Waves Flashcards
what is a progressive wave
carries energy from one place to another without transferring any material. its caused by something vibrating the particles near the source.
what is a transverse wave and examples
Oscillations of particles are perpendicular to direction of energy transfer
-electromagnetic waves/water waves/ s-waves
what is a longitudinal wave and examples
oscillations of particles are parallel to direction of energy transfer
-sound waves/ p-waves
what is it does mean when light is polarised
the particles in the wave are oscillating in more than one plane
what type of wave can be polarised
transverse
how do you polarise light
by using a polarising filter, a square with small openings along through it which prevents light oscillating in more than one plane
what happens when 2 filters 90 degrees apart are put in front of light
no light gets through
what happens when 2 filters 45 degrees apart are put in front of light
the light doesn’t get through as much making it look darker
what are uses of polarised light
tv antennas- light is sent polarised which is why they need to be in a certain position to be reserved
sunglasses- they are coated with polarising filter so some of the light is blocked
what are some natural polarisers
water/glass/tarmac
went light hits them the light reflected off is partially polarised making them easier to be polarised with a filter
what is the wave speed experiment (microphones)
two microphones connected to a computer are put at a fixed distance apart. the computer records when the first microphone hears it then the second.
this gives you a distance and a time
what is the wave speed experiment (echo)
someone makes aloud noise at a fixed distance to a wall. then measure the time to hear the echo again. make sure to double the distance.
what is the wave speed experiment (gunshot)
have someone with a starter pistol and a great distance with a stop watch. start the timer when you see the flash and stop it when you hear the gunshot.
what is superposition
when two or more waves combine together
what are the three types of superposition
constructive interference- pulses combine
destructive interference- pulses partially cancel out
total destructive interference- pulses cancel out
what are the conditions for producing a stationary wave
-it is the superposition of two progressive waves travelling in opposite direction
-they have the same frequency and wavelength
-similar amplitude
what is a stationary wave
a wave where no energy is transferred. you can have both longitudinal and transverse stationary waves
what are nodes and anti-nodes
nodes-points of no displacement, total destructive interference is always occurring here
anti-nodes- points of maximum displacement, constructive interference is happening here
what are the distances between 2 nodes and 2 antinodes
they are both half a wavelength apart from each other
what are examples of stationary waves in real life
strings on guitars and pianos are transverse stationary waves
wind instruments are longitudinal stationary waves
how are microwaves an example of how stationary waves work
an easy way for a stationary wave to be formed is to reflect back on itself
in a microwave the food has to spin or the food doesn’t cook at the nodes
what are harmonics
the certain frequencies where stationary waves can form. this is when there is a whole number of wavelengths.
what is the fundamental frequency
the Lowest frequency that a harmonic can be, only fitting 1/2 a wavelength
how can you create the fundamental frequency
use a signal generator to vibrate a string up and down, the string will reflect back on itself due to a moveable bridge and pulley with masses of the end.
what 3 things affect the fundamental frequency
length
mass per unit length
tension
how does length affect the fundamental frequency
you can change the length of the string by moving the bridge.
because wave speed is constant if the wavelength increases the frequency will decrease.
wavelength is inversely proportional to 2x length
how does tension affect the fundamental frequency
by changing the mass on the string this will increase the tension which will further increase the wave speed.
because wavelength is constant this means the the more tension, the higher the frequency
tension can be found T=mg
how does mass per unit length affect the fundamental frequency
by increasing the mass of the string it will create slower waves therefore will have a lower frequency
calculate by mass/ length of whole string (not just the vibrating part)
how to calculate mass per unit length using density
mass per unit length = density x area
what affects the amount of diffraction (examples)
the gap its trying to diffract through
- if gap is significantly larger- no diffraction
-if gap is slightly larger -some diffraction
-if gap is same as wavelength- max diffraction
-if gap is smaller than wavelength- no diffraction
how else can diffraction be achieved (not through a gap)
by passing round an object, each corner being a source of diffraction
what is monochromatic light
light of the same wavelength so all diffraction will be the same
what is coherent light
has the same frequency and phase difference, achieved by using a laser
what happens when you shine a laser through a single slit
an interference pattern is produced
bright fringes are constructive interference where light arrives in phase
darl fringes are total destructive interference where light arrives in anti phase
describe the intensity graph pattern
its brightest in the middle (central maxima)
the central maxima is twice the width off all the other maxima
what happen when white light is shone through a single slit
all the colours of the rainbow produce an interference pattern with red being the furthest from the maxima (longest wavelength)
what is phase difference
the amount by how much one wave lags behind another. one wavelength is 360/2pi
when is are waves in phase (progressive)
when they have a phase difference of 0/360/720 etc
when is a wave in anti phase (progressive)
when there is a phase difference of 180/540 etc
what are the differences between phase difference in stationary and progressive waves
progressive- phase differences between two points can be any value
stationary- an only be in phase or anti phase
what happens in two source interference
its when waves from 2 different sources overlap eg. speakers.
