Waves Flashcards
What is a wave?
Continuous transfer of energy without a net transfer of mass
What is the difference between mechanical and electromagnetic waves?
Mechanical waves require a medium too pass through
EM waves are oscillations in electric and magnetic fields that can travel though a vacuum
What is the time period?
The time for one whole wave too pass a point
What do displacement time graphs show?
A single point on a wave through time
What do displacement distance graphs show?
The whole wave at a single point in time
What is a phase difference?
The fraction of a cycle difference between two point on a wave
When are two points along a progressive wave in antiphase?
Odd integer multiples of pi or 180 degrees phase difference
OR
Multiple of a half the wavelength phase difference
When are two points along a progressive wave in phase?
Even integer multiples of pi or 360 degrees phase difference
OR
Integer multiple of the wavelength phase difference
What speed do EM waves travel?
C, speed of light
What do filters do?
Blocks light of a specific properties
E.g. a blue filter absorbs all wavelengths other than those in the blue range.
How does a polarising lens work?
Only waves with oscillation in the plane of polarisation can pass through. They are restricted too a single plane.
What type of wave can be polarised?
Transverse only as oscillations are perpendicular too energy transfer
What is the principle of superposition?
When two waves meet, the resultant amplitude is the sum of their amplitudes
What is a coherent source?
The waves have the same frequency and a constant phase relationship.
What does super position in phase and in antiphase lead too?
Constructive and destructive interference (the amplitudes cancel one another as they are summed)
What is the interference pattern like for double slit diffraction?
Same width fringes with different intensities and a brightest central fringe
What is the double slit interference pattern for white light?
Bright white central maxima
Multicoloured fringes with the longer wavelengths on the outside fringes
Different wavelengths cause some fringes too overlap
What is the requirement for an interference pattern to occur through double slit diffraction?
A coherent light source
What is the interference pattern like for single slit diffraction of white light and why?
Even width and intensity outer fringes appearing as spectra with a wider and brighter central maxima of white light.
Each point on a wave produces wavelets (Huygens theory) and the pattern comes from the wavelets interfering.
What is the equation for the maximum order of light through a diffraction grating?
distance between slits / wavelength
d/ λ
What a stationary waves?
Two waves with the same frequency travelling in opposite directions superpose.
No net energy transfer.
When are antinodes formed?
Two waves superpose in phase and create a point of maximum amplitude
When are nodes formed?
Two waves superpose in antiphase and create a point of minimum amplitude
How is a standing wave formed on a string?
Ends tied down form nodes and the waves reflect off of them and superpose forming stationary waves.
What are the number of nodes, ant-nodes, wavelength and frequency of a wave in the nth harmonic on a string length, l?
Nodes : n+1
Anti-nodes: n
Wavelength: 2/n * l
Frequency: n.V / 2l
What is the frequency of any harmonic n?
frequency = n * fundamental frequency (f0)
What is the speed of a standing wave on a string?
v = √ (T/μ) from given f0 formula
In a pipe, what is a closed end and open end in relation to the stationary wave formed?
closed end = node
open end = anti-node
What is refraction?
The bending of light as it moves into a material with a different refractive index.
If a wave travels into a smaller refractive index which wave properties change and which stay the same?
Wave speed and wavelength decrease but frequency remains the same
What is the formula for the refractive index?
Ratios of : C/ Cs Cs - C in substance
λ/ λs
If N1>N2 …
θ2 > θ1 and so the light bends away from the normal
If N2>N1 …
θ1 > θ2 and so the light bends towards the normal
What are the conditions for total internal refraction too occur?
N1 > N2 and the θ incident > θc
What happens if θi is equal too θc ?
The light reflects along the boundary
If light is normally incident too a boundary there is …
No refraction
What is the purpose of cladding in a fibre optic cable?
Low optical density so promotes TIR and prevents any data/ light from escaping
Protects the core
What is material dispersion?
Different frequencies of light taking the same path will take different amounts of time to travel down the fibre causing pulse broadening
How do you prevent material dispersion?
Use a smaller range of different wavelengths/ frequencies or use monochromatic light.
What is modal dispersion?
Waves can take different paths down the core taking different amounts of time causing pulse broadening
How do you prevent modal dispersion?
Use narrow ‘single mode’ fibres reducing the number of possible paths too one
Make Ncladding closer too Ncore so the θc is larger and TIR occurs at shallower angles so there is less difference in the lengths of the paths down the core
