waves Flashcards
what is a progressive wave?
a wave that transfers energy from one point to another without transferring matter
define displacement of a wave
how far a point on a wave has moved from its equilibrium
define amplitude of a wave
maximum magnitude of the displacement
define wavelength of a wave
the distance between neighbouring points on two waves
define period of a wave
time taken for a whole cycle to complete
define frequency of a wave
number of cycles per second passing a given point
define phase of a wave
measurement of the position of a certain point along the wave cycle
define phase difference
the amount one wave lags behind another
how can you work out the frequency of a wave?
f=1/time period
give the wave equation
v= fλ
what does an oscilloscope do?
displays waves from a signal generator as a function of voltage over time
what is a transverse wave?
a wave where oscillations are perpendicular to the direction of energy transfer
give an example of a transverse wave
electromagnetic waves
what is a longitudinal wave?
a wave where oscillations are parallel to the direction of energy transfer
give a similarity and difference between a transverse and longitudinal wave
similarity- both transfer energy
difference- transverse have peaks/troughs whereas longitudinal have compressions and rarefactions
give an example of a longitudinal wave
sound waves
what is intensity?
rate of flow of energy per unit area at right angles to the direction of travel of the wave
give the intensity equation
I=power/area
what is the relationship between intensity and amplitude
intensity is directly proportional to amplitude squared
give a property of all EM waves
all travel at speed of light (3.8x10^8) in a vacuum
what are EM waves?
transverse waves that consist of vibrating magnetic and electric fields at right angles to each other and direction of travel
give a use of radio waves
radio transmission
give a use of microwaves
cooking
radar
tv transmissions
give a use of infrared radiation
heat detectors
night vision cameras
optical fibres
give a use of visible light
human sight
optical fibres
give a use of ultraviolet light
sunbeds
security
give a use of x-rays
see damage to bones and teeth
airport security scanners
kill cancer cells
give a use of gamma rays
kill cancer cells
sterilisation of medical equipment
give the EM spectrum in order of increasing frequency and decreasing wavelength
radiowaves microwaves infrared radiation visible light ultraviolet x-rays gamma rays
what is a polarised wave?
a wave that only oscillates in one direction
what is the plane of polarisation?
the plane in which a wave vibrates in
what is plane polarisation?
polarising a wave so it only oscillates in one direction
how can a wave be polarised?
using a polarising filter that only transmits vibrations in one direction
can longitudinal waves be polarised?
no, only transverse waves can
how can you polarise microwaves?
using metal grilles - polarising filters won’t work
describe why the intensity of microwaves drops to 0 when the wires of the metal grille are aligned with the direction of the microwaves
- the vibrating electric field of the microwave excites electrons in the metal grille
- energy of incoming microwaves is absorbed and re-emitted in all directions
- only a few re-emitted waves are vibrating in direction of microwave receiver
- only microwaves in one plane are received
- when wires and vibrations of the waves are aligned, more electrons are excited that when the grille and vibration of the waves are at right angles to each other- causes drop in intensity
define diffraction
the spreading out of waves as they pass through a narrow gap or go round an obstacle
what does the amount of diffraction depend on?
size of the gap compared to the λ
when do you get the most diffraction?
when the gap is the same size as the wavelength
how can you demonstrate diffraction in light using laser light?
-shining light through a narrow slit onto a screen. you can alter amount of diffraction by changing the width of the slit
or
-use a white light source and a set of coloured filters to see how wavelength varies for each colour and how diffraction changes
what does the diffraction pattern look like when the wavelength of a light wave is similar to the size of the slit?
pattern of light and dark fringes are seen
what is reflection of a wave?
wave is bounced back when it hits a boundary.
angle of incidence = angle of reflection
what is refraction of a wave?
wave changes direction when entering a different medium as a result of the wave slowing down or speeding up
if a wave slows down, does the wave bend towards or away from the normal?
towards
if a wave speeds up, does the wave bend towards or away from the normal?
away
what is the refractive index of a material?
measure of how much a material slows down light
define the absolute refractive index of a material
the ratio between the speed of light in a vacuum and the speed of light in that material
what is the equation for refractive index?
n=c/v
what is the refractive index of air?
1
what equation is given for a light ray passing across a boundary between 2 materials?
nsinθ = constant
give Snell’s law
n1sinθ1 = n2sinθ2
when the angle os refraction is 90 degrees, what is the angle of incidence?
critical
what is total internal reflection?
when all light is reflected back into the material
how can you find the critical angle?
sinC= 1/n
define superposition
when two or more waves pass through each other
give the principle of superposition
when two or more waves, cross, the resultant displacement equals the vector sum of the individual displacements
what are the two types of interference?
constructive and destructive
explain constructive interference
two points on a wave meet to create a larger displacement. e.g crest + crest gives a bigger crest
explain destructive interference
two points on a wave meet to give no displacement
e.g crest + trough meet to give nothing
describe what is meant when a wave is ‘in phase’
- points on adjacent waves on at the same position, have the same displacement and velocity
- two points in phase interfere constructively
- phase difference of 0 or a multiple of 360 degrees is in phase (2pi)
describe what is meant when a wave is ‘out of phase’
- points on adjacent waves that are not in the same position, have different displacements and different velocities
- phase difference of odd number multiples of 180 (pi radians)
how can you get interference patterns from two sources?
the two sources must be coherent
define coherent
same wavelength
same frequency
fixed phase difference
what does constructive or destructive interference depend on?
path difference
when would constructive interference occur?
when path difference = nλ (n=integer)
when would destructive interference occur?
when path difference = (2n+1)λ/2
or (n+0.5)λ
how can you demonstrate interference for EM radiation?
Young’s double slit experiment
describe Young’s double slit experiment
-laser light is coherent and monochromatic
-slits must be same size as wavelength of laser light so it is diffracted
-pattern of light and dark fringes is created depending on whether constructive or destructive interference occurs
-work out wavelength with Young’s double slit formula
fringe spacing, x = λD/a
does young’s experiment support the wave or particle model of light?
wave
what other practical can be used to work out the wavelength of light?
diffraction grating
creates bright central line called zero order
and other maxima and minima are first, second orders etc.
