waves Flashcards
what’s a progressive wave?
carries energy from one place to another without transferring any material.
example of a progressive wave
a buoy bobbing up and down on a water wave. the buoy doesn’t move from its location just up and down.
what are waves caused by?
something that makes particles or fields oscillate or vibrate at the source
what do oscillations of a wave pass through?
a medium or field
where is energy transferred in waves?
away from the source
what are the 4 ways we can tell that waves carry energy?
- electromagnetic waves cause things to heat up
- X-rays and gamma rays knock electrons out of their orbits causing ionisation
- loud sounds cause large oscillations of air particles which can make things vibrate
- wave power can be used to generate electricity
how does reflection of a wave occur?
the wave bounces back when it hits a boundary. eg. mirrors
how does refraction of waves occur?
the wave changes direction as it enters a different medium as a result of it changing speed.
how does diffraction of waves occur?
the waves spread out as it passes through a gap or around an obstacle
what are crests and troughs of waves?
crests- greatest possible positive displacement
troughs- greatest possible negative displacement
what speed do all electromagnetic waves travel at in a vacuum?
the speed of light / c
how can we measure the speed of sound?
use two microphones in a straight line of distance d apart.
the microphones should have separate inputs so each signal gets recorded separately
then use a signal generator to produce a sound from the loudspeaker
use a computer to record the time between the first and second microphone picking up the sound
do this by measuring the time delay between the first peak of the signal received by each microphone of a graph of voltage against time.
speed= d/time
how do we measure wave speed in water?
start by recording the depth of the water in the tank using a ruler
use a ripple tank dipper to create vibrations with a regular frequency in the tank.
dim the main lights and turn on strobe lights
increase the frequency of the strobe light from zero until the waves appear to be stationary
use a ruler on white paper under the tank to measure the distance between two adjacent peaks which equals wavelength
then use c=f x wavelength.
f=f of strobe light
repeat for a range of water depths
what are transverse waves?
where the displacement of the particles or field is at right angles to direction of energy transfer.
examples of transverse waves
all electromagnetic waves
ripples on water
waves on string
S-waves from earthquakes
what are longitudinal waves?
the displacement of the particles or fields is parallel to direction of energy transfer. contains compressions and rarefactions
examples of longitudinal waves
sound waves
P-waves from earthquakes
can be demonstrated using springs
what’s a polarised wave?
only oscillates in one direction
what would happen if you had two polarising filters at right angles to each other?
no light would get through
what are some real life example of polarising light?
glare reduction
improving tv and radio signals
what is superposition of waves?
when two or more waves pass through each other, they combine their displacements.
could lead to them cancelling out
what is constructive interference?
when two waves meet with displacements in the same direction.
the displacements combine to give a bigger displacement
what’s destructive interference?
when a wave with a positive displacement (crest) meets a wave with a negative displacement (trough), they will cancel each other out and interfere destructively.
what is total destructive interference?
when two waves with equal and opposite displacements meet. they completely cancel each other out.
what does it mean if two points on a wave are in phase?
they are both in the same place of the wave cycle
have the same displacement and velocity
phase difference of 0 or 360 degrees
what is a stationary wave?
the superposition of two progressive waves with the same frequency and amplitude, moving in opposite directions
how can you demonstrate a stationary wave?
setting up a driving oscillator at one end of a stretched string with the other end fixed.
the wave generated by the oscillator is reflected back and forth.
set frequency to resonant frequency
what are nodes on a wave?
points on the wave where the amplitude of vibration is zero
what are antinodes on a wave?
points of maximum amplitude
what is resonance frequency?
when an exact number of half wavelengths fits on a string
what’s the first harmonic?
when the stationary wave is vibrating at its lowest possible resonance.
one node at each end
half a wavelength fits on the string
what’s the second harmonic?
it has twice the frequency of the first harmonic.
there is two nodes both ends and another in the middle
the wavelength is the length of the string
what other ways can you use to set up a stationary wave other than a string?
stationary microwaves
stationary sound waves
how do we observe stationary microwaves?
reflect a microwave beam at a metal plate
the superposition of the wave and its reflection produces a stationary wave.
how do we observe stationary sound waves?
power in a tube of air can show stationary sound waves
use a loudspeaker to produce sound waves, reflected at the end of the tube
superposition causes wave to become stationary by cancelling
power at bottom is shaken away from antinodes and left undisturbed at the nodes.
explain how a microwave transmitter and vertical rod detector can be used to demonstrate waves are vertically polarised.
you need to rotate the aerial in the vertical plane.
