Waves Flashcards
what is a progressive wave?
A wave that carries energy from one place to another without transferring any material.
what is the difference between transverse and longitudinal waves
transverse waves oscillate perpendicular to the direction of energy propagation whereas longitudinal waves oscillate parallel to the direction of energy propagation
Give examples of longitudinal waves
Sound waves,
P-waves,
Slinky spring pushed on end
Give examples of transverse waves
All EM waves,
Water ripples,
Waves on strings,
A slinky spring wriggled up and down, S-Waves
What happens to air particles when a sound wave travels through air
the particles in the medium exert forces on each other causing rarefactions and compressions, this transfers energy
what does a displaced air particle experience
A restoring force from its neighbours and is pulled back to its original position
What is a peak and trough?
the peak (top) and trough (bottom) are where the particles are at maximum displacement from their equilibrium.
Definition of displacement
how far a point on a wave has move from its undisturbed position.
definition of amplitude
the maximum magnitude of displacement
define wavelength
the length of one whole wave oscillation
define period
The time taken for one whole wave cycle
define frequency
the number of whole wave cycles per second passing a given point
define phase
A measurement of the position of a certain point along the wave cycle
define phase difference
The amount one wave lags behind another
similarity between transverse and longitudinal waves
both waves transfer energy without transferring matter
what Is the relationship between frequency (f) and period (T)
f = T-1
formula for wave speed
v = f λ
formula for intensity
I=P/A
relationship between intensity and amplitude
intensity ∝ (amplitude)2
at what speed do EM wavs travel at in a vacuum?
3x108
as the frequency increases the waves carry more
energy
towards the end of the UV waves on the spectrum waves become energetic enough to
cause ionisation, this is where they transfer so much energy to an atomic electron it is moved from the atom.
define polarisation of waves
polarisation means that the particles oscillate along one direction only which means that the wave is confined to a single plane
What is the plane of polarisation?
the plane in which a plane polarised wave vibrates is called the plane of polarisation.
Why can’t longitudinal waves be polarised?
Because their oscillations are already limited to one plane, parallel to direction of energy transfer
Define the law of reflection
The angle of reflection equals the angle of incidence
what is reflection?
when a wave is bounced back when it hits a boundary
what is refraction?
When a wave crosses a boundary between different medium at an angle.
what does refraction affect?
as refraction affects the speed, we can derive from v=fλ, that as frequency is a constant then the wavelength is affected.
if the wave bends towards the normal then
the wave is slowing down
if the wave bends away from the normal it is
speeding up
Define refractive index
Ratio of the speed of light in free space to its speed in a given material
the more optically dense a material is the …..
more light slows down when it enters
what is the formula of refractive index?
n = c/v
what is the refractive index of air and why?
we can assume it is 1 because light travels at a very similar speed in air as it does in a vacuum.
Define Snell’s law.
The change in medium results in a change of speed and change in direction.
formula for snells law?
(n1)sin(θ1) = (n2)sin(θ2)
how many radians are in a circle?
2π radians
what is diffraction?
The bending of a wave as it moves around an obstacle or passes through a gap.
when is diffraction greatest?
When the wavelength is about the same size as the gap or obstacle.
what is the formula to calculate the critical angle?
sin(θ) = n2/n1
*where n2 is the refractive index of less optically dense material
If the angle of incidence is equal to the critical angle…
the ray of light will travel along the boundary of the two mediums
if the angle of incidence is larger than critical angle…
this leads to total internal reflection , this means that all of the light is reflected
what is internal reflection
light speeds up when entering a less optically dense mediumwhen this happens some light is refracted and some light is reflected this is
what is the principle of superposition?
when two waves meet at a point the resultant displacement at that point is equal to the sum of the displacements of the individual waves.
what is constructive intereference
if the two waves are in phase then the maximum positive displacements from each wave line up creating a resultant displacement with increased amplitude
- as intensity = amplitude2 increased amplitude means increased intensity
what is destructive interference
if two progressive waves are in antiphase then the maximum positive displacement from one wave lines up with the maximum negative displacement from the other the resultant displacement is smalller than for each individual wave.