what is the condition for two source interference
the sources need to be coherent
how are maxima and minima formed (2 source)
when peaks or troughs meet constructive interference happens and a maxima is produced
when a peak meets a trough total destructive interference happens and a minima is produced
what’s the difference between the interference pattern in a single slit and a double slit
all the maxima’s on a double slit are the same length
what is path difference
is a measure of how much further one wave has travelled than another.
eg. if one source is set up 5m from a wall and another 4m from the wall, path difference is 1m
how does path difference explain maxima and minima
if the path difference is 1 wavelength the light arrives in phase resulting in a bright spot
if the path difference is 1/2 a wavelength the light arrive in anti phase resulting in a dark spot
what’s the path difference for a double slit experiment producing a bright spot
0 or 1 wavelength
what’s the path difference for a double slit experiment producing a dark spot
1/2 wavelength
what’s the condition for Youngs double slit formula
if the distance between source and screen is much larger than the slit separation
what are some laser precautions
never shine in someones eyes
wear protective goggles
avoid reflective surfaces
have warning sign
turn off laser when not in use
what are the interference patterns like for single slit/double slit and diffraction gratings
single- blurry
double- more intense
grating- very intense due to 100s of slits
what is a diffraction grating
a square containing loads of tiny slits very close together (100s per mm)
this makes it easier to take measurements from
how to calculate the distance between slits
d=1/N
N- number of slits per m
what are bright spots called when using a diffraction grating
orders
the central maxima is the zero order
the spots either side are the first order etc
what’s the diffraction grating equation
d x sin(theta)=wavelength x order
how do you derive the diffraction grating formula
use the drawing
create triangle for Pythagoras
(use century to check)
what changes can be made that affects the diffraction grating interference patterns
wavelength
slit size
how many orders possible
what happens when wavelength is changed (diffraction grating)
when wavelength increases, more diffraction occurs so the patterns widen
what happens when the slit size changes (diffraction grating)
when there’s less slits per mm so gaps are wider therefore the diffraction is a lot narrower so the pattern shortens
is there a limit to how many orders you can get
yes, there cant be orders past 90 degrees
how do you calculate maximum orders
n(max)= wavelength/ distance between slits
(when calculating always round orders down)
what happens when white light is shone through a diffraction grating
the central maxima will appear white
all other maximas will be a continuous spectra
what are some applications of diffraction gratings
it does the same job as a prism, splitting white light can let us see if there’s anything missing AKA an absorption spectra
this can help us identify elements
what is refraction
occurs when a wave passes through one medium into another
this causes a wave to change speed and direction
what is the refractive index
the ratio of how fast light travels in vacuum compared to that material
visible light travels at different speeds dependent on the transparent material
how do you calculate a materials refractive index
n= c/cs
c- speed of light in a vacuum
cs- speed of light in that material
what’s the refractive index of air
1
what happens when a wave passes into a denser material
it cause the wave to slow down and the wavelength to reduce
the number of waves passing a point per second (frequency) stays the same
how do you calculate the relative refractive index of 2 materials
1n2=n2/n1
going from material 1 into material 2
what is snells law
the amount lights direction changes when passing through mediums
what’s snells law of refraction (equation)
n1sin(theta)1=n2sin(theta)2
what happens when light goes from a low to high refractive index
it bends toward the normal
what happens when light goes from a high to low refractive index
it bends away from the normal and some of the light is reflected back
the angle of incidence is the same as the angle of reflection
according to snells law what happens when you increase the angle of incidence (critical angle)
the angle of refraction becomes even larger eventually becoming 90 degrees and not escaping material 1 this is the critical angle
what are the conditions for the critical angle
only occurs when going from a high to low refractive index
when the angle of refraction is 90 degrees
-this is different for different materials
what happens when the angle of incidence become higher than the critical angle
you achieve total internal reflection
what are the conditions for total internal reflection
light travels from a high to low refractive index
the angle of incidence is greater than the critical angle between the two materials
what are fibre optics
cables used to send messages at the speed of light from one side of the planet to the other
how do fibre optics work
the core has a very high refractive index and the cladding has a very low refractive index
so the light always hits at an angle greater than the critical angle so TIR always occurs
what are some advantages of fibre optics
-signal can carry more info due to higher frequency
-light doesn’t heat up cable so no energy loss
-no electrical interference
-much cheaper to produce
-signal can travel far without signal loss
what are the 3 disadvantages of fibre optics (detailed)
-sent as pulses of 0s and 1s, as it travels some energy is lost due to scattering and absorption, to resolve this the signal can be boosted every 100km
-overtime pulses can get wider (broadening) modal dispersion is where light enters at different angles arriving at different times, solution is again build signal boosters
-material dispersion, different wavelengths rare refractive by different amounts causing them to arrive at different times. Solution is to use monochromatic light
what is displacement
distance and direction of a particle from the equilibrium
what is amplitude
the maximum displacement of a particle on a wave from the equilibrium
what is time period
time taken for one whole wave cycle
what is frequency
the number of whole wave cycles per second passing a given point
