Waves Flashcards
longitudinal wave
particles oscillation is parallel to direction of wave propagation
examples of longitudinal waves
sound waves, seismic waves
transverse wave
particle oscillation is perpendicular to the direction of wave propagation, only transverse waves can be polarised
examples of a transverse wave
electromagnetic radiation
particle displacement
the distance of a particle from its equilibrium position in given direction
amplitude
the maximum displacement of a particle from its equilibrium position
wave speed
frequency x wavelength
phase
the position of a certain point along a wave cycle
phase difference
the amount one wave lags behind another in radians
frequency
number of oscillations of a particle per second
time period
the time taken for one complete oscillation
wavelength
shortest distance between two point in phase
wavelength for stationary wave
distance between alternate nodes or distance from peak to peak/ trough to trough
diffraction
the spreading out of a wave (when it passes through a gap or past the edge of an object
refraction
wave bends/ changes diffraction when entering a media with a different RI due to changing speed.
polarisation
when a transverse wave only oscillates in one plane
application of polarisation in sunglasses
- light reflected from surfaces is weakly polarised in one plane
- polaroid in sunglasses can be orientated at a certain angle to remove this reflected light reducing the glare
application of polarisation in tv transmitters and aerials
- signals from tv transmitter are polarised
- aerials need to be orientated so they are in the same plane as the transmitted signal for maximum strength
superposition
where two or more waves meet, the resultant displacement equals the vector sum of the individual displacements
conditions for formation of stationary waves
- two waves travelling past each other in opposite directions
- with the same wavelength and frequency
- similar amplitudes and oscillating in the same plane
monochromatic
single wavelength
safety with a laser
- avoid looking along the beam of a laser
- wear laser safety goggles
- avoid reflections
- put up a warning sign that a laser is in use
properties of laser light
- monochromatic - single wavelength
- coherent - waves have a constant phase difference and the same wavelength/frequency
- collimated - produces an approximately parallel beam
Stationary wave formation in a microwave
- waves travelling from the transmitter reflect off the walls of the microwave.
- the waves from the transmitter are oscillating in the opposite direction but the same plane as the reflected waves with the same frequency and wavelength hence form a stationary wave.
frequency of 1st harmonic
f=1/2l x (T/µ)½
wavelength of stationary waves for first 4 harmonics
length of string = L
1st harmonic: λ = 2 L
2nd harmonic: λ = L
3rd harmonic: λ = 2/3 L
4th harmonic: λ = 1/2 L
mass per unit length
µ = mass/length = density x Area
the difference in resonant frequency due to altering the properties of the string
longer string = lower resonant frequency as half wavelength is longer
heavier string = lower resonant frequency as waves travel slower
lower string = lower resonant frequency as waves travel slower
nodes and antinodes
nodes - points of no oscillation / zero amplitude
antinodes - points of maximum amplitude
path difference
the extra distance travelled by one wave than another resulting in constructive or destructive interference
coherent sources
waves from two sources that have:
- constant phase difference
- same wavelength and frequency
explanation of formation of fringes with Young’s slit
- the light from the two slits is coherent as the slits are an equal distance from the light source and the light is coherent as it is filtered so has the same wavelength and zero phase difference
- interference fringes are formed where the light from the two slits overlaps
- bright fringes are formed by constructive interference because light from the two slits are in phase
- dark fringes formed by destructive interference because light from the two slits is in anti-phase
appearance of interference fringes from two vertical slit illuminated with yellow light
- vertical or parallel
- equally spaced
- black and yellow bands
fringe width changes
- slits closer together - w increases
- screen further away - w increases
- shorter wavelength - w decreases
width of slits is reduced
fringe separation increases as s decreases
appearance of white light trough Young’s slits
- central fringe is white
- side fringes are continuous spectra
- bright fringe would be blue on the side nearest the central fringes and red furthest away due to difference in λ
- bright fringes merge furtehr away from centre. fringes are of equal width but the central fringe is the brightest by far
appearance of diffraction pattern from a single slit
- central bright fringe has twice width of other bright fringes
- the other bright fringes have a much lower intensity and are equally spaced
single slit pattern changes
narrower slit width = wider pattern/ increased separation and reduced intensity
shorter wavelength = narrower pattern/ reduced separation and greater intensity
lines per mm of a grating
spacing, d, of slits on a diffraction grating given by d = 1/number of lines per mm. Units: mm
derivation of nλ = dsinθ
between slits a and b:
- path difference to nth maximum = nλ
- from trigonometry sinθ = nλ/d
applications of gratings to spectral analysis of light from stars
- dark lines in spectrum from a star
- reveal the composition of elements present in the start’s atmosphere
how does light change moving from air to glass
- speed- decreases
- wavelength - decreases
- frequency - remains constant
conditions for total internal reflection
- angle of incidence is greater than the critical angle
- the refractive index of the material light is going from is greater than the refractive index of the material the light is going to
total internal reflection
when all the light is reflected back into the material
critical angle
angle of incidence which produces an angle of refraction of 90 degrees
Structure of an optical fibre
Central core, surrounded by cladding. Refractive index of core must be greater than refractive index of cladding (to ensure total internal reflection)
purpose of cladding
- prevents crossover of signal/data to other fibres
- prevents scratching of the core
- reduces pulse broadening/ dispersion
use of optical fibres
- communication - improve transmission of data/high speed internet
- endoscopes - improved medical diagnosis
how do pulses of light change travelling down optical fibres
- reduced amplitude due to absorption/ energy loss and scattering within fibre
- pulse broadening due to multipath dispersion from rays taking different paths and different times to travel down same fiber
how is multipath dispersion reduced
core of fibre is made very narrow/thin and the signal is regenerated every so often
