Waves A Flashcards
Definitions. Wave properties.
Define wavelength.
Define amplitude.
Define displacement.
Define period.
Define frequency.
Progressive and stationary waves.
Give 3 differences between them.
A progressive wave transfers energy between two points without transferring matter
while a stationary wave stores energy
A progressive wave moves through space
while a stationary wave does not
A progressive wave has the same amplitude of oscillation for all points
while a stationary wave has varying amplitude from 0 to max
Transverse vs Longitudinal waves.
Differences and key features
Examples of both waves
Transverse waves:
have oscillations that are perpendicular to the direction of energy transfer
have peaks and troughs
Longitudinal waves:
have oscillations that are parallel to the direction of energy transfer.
have compressions and rarefactions.
Examples of both:
transverse - EM waves, waves on a rope, surface water waves
longitudinal - sound waves, seismic P-waves
Polarised and Unpolarised waves.
Define what a polarised and unpolarised wave is.
Explain why longitudinal waves cannot be polarised.
A polarised wave is a wave with oscillations in only one plane that is perpendicular to the direction of wave travel.
An unpolarised wave is a wave with oscillations in more than one plane that is perpendicular to the direction of wave travel.
Longitudinal waves have oscillations that are parallel to the direction of wave travel, so it is impossible by definition
Polarising filters.
Describe and explain the uses of polarising filters in:
-polaroid sunglasses
-polaroid photography
-radio and broadcast
Polaroid sunglasses
- vertical filters that absorb horizontally polarised light, while transmitting vertically polarised light.
- reflected light from surface of wet roads is partially horizontally polarised and is absorbed
- glare reduced
Polaroid photography
-vertical filters absorb all horizontally polarised light, while transmitting vertically polarised light
- reflections from surface of water are partially horizontally polarised and absorbed
- light from underwater object is refracted and not polarised, so transmitted
Radio and broadcast
- all television services are broadcast using polarised radio/microwaves
- aerial must be aligned along the plane of polarisation for maximum intensity pick-up
Stationary waves.
Conditions required for a stationary wave. (Why does a stationary wave form in a microwave?)
Explain which points are in-phase/anti-phase.
Variation of amplitude across a stationary wave?
What are nodes and anti-nodes?
Context-specific formation of stationary waves (formation, reflection, superposition, fixed ends!)
Why do only specific frequencies produce stationary waves on taut strings?
Definition of monochromatic
Conditions for a stationary wave to be formed:
- two progressive waves which move in opposite directions, with same amplitude, same wavespeed, frequency and wavelength.
Which points in-phase, which anti-phase?
- any two points between two nodes/anti-nodes are always in phase
- any two points on directly opposite sides of the same adjacent node/antinode are in anti-phase
Context-specific formation of stat.
- progressive waves produced by a source, travel down rope
- reflected at the fixed boundaries
- incident and reflected waves superpose and interfere
- conditions for stationary waves satisfied
Why do only specific frequencies produce stationary waves on taut strings?
- the fixed ends of the string must be nodes, so only an integer number of half wavelengths can fit on the string
Definition of monochromatic
- light consisting of only one frequency or wavelength
Harmonics on a string.
wavelength of the nth harmonic w/r to L
frequency of the nth harmonic
factors affecting fundamental frequency
wavelength of nth harmonic 2L/N
frequency is N * f
factors are length, mass per unit length, and tension
mass per unit length: string gets heavier, waves travel slower
Young’s Double Slit
How are bright and dark fringes formed?
What does the pattern look like?
Bright fringes:
-light diffracts through both slits
-light from both slits interferes and superposes
-for different points on the screen, the path difference for light arriving from the two slits is different
-when path difference is n lambda, the two waves arrive in-phase and constructive interference occurs, so resultant intensity of light is maximum
-when path difference is n+1/2 lambda, the two waves arrive in anti-phase and destructive interference occurs, so resultant intensity of light is 0.
Pattern for double slits:
-alternating bright and dark fringes parallel to the slits
-intensity of the bright fringes decreases with distance
-central bright fringe
Young’s Double slit
What does the pattern look like for white light?
-central bright fringe is white
-outer fringes are spectrums of colour with violet closest to the centre and red furthest away
-outer fringes are wider due to overlap of patterns, so dark fringes may no longer be visible
Young’s Double Slit:
What is the effect of changing the wavelength of light on the pattern?
- the fringe spacing changes with wavelength of light.
Safety & Lasers
Lasers can cause permanent eye damage, and must be used carefully.
Give as many precautions as you possibly can for using lasers.
- put a “laser in use” sign outside lab
- do not look directly into a laser/ shine the laser at someone
- wear laser safety goggles
- stand behind the laser
Single slit diffraction.
Describe the pattern produced in single slit diffraction.
- alternating bright and dark fringes parallel to the slits
- central bright fringe, which is double the width of outer fringes and much higher in intensity
- intensity of bright fringes drops off very quickly with distance
Single slit diffraction.
Describe the pattern produced when white light is used.
- central bright fringe is white, and still double the width and higher in intensity
- outer bright fringes are spectrum of colours with violet closest and red furthest away
- outer bright fringes are wider and less intense than for monochromatic light
- fringe spacing decreases with distance
Structure of step index optical fibres.
Features of the core - material? diameter?
Purpose of the cladding - talk about TIR, protection, structure and security.
The CORE:
- is made of plastic or glass
- is very narrow (to decrease modal dispersion)
The CLADDING:
- has a lower optical density (n) than the core, to allow TIR
- protects the core from scratches (that would let light escape the core)
- provides structural strength (preventing breakage of the narrow core)
- prevents crossover of signals between touching optical fibres (for security of communication)
Overview of how a fibre optic cable works.
- light, refracts when it enters the optical fibre at one end
- it then undergoes repeated total internal reflection off the core-cladding boundary until reaching the other end
- where it is refracted back out
Applications of fibre optic cables.
- endoscopies for better and more accurate medical diagnoses
- high-speed communication for faster information transfer
Material and modal dispersion.
What is material dispersion and how can it be prevented?
What is modal dispersion and how can it be prevented?
Material dispersion is the broadening of a pulse in fibre optic cable, because:
- different wavelengths of light have different refractive indices and travel at different speeds in the core
- so take different times to reach the end of the fibre
Modal dispersion is the broadening of a pulse in a fibre optic cable, because
- rays of light enter the core at different angles, so take different paths in the cable
- longer paths mean longer time to travel to the end
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