What is coherence
it refers to waves emitted from two sources having a constant phase difference, for such to be achieved they must have the same frequency.
intereference patterns contain a maxima and minima, what does this mean?
the waves interfere constructivley at a maximum and interfere destructivley at a minima
how are maxima and minima caused?
it is a result of the two waves having travelled different distances from their sources, the difference in the distance travlled is called the path difference
What are the conditions of constructive intereference?
if the path difference to a point is zero or a whole number of wavelengths (0, λ, 2λ,..), the two waves will arrive in phase and intefere constructivley
what are the conditions of destructive intereference
if the paths difference to a point is an odd number of half wavelengths (½λ,3/2λ, (n+½)λ) the waves will meet in antiphase and always produce destructive interference
what is the path difference at the 1st order max/min and 2nd order maxi/mini
A diagram of Young’s double slit experiment ( press 3 )
Why does young use a monochromatic source of light ?
two coherent waves are needed to form an intereference pattern, he achieved this by using a colour filter that allows only a specific frequency of light to pass and a narrow single slit to diffract light
explain what occurs to the light in youngs double slit experiment
light diffracting from the single slit arrives at the double slit in phase it then diffracts again from the double slit. each slit acts as a source of coherent waves which spread from each slit, overlapping and forming an intereference pattern that can be seen on a screen as bright or dark fringes
fromula to calculate the wavelength of light used to form the intereference pattern
λ = ax/ D
a = distance between slits
x = seperation between the adjacent same types of fringes (from bright to bright) (dark to dark)
D = distance of the screen from source of light
How is a stationary wave formed?
two progressive waves with the same frequency travelling in opposite directions are superposed. there is no net energy transfer as the waves are travelling in opposite directions
What is a node
as the waves have the same frequency at certain points they are in antiphase at these points their displacement cancels out. this forms a node where their displacement is always 0.
What is an antinode
at other poitns when the two waves are always in phase an antinode is formed, the point of greatest amplitude.
what is the seperation between two adjacent nodes?
it is equal to half the wavelength of the original progressive wave and the frequency is the same as that of the original waves.
explain phase difference along a stationary wave
- inbetween adjacent nodes all the particles in a stationary waave are sociallting in phase with each other they all reach their maximum positive displacement at the same time
- on different sides of a node the particles are in antiphas. the particles on one side of a node reach their maximum positive displacement at the same time as those on the other reach their maximum negative displacement
explain what happens when a wave is stretched between two fixed points
the points act as nodes. when the string is plucked a progressive wave travels along the string and reflects off its ends this creates two progressive waves travelling in opposite directions that form a stationary wave
what is meant by the fundamental frequency f0
it is the minimum frequency of a stationary wave for a string, which is the fundamental mode of vibration.
explain how the frequency effects the wavelength in a stationary wave along a string
- v = fλ, the speed of progressive waves along the string is constant.
- as frequency increases the wavelength must decrease in proportion at a frequency of 2f0 the λ is half the wavelength at f0
different harmonics
conditions for stationary wave in a tube closed at one end
- there must be an antinode at the open end and a node at the closed end.
- the air at the closed end cannot move and so must form a node.
- at the open end the oscillations of the air are at their greatest amplitude so must be an antinode
- the frequencies of the harmonics in tubes closed at one end are always odd multiples of f0
conditions for stationary wave in open tubes
- must have an antinode at each end in order to form a stationary wave.
- harmonics at all integer multiples of f0 are possible
formula for energy
E= hf = hc/λ = eV
what is an electron volt
the energy of 1 eV is defined as the energy transferred to or from an electron when it moves through a potential difference of 1v
- 1eV = 1.6 x 10-19J
what is the photoelectric effect
occurs when electromagnet radiation is incident on the surface of a metal causing photoelectrons to be emitted
three key observations from photoelectric effect
- photoelectrons are only emitted if the incident radiation was above a certain frequency for each metal
- if the incident radiation was above the threshold frequency emission of photoelectrons was instantaneous
- if the incident radiation was above the threshold frequency, increasing the intensity of radiation did not increase the max ke of photoelectrons instead more are emitted
Einsteins photoelectric effect equation
hf = ∅ + KEmax
- energy of single photon = minimum energy required to free a single electron from the metal surface + maximum kinetic energy of the emitted electron
wave-particle duality
all matter has both wave and particle properties
de Broglie equation
λ = h/p
- h = planc’s constant
- p = momentum the particle
