Wave-Particle Duality Flashcards
What was Newton’s Corpuscular Theory of Light
Light was made of small particle-like bodies called corpuscles, emitted by luminous objects
What was one of the predictions of Newton’s Corpuscular Theory of Light
Objects emitting light were losing mass slowly
What phenomena could Newton’s Corpuscular Theory of Light explain
Reflection, refraction and dispersion but NOT diffraction
How did Newton’s Corpuscular Theory of Light explain reflection
The corpuscles hit the reflective surface and experienced an equal and opposite repulsive force from the surface due to N3L
This is because corpuscular theory treated corspuscles like solid, elastic spheres
How did Newton’s Corpuscular Theory of Light explain refraction
At a boundary between air and denser medium, there was a resultant force on the corpuscles acting perpendicular to the boundary.
This meant light travelled faster in a more dense medium
Corpuscles were attracted to the denser medium
Where does the boundary force arise in Newtons explanation of refraction
There is an attraction to a greater amount of matter, increasing the vertical component of velocity
What did Huygen propose about light
Light was a wave
Huygen’s theory of light
Light travelled in wavefront.
The wavefront were emitted from a point source
Any point of the wavefront acted as a secondary point source from which wavelets could propagate.
Wavelets joined together to form new wavefront.
How did Huygens theory of light explain reflection
When wavefront hits a reflective surface, point of reflection becomes secondary point source for wavelets.
Different parts of wavefronts hit reflective surface at different times, so new wavefront formed in new direction
How did Huygens theory of light explain refraction
Relied on light travelling slower in denser mediums.
Different parts of wavefront hit boundary at different times.
Part of wavefront which reaches boundary first slows down first and causes it to change direction
Similarities between Newtons and Huygens theories of light
Both explained :
reflection
refraction
dispersion
Differences between Newtons and Huygens theories of light
Corpuscular theory said light travelled faster in denser mediums, wave theory said light travelled slower in denser mediums.
Corpuscular theory said light was composed of particles with mass, wave theory said it was a wave travelling through a massless medium.
Corpuscular theory had no explanation for diffraction or interference
Why was Newtons theory more accepted than Huygens
Newton was more widely known and respected.
There was also no way of measuring speed of light or observing diffraction at the time.
What does Young’s double slit experiment demonstrate
How light waves produce a diffraction pattern
Describe the set up for Young’s double slit experiment
Monochromatic light source
Single slit
Double slit
Screen
What pattern did Young’s double slit experiment show
A interference pattern
What did corpuscular theory predict for Young’s double slit experiment
Only 2 bright regions
What was evidence of Huygens wave theory of light
Light diffracts through slits to form an interference pattern
What are electromagnetic waves
Oscillating electric and magnetic fields which propagated each other
Direction of electric field on an accelerating charge
Perpendicular to the particles motion
What does the alternating electric field of a charged particles produce
A perpendicular alternating magnetic field
Does light need a medium to travel
No
State Maxwell’s formula for the speed of EM waves in a vacuum and explain why speed of light is constant.
c = 1 / √(μ₀ε₀)
All values are constants therefore speed of EM waves in a vacuum is constant
What does μ₀ relate to
Magnetic flux density due to a current carrying wire in free space
What does ε₀ relate to
The electric field strength due to a charged object in free space
What did Hertz discover
Radiowaves
How did Hertz discover radiowaves
He made a short air gap between wires and put a large pd across this gap
The high pd spark bridged the gap.
The sparks generated radiowaves
Describe the equipment Hertz used to detect radiowaves
A circular wire with a small break in the circuit which produced spark across the break when near the source.
A concave metal sheet with 2 parallel metal rods at the centre with an oscillating potential difference induced across them by the radio waves alternating magnetic field
Describe how Hertz measured the speed of radio waves
He reflected radio waves from the transmