WAVES Flashcards
Define Amplitude:
Maximum displacement from equilibrium position
Define Frequency:
Number of complete oscillations per unit time
Define Period:
Time taken for one complete oscillation
Define Wavelength:
Distance between one point on a wave and a point with the same phase
on the next wave
Define Longitudinal wave:
Oscillations of the molecules are parallel to the direction of energy transfer, producing compressions and rarefactions
Define Transverse Wave:
Oscillations of the molecules/fields are perpendicular to the direction of energy transfer
Define Coherance:
When two waves have a constant phase relationship and the same
frequency
Define Path Difference:
The difference in the path lengths from the source to a point
Define Superposition:
When two or more waves meet at a point the resulting displacement is
equal to the vector sum of the individual displacements
Define Inteference:
When two coherent waves meet at a point and undergo superposition – the resultant amplitude is equal to the vector sum of the individual amplitudes
Define Constructive Inteference:
When the resultant amplitude is maximum (as the waves meet in phase)
Define Destructive inteference:
When the resultant amplitude is zero (as the waves meet pi out of phase)
Explain how an interference pattern is produced (eg. of bright and dark lines/loud and quiet
regions)
- Waves spread out from source, meet at points in undergo superposition
- When the waves meet in phase, with a path difference of n lambda, constructive interference occurs
- This constitutes a maximum amplitude of oscillation and bright spot/loud sound
- When the waves meet in antiphase/ 180 degrees out of phase, with a path difference of (n+1/2) lambda, destructive interference occurs
- This constitutes zero amplitude of oscillation and there is a dark spot/no sound
Define phase
The fraction of the wave cycle that has been completed relative to the origin
Define Path Difference
The difference in the distance travelled by two waves to a single point.
State the relationship between path difference and phase difference:
Path difference= (ie. multiply the fractional difference of the cycle completed by the wavelength)
Explain how a standing wave is set up:
- Two waves travelling in opposite directions
- Of equal frequency and similar amplitude
- Undergo superposition when they meet
- Constructive interference produces antinodes – positions of maximum amplitude
- Where waves meet in phase
- Destructive interference produces nodes – positions of zero amplitude
- Where waves meet in antiphase
Define node:
Positions of zero amplitude
Define antinode:
Position of maximum amplitude
State the conditions for a standing wave to be set up between a speaker and a wall
- The emitted and reflected wave must have similar amplitudes in order to completely destructively interfere and produce nodes
- So the wall should not absorb the wave
- The distance between the speaker and wall must be a whole number of half
wavelengths - There must not be multiple reflections from other parts of the room
State the distance between nodes in terms of the wavelength
Distance between nodes = λ/2
State the distance between antinodes in terms of the wavelength
Distance between antinodes = λ/2
Define diffraction
The spreading out of a wave as it passes through a gap
State Huygen’s construction
Each point on a wavefront acts as a source of secondary circular
wavelets. The new wavefront is the surface tangential to the secondary wavelets.
Define each of the quantities in nλ = dsinθ
- n is the order
- λ is the wavelength of light
- nλ = the path difference between the central order n=0 order and the nth order
- d is the separation between the slits ( d= 1/N where N is the number of slits per m and
d=10 -3 /N where N is the number of slits per mm) - θ is the angle between the n=0 order and the nth order
Define the intensity of light in the wave model of light
- Intensity is power/area
2. Intensity is proportional to the amplitude of the wave squared
Define photon
Packet of electromagnetic energy
Describe what is meant by the wave particle duality of light
- Light exhibits wave behaviour (eg. diffraction, intereference, superposition, polarisation)
- Light exhibits particle behaviour (eg. absorption and emission line spectra, photoelectric
effect) - So light exhibits both wave and particle behaviour: it exhibits wave particle duality
Define work function
Minimum energy required for electron to be released from the surface of
a metal
Define threshold frequency
Minimum photon frequency required for electron to be released
from the surface of a metal (by absorbing the photon)
Explain why kinetic energy of electron is maximum kinetic energy
- The work function is the minimum energy required for electron to be released,
- Thus in the equation hf = work function + ke max, the kinetic energy is the maximum
possible kinetic energy. - Most electrons will have less kinetic energy than this as they lose more energy as they
leave the surface of the metal.
Explain how the photoelectric effect indicates that light is a particle
- It is observed in the photoelectric effect that only EM radiation above a certain threshold
frequency will release electrons (no matter what the intensity) - So the energy of the photon, E=hf is proportional to frequency
- In wave model energy is proportional to intensity, so in the wave model high intensity
light of any frequency should emit electrons – this is not observed - Instead, electrons are always released from surface of metal when energy of photon is
larger than work function of metal and with frequency above the threshold - It is also observed in the photoelectric effect that electrons are emitted instantaneously
from the metal - This indicates that energy is absorbed instantaneously – comes in packets of energy
called photons - One photon is absorbed by one electron and photon energy E=hf depends on frequency
- However, in the wave model energy would be absorbed over time and not release
electrons instantaneously - In the particle model, the higher the intensity, more photons would be absorbed each
section and so there would be more electrons emitted per second – this is what is
observed
Define the intensity of light in the particle model of light:
- Intensity = power / area = rate of energy transfer per second/area
- Intensity = number of photons x energy of one photon/ time x area
- I = Nhf/tA
- So the intensity is directly proportional to the number of photons per second
State the effect on the maximum kinetic Energy of photoelectrons emitted when the intensity of light increases
No effect
State the effect on the number of photoelectrons emitted per second when the intensity of light
increases
The number of photoelectrons emitted per second increases in proportion with the
intensity increase
Define stopping potential:
The potential difference at which photoelectrons do not have enough kinetic energy to
pass across the gap
vs = KE max /e
Define the de Broglie wavelength
The wavelength of a particle that has a momentum
Define energy level
The discrete allowed energy of an electron within an atom
Explain why energy levels of electrons in atoms are negative
- just free electron has zero energy
2. In order for the electron to move up energy levels to be released it must gain energy
Explain origin of line spectra (spectral lines/ emission lines) specific to certain elements
- Electrons exist in discrete energy levels
- Electron within atom excited to higher energy level when: fast moving electron collides
with atom, transferring its kinetic energy or current is passed through OR gas is heated - The electron then falls back down to lower energy level
- Emitting a photon with an energy E equal to the energy difference between the two
electron levels - E=hf is the energy of the photon
- The photon is emitted with a specific frequency f = E/h where E is the energy difference
between the levels - There are only a limited number of energy differences between levels and there only a
corresponding limited number of frequencies emitted - Different elements have different energy differences between levels so produce different
spectral lines
Explain origin of absorption spectra
- Electrons exist in discrete energy levels
- Electron within atom (in atmosphere of sun) excited to higher energy level when it
absorbs a photon - E=hf is the energy of the photon
- The photon absorbed must have a specific frequency f = E/h where E is the energy
difference between the levels - There are only a limited number of energy differences between levels and only a
corresponding limited number of frequencies of photons absorbed - Different elements have different energy differences between levels so produce different
absorption lines at specific frequencies
Define polarised light
- Where the oscillations of the field are only in one plane
2. And that plane also contains the direction of energy transfer
Explain how the pulse-echo technique can be used to determine the distance to an object
- A pulse is sent out. When the medium’s density changes, some of the pulse is reflected
off the density change boundary. - The reflected wave is picked up and the time between the emission and return is
recorded. - If the speed in the medium is known, use distance = time for pulse to return x speed/ 2
(as time between emission and reflected wave returning is time taken to travel twice the
distance)
Explain why the pulse length must be less than the time taken for the pulse to return and
explain the condition placed on the time between pulses
- The pulse length is the time that the pulse lasts for. This must be less than the time
taken for the pulse to return after being reflected so that the reflected pulse does not
overlap with the emitted pulse. - The time between each pulse must be long enough that the reflected pulse does not
overlap with the emitted pulse.
Explain how the pulse-echo technique can be used to determine the speed of a moving object
- A pulse is sent out. It reflects off a boundary to an object with a different density.
- If the object is moving then the reflected ultrasound will have a shifted frequency/wavelength due to the Doppler effect
- If the object is moving towards the receiver then the frequency will increase, if the object
is moving away from the receiver the frequency will decrease - The greater the shift in frequency the greater the speed of the object
Explain what is meant by refraction
Refraction is the change in direction of a wave as it changes speed due to a change in
the density of the medium that it is travelling in
Define the refractive index
Refractive index, n = c/v – this is the ratio of the speed of light in a vacuum to the speed
of light in the medium
Define the critical angle
The angle of incidence in the denser medium at which the angle of refraction is 90
degrees in the less dense medium: sin c = 1/n
Describe the condition for total internal reflection to occur
Total internal reflection will occur when the angle of incidence is larger than the critical
angle
Describe how an atomic emission line spectrum is formed
- Electrons exist in discrete energy levels.
- The electron will become excited and move up energy levels.
- The electron then drops down an energy level emitting a photon
- The photon energy, E=hf is equal to the energy difference between levels
- As there are only a limited number of specific energy differences between energy levels only specific frequencies of photons can be emmitted
When does total internal reflection occur
Light travelling from a more dense medium to less dense medium.
What is snells law
n1sin0=n2sin02
Polarised light is passed through a second polarising filter. No light is observed. Explain this observation, and explain how the intensity of light changes through the polarising filter as the filter is rotated through 180 degrees.
- Polarised light has oscillations of the electromagnetic fields only in one plane that contains the direction of energy transfer.
- Initially, the intensity of light is zero because the second polarising filter’s plane of transmission is perpendicular to the plane of polarisation of the light. All light is absorbed.
- As the second filter is rotated, components of the polarised light that oscillate in the direction of transmission of the second filter are transmitted, so the intensity increases.
- When the plane of transmission of the filter is parallel to the plane of oscillation of the polarised light, all of the light is transmitted and the intensity is maximum – this is when the filter has rotated 90 degrees.
- The intensity will reduce to zero again after the 180 degree rotation as the plane of transmission of the polarising filter is perpendicular to the plane of oscillation of the light.
What is the equation to find the frequency of the first harmonic?
f=1/2l square root T/u
- l being the length of the string
- u being mass per unit length
How do you calculate the distance between bright fringes in youngs experiment
w=lambda x D/s where: w= fringe spacing D= distance from slits to screen s= spacing between slits
Normally fringe spacing is very small so to ensure our measurement is accurate we measure across lots of fringes and divide by the number of fringe widths to find an average