when the aerial is vertical, signal is at a maximum
when the aerial is horizontal, signal is at a minimum
maximum occurs when aerial is aligned with the plane of polarisation of the microwave.
how to find the resonance frequency RP1
start by measuring the mass and lengths of string of different types using a mass balance and ruler.
then find the mass per unit length for each string
set up by having a signal generator and vibration transducer on a bench, attached to one side of the string. the string then goes along over a pulley off the bench with mass attached to the other end.
turn on the signal generator and vary the frequency of the vibration transducer.
find the first harmonic
can change length, tension and mass per unit length to see how frequency changes
what are the factors effecting resonance frequency?
the longer the string, the lower the resonance frequency
the heavier the string, the lower the resonance frequency because the waves travel more slowly down the spring
the lower the tension, the lower the resonance frequency
what’s the equation for calculating resonant frequency?
f= 1/ 2xlength (square root tension/mass per unit length (mu)
on formula sheet
what’s diffraction?
the way that waves spread out as they come through a narrow gap or go round obstacles
what size gap causes maximum diffraction?
gap the same size as the wavelength of the wave.
how does the size of an obstacle effect diffraction?
the wider the obstacle, the less diffraction you get
what type of light should you use to observe a clear diffraction pattern?
monochromatic, coherent light source
put a colour filter over white light to make it single wavelength
use a laser
what’s monochromatic light?
light of a single wavelength and frequency and also a single colour.
what will occur if you use light which is not monochromatic to show a diffraction pattern?
the different wavelengths will diffract by different amounts and the pattern produced won’t be very clear
what will occur if the wavelength of a light wave is roughly the same as the size of the aperture (slit)?
you get a diffraction pattern of light and dark fringes
what are bright fringes of diffraction patterns due to?
constructive interference
the waves from across the width of the slit arrive at the screen in phase
what are the dark fringes of diffraction patterns due to?
total destructive interference
where the waves from across the width of the slit arrive at the screen completely out of phase
what do you observe when white light diffracts through a slit?
you get a spectrum of colours
where’s light intensity highest in single slit diffraction and what does this mean we observe?
in the centre, the brightest part of the pattern is in the central maximum
what happens to the width of the central maximum if you increase the slit width?
central maximum becomes narrower and the intensity of it is higher due to less diffraction
what’s two source interference?
when waves from two sources interfere to make a pattern
what causes constructive interference with two sources?
when the light source is coherent and in phase
what is path difference?
the amount by which the path travelled by one wave is longer than the path travelled by the other wave.
where does constructive interference occur?
for any points equal distance from two sources in phase (maxima)
and where the path difference is a whole number of wavelengths
how can you demonstrate a two-source interference pattern? RP2
for water, use one vibrator which drives two dippers
for sound, connect one oscillator to two loudspeakers
for microwaves, attach two microwave transmitter cones to a signal generator.
what type of material causes light to slow down the most when it enters it?
when the material is more optically dense
where does light travel the fastest?
in a vacuum
what’s the optical density of a material measured by?
its refractive index (the higher its optical density, the higher its refractive index)
what’s the absolute refractive index in a material?
the ratio between the speed of light in a vacuum and the speed of light in that material.
what can we assume the refractive index of air to be?
1
what’s the relative refractive index between two materials?
the ratio of the speed of light in material 1 to the speed of light in material 2
what’s snells law of refraction between two materials given by?
n1sin01= n2sin02
when would light refract towards the normal while travelling from one material to another?
less optically dense material to a more optically dense
how to set up the young’s modulus double slit experiment RP2
either use two coherent light sources or shine a laser through two slits.
the slits have to be around the same size as the wavelength of the laser light so that its diffracted
what will you see on the screen from the double slit experiment
A pattern of light and dark fringes, depending on whether destructive or constructive interference is taking place
what would happen if you use white light in the double slit experiment?
the diffraction pattern would be less intense, with a wider maxima. the pattern would also contain different colours with a central wide fringe due to a mixture of frequencies in white light.
laser safety precautions
lasers can be very dangerous as laser light is very direct, powerful beam of monochromatic light
to avoid damage-
never shine laser towards a person
wear laser safety goggles
avoid shining laser beam on a reflective surface
turn off laser when not needed
in the double slit formula w=landaD/s what do all the letters stand for?
w- fringe spacing, distance between two adjacent maxima in metres
landa- wavelength in metres
D- distance between slits and screen in metres
s- distance between slits in metres
what relationships can you investigate using. Youngs double slit experiment?
varying D to see how it effects w
varying s to see how it effects w
varying wavelength/ colour of light to see how it effects w
what did Youngs double slit experiment show?
that light can have the nature of a wave as it can diffract through two narrow gaps to form an interference pattern.
how do you get the critical angle of light at a boundary?
you need to be going from a higher refractive index to lower
gradually increase the angle of incidence until the angle of refraction reaches 90 degrees, this is the critical angle, light is refracted across the boundary
when does total internal reflection (TIR) occur and what is it?
occurs at angles of incidence greater then the critical angle, refraction can’t happen meaning all the light is reflected back into the material
whats an optical fibre?
a very thin flexible tube of glass or plastic fibre that can carry light signals over long distances and round corners using TIR.
what are step-index optical fibres? (only type you need to know)
have a high refractive index
core is surrounded by cladding which has a lower refractive index to allow TIR.
cladding also protects the fibre from scratches which could allow light to escape
why does TIR occur in optical fibres?
light is shone in at one end of the fibre. the fibre is so narrow that the light always hits the boundary between fibre and cladding at an angle greater then the critical angle. so all light is totally internally reflected.
what are optical fibres used for?
transmitting phone and cable TV signals
why are optical fibres better than the old system (electricity flowing through copper wires)?
the signal can carry more information because light has a high frequency
light doesn’t heat up the fibre (almost no heat energy lost)
there’s no electrical interference
fibre-optic cables are much cheaper to produce
the signal can travel a long way, very quickly, with minimal signal loss.
what are the two ways the signals can be degraded?
absorption and dispersion
why is signal degradation bad?
it can cause information to be lost
how can a signal be degraded by absorption?
some of the signals energy is absorbed by the material the fibre is made from. this energy loss results in the amplitude of the signal being reduced
what happens when a signal is degraded by dispersion?
results in pulse broadening- the signal received is broader then the initial signal
broadened signals can overlap each other leading to information loss
what are the two types of dispersion?
modal and material dispersion
what causes modal dispersion?
light rays entering the optical fibre at different angles, this causes them to take different paths down the fibre so will take different amounts of time
how to reduce modal dispersion
use a single-mode fibre, light is only allowed a very narrow path
whats material dispersion caused by?
different amounts of refraction experienced by different wavelengths of light. travel at different speeds so take a different amount of time
how to stop material dispersion from occurring
using monochromatic light
what can be used to reduce both types of signal degradation?
optical fibre repeaters are used to regenerate the signal every so often.
what is a diffraction grating and what does it produce at the screen?
contains lots of equally spaced slits very close together
produces a very sharp interference pattern. (equally spaced bright fringes with dark regions between)
why does a diffraction grating cause a sharp interference pattern?
because there’re so many beams reinforcing the pattern
why is a sharper interference pattern good?
means results are more accurate
what is the zero order line?
a line of maximum brightness at the centre, in the same direction as the beam of incident on the grating
where are the first order lines situated?
either side of the zero order line
how to derive the diffraction grating equation
consider the first order maximum n=1
call the angle between the angle of refraction and the incoming light
use trigonometry, d=split spacing (hypotenuse) , landa= the path difference (opposite).
to get dsin(angle)=n(landa)
if landa is larger, will the diffraction pattern spread out more, why?
yes it will because sin(angle) will be bigger so angle will be bigger so wavelength will be larger
how can you work out the fringe spacing of a certain order maximum? (diffraction grating) RP2
measure using a ruler
by using different laser sources or coloured filters to select certain wavelengths of white light
to you could change the diffraction grating to vary d
or change D by moving observation screen
what should you find the relationship between landa and distance to be when investigating diffraction grating interference? RP2
as landa increased, so should the distance
when would you use small angle diffraction and what is it?
for investigation diffraction grating interference because the angle is small
sin(0)=0, tan(0)=0, so sin(0)=x/D
and n(landa)=d(x/D)
if you used white light through a diffraction grating what would you see?
the pattern become a spectrum
red on the outside, violet of the inside and the zero order maximum will always be white because all the wavelengths pass straight through
what would you see when you split up light from a star using a diffraction grating?
you can see a line absorption spectra
spectra with dark lines corresponding to different wavelengths of light that have be absorbed.
what are some used for diffraction gratings?
astronomers analyse the spectrum of stars and chemists analyse the spectra of certain materials to see what elements are present.
X-ray crystallography- the average wavelength of X-rays is of similar scale to the spacing between atoms in crystalline solids (when X-rays are directed at thin crystal a diffraction pattern will occur
