Waves (W) Flashcards
progressive waves:
a wave that transfers energy from one point to another without transferring the medium itself
displacement:
vector quantity, distance a wave is from its equilibrium point
amplitude:
maximum vertical displacement of wave from equilibrium position
wavelength:
distance between successive oscillations of a wave
period:
time for one complete oscillation
frequency:
number of oscillations per unit time
equation relating frequency and time period:
F=1/T
Phase difference:
how much a point on a wave is in front/behind another
When are waves in phase?
when crests/troughs are aligned
When are waves in anti phase?
when the crest of one aligns with the trough of another
How can you express phase difference in degrees?
The fraction of the wavelength x 360
How can you express phase difference in radians?
Fraction of wavelength x ∏/2
How many degrees is in phase?
360
How many radians is in phase?
2∏
How many degrees is anti phase?
180
How many radians is anti phase?
∏
Transverse wave:
a wave in which the particles oscillate perpendicular to the direction of travel and energy transfer
Longitudinal wave:
When particles oscillate parallel to direction of wave travel and energy transfer.
example of transverse wave:
radio wave, visible light, UV
example of longitudinal wave:
ultrasound, sound
What are the areas of increases pressure in longitudinal waves called?
compressions
What are the areas of decreased pressure called in longitudinal waves?
rarefactions
Which type of wave cannot be polarised?
Longitudinal
Polarisation:
When particle oscillations only occur in one of the directions perpendicular to the direction of wave propagation.
Through what can polarising occur?
polarising filter or polariser.
how are stationary waves produced?
By the superposition of two waves of the same frequency and amplitude travelling in opposite directions
How are stationary waves USUALLY produced?
By a travelling wave and its reflection.
Node:
regions of no vibrations
antinodes:
regions where vibrations are at their maximum amplitude
Points between nodes are……
in phase
points that have an even number of nodes between them are…..
in phase
points that have an odd number of nodes between them are …….
out of phase
What is the principle of superposition?
When two or more waves with the same frequency arrive at a point, the resultant displacement is the sum of displacements of each wave.
What causes constructive interference?
In phase points (peaks line up/troughs line up = 2x amplitude)
What causes destructive interference?
Antiphase points (peaks and troughs line up = no amplitude)
Stationary waves can have different patterns, know as…..
harmonics
What do harmonics depend on?
The frequency of vibrations and situation in which they are created.
Where can harmonics be observed?
On strings with fixed ends
Wavelength of first harmonic in terms of L:
2L
Wavelength of second harmonic in terms of L:
L
Wavelength of third harmonic in terms of L:
2/3 L
What is the equation for the speed a wave whilst travelling along a string with two fixed ends?
v = root of: tension in string (N) / mu, mass per unit length of string (Kgm-1)
When are waves coherent?
If they have the same frequency and constant phase difference.
Path difference:
Difference in distance travelled by waves from their sources to a point where they meet.
If the path difference is nλ…
constructive interference
If the path difference in (n+1/2)λ
Destructive interference
What two conditions must there be in order for two source interference to be observed?
Coherent waves, monochromatic
In Young’s double slit experiment, where are dark fringes observed?
Where path difference is (n+1/2)λ, so destructive interference occurs
What does Young’s double slit experiment demonstrate?
How light waves can produce an interference pattern
In Young’s double slit experiment, where are light fringes produced?
Where path difference in nλ so constructive interference occurs
What is the first maxima labelled?
n=0
Why is it that in young’s double slit experiment both light sources originate from the same primary source?
So that the light is coherent
What is the fringe spacing equation for Young’s double slit experiment?
fringe width = (wavelength of source x distance from double slit to screen) / distance between centres of slits
What is diffraction?
The spreading out of waves when they pass an obstruction.
What does extent of diffraction depend on?
The width of the gap compared to the wavelength
What changes about a wave when it is diffracted?
The amplitude as some of the energy is dissipated
(diffraction) If gap size»_space; wavelength…
Then the effect becomes less pronounced
(diffraction) if slit was made narrower…
intensity of diffraction would increase and fringe spacing would be wider
If the gap size remained the same, which would diffract more and why: red light or blue light?
Red light as it has a larger wavelength making the width of the gap compared to the wavelength relatively smaller so the fringes become more spaced out.
What is a diffraction grating?
A plate on which there are a very large number or parallel identical close spaced slits.
What happens when light is put through a diffraction grating?
A pattern of narrow bright fringes are created.
What is the diffraction grating equation?
spacing between adjacent slits x sin x θ = order of maxima x wavelength
How can lines per mm be converted into d (as in dsinθ=nλ)
d = 1/ lines per metre
What is the angular separation?
The angle from the central maxima (n=0)
In diffraction, at what angle has the maximum angle to see orders of maxima been reached?
when theta = 90 or sin theta = 1
When does refraction occur?
When light passes a boundary between two different transparent media.
Why does refraction occur?
Light will bend towards the normal if it enters a more optically dense material as the wave will be slowed down (and vice versa)
How can you calculate refractive index?
n = speed of light in vacuum / speed of light in a substance
What is refractive index?
Property of light which measures how much light slows down
If an object is optically very dense it will have a ……
high refractive index
Refractive index will always be…
n < 1
What is refractive index of air said to approximately be?
1
What is snell’s law?
n1sinθ1=n2sinθ2 (where 1 is first material and 2 is material it refracts in and theta is angle form normal)
What happens when angle of incidence is critical angle?
