intense central maximum
What safety precautions should you take when using lasers?
- Never shine directly at a person
- Wear safety goggles
- Avoid shining at a reflective surface
- Have a warning sign on display
- Turn the laser off when not needed
What does young’s double slit experiment prove?
EM radiation’a wave nature
In the equation, lambda = wD/s, state what the components are
lambda = wavelength w = fringe spacing D = distance from slits to screen s = slit separation
What happens to interference patterns when you diffract light through more than 2 slits?
They get sharper - bright bands are brighter and narrower, dark areas are darker
In the equation, n lambda = d sin theta, state what the components are
d = grating with slits d apart
sin theta = sin of angle between zero order and n order
n = order of line
lambda = wavelength
from the n lambda = d sin theta experiment, what conclusions can be drawn?
- If lambda is bigger, theta is bigger-> longer wavelength the more the pattern spreads out
- d is bigger, sin theta is smaller -> coarser the grating, the less it spreads out
- for certain n values, sin theta is >0 therefore don’t exist
What do diffraction help identify?
Elements and calculate atomic spacing
What does the refractive index or a material measure?
How much it slows down light
What happens when light enters a more optically dense medium?
It slows down and refracts towards the normal
What happens when light enters a less optically dense medium?
It speeds up and refracts towards the normal
What is an optical fibre?
A thin, flexible tube of glass or plastic than can carry light signals
What does an optical fibre consist of?
A core and cladding
What is the job of the cladding in an optic fibre?
It allows TIR as the refractive index is lower than the core, and it protects the core from scratches which could lead to signal leakage
In terms of signal, what does dispersion and absorption cause?
Signal degredation
What does absorption do to a wave?
Causes a loss in amplitude
What does dispersion do to a signal?
Cause pulse broadening
What is model dispersion?
When light rays enter the fibre at different angles and take different paths. Some rays take longer paths than others that go down the middle
What is the solution to model dispersion?
Use a single mode fibre which only lets light take one path
What is material dispersion?
Light consists of different wavelengths which travel at different speeds. Some travel and reach the end faster than others
What is the solution to material dispersion?
Use monochromatic light
What are optical fibre repeaters?
Something that boosts and regenerates the signal every so often. Can reduce signal degradation caused by absorption and dispersion