The angle of refraction will equal 90 and light will refract across the boundary
When does total internal reflection occur?
When the angle of incidence is greater than the critical angle
What does the angle of refraction equal in total internal reflection?
the angle of incidence
What do fibre optics utilise?
Total internal reflection
What do fibre optics do?
Send high speed light signals over large distances
Why can total internal reflection occur in fibre optics?
Because n cladding < n core
What are the three main components of fibre optics?
optically dense core, lower optical density cladding, an outer sheath
What is the purpose of the outer sheath?
Prevents physical damage and strengthens fibre
What is the purpose of the cladding in fibre optics?
Protects core from damage, keeps signal secure
What is pulse broadening?
When light pulses in optical fibres spread out due to different angles of incidence.
What are three advantages of a narrow core of a fibre optic?
less light lost by refraction out of core, less overlapping pulses, angle of incidence less likely to drop below critical angle as there is smaller change in angle between each reflection
What can be done to reduce absorption in optic fibres?
Using a transparent core and using optical fibre repeaters so that the pulse is regenerated before significant absorption occurs.
how can you calculate the critical angle?
n2/n1 where n1 is the more optically dense material
When does material dispersion occur?
The refractive index of the fibre varies with frequency. Different wavelengths of light in the signal travel at different speeds. This causes a sharp pulse to spread into a broader signal. So the duration of each pulse increases.
What is material dispersion? short
the spreading of a signal caused by the variation of refractive index with wavelength
What is modal dispersion?
the spreading fo a signal caused by rays taking a slightly different path in the fibre
What is absorption in terms of fibre optics?
when energy from a signal is absorbed by the optical fibre in which it travels
When does modal dispersion occur?
When rays inside and optical fibre take slightly different paths so rays taking longer paths take longer to travel through the fibre broadening the pulse
What is a photon?
A discrete packet of energy
how can you calculate energy of a photon?
E=hf
How can you calculate energy of a photon not using frequency?
E=hc/λ
What is light intensity determined by?
the number of photons
What is the frequency of light determined by?
the energy of photons
How many joules is an electron volt?
1.6 x 10 -19
How can you calculate power of a light source?
number of photons x hf
what is the photoelectric effect?
Where photoelectrons are emitted from the surface of a metal after light above a certain frequency is shone on it. this certain frequency is the threshold frequency and it differs for each metal.
Why couldn’t the photoelectric effect be explained by wave theory?
Because it said that any frequency of light should be able to cause the emission of photoelectrons as the energy absorbed by each electron will gradually increase with each incoming wave.
What is the photon model of light?
- Em waves travel in discrete packets of energy called photons which each have an energy directly proportional to the frequency
- Each electron can absorb a single photon, therefore a photoelectron is only emitted if frequency is above threshold frequency
- If the intensity of light is increased, if the frequency is above threshold, more photoelectrons are emitted per second
What is the work function of a metal?
Φ, is the minimum energy required for electrons to be emitted from the surface of the metal
What is the stopping potential?
The potential difference you would need to apply across the metal to stop photoelectrons with the maximum kinetic energy.
What is the equation for stopping potential?
Ekmax = charge of electron x stopping potetnial
What is the photoelectric equation?
KE(max) = hf - threshold frequency
Electrons in atoms can only exist in….
discrete energy levels
What happens when electrons gain energy?
If an electron gains energy form say colliding with another free electron, it will move up an energy level. This is known as excitation.
What can happen if an electron gains enough energy?
It can be removed from the atom entirely, which is called ionisation. This occurs if the energy of the free electron is greater than the ionisation energy.
What happens immediately if an electron becomes excited?
It will quickly return to its original energy level (ground state) and therefore release the energy it gained in the form of a photon.
What is an example of excitation?
Fluorescent tubes
How do fluorescent tubes work?
The voltage across it accelerates free electrons through the tube which collide with the mercury vapour atoms, causing them to become ionised, releasing more free electrons. The free electrons then collide further with the mercury atoms causing them to become excited. When they de-excite they will release photons, most of which are in the UV range. The fluorescent coating in the tube will absorb these UV photons and electrons in the atoms of the coating become excited and de-excite releasing photons in the form of visible light.
How do you convert from eV to joules?
x 1.6 x 10-19
How do you convert from joules to eV?
divide by 1.6 x 10-19
How can a fluorescent tube produce a line spectrum?
By looking at it through a diffraction grating or prism.
What does each line in a light spectrum represent?
Each line represents a different wavelength of light emitted by the tube.
Is a photon emission spectrum discrete or continuous?
Discrete. The only photon energies emitted will correspond to these wavelengths, therefore this is evidence to show that electrons in atoms can only transition between discrete energy levels.
What is wave particle duality?
Light can be shown as having both wave and particle properties.
What are examples of light acting as a wave?
diffraction and interference
What is an example of light acting as a particle?
photoelectric effect
What is an example of wave particle duality?
The wave nature of electrons can be observed through electron diffraction, as only waves can experience diffraction.
What is the de Broglie wavelength equation?
λ = h/mv
What does the de Broglie wavelength allow you to do?
how the amount of diffraction changes as a particle’s momentum changes
What happens when the momentum of a particle is increased?
When momentum increases, the wavelength decreases and therefore the amount of diffraction decreases, so the concentric rings of interference come closer.
What happens when the momentum of a particle is decreased?
The wavelength will increase and the diffraction will increase so the concentric rings of interference will move further apart.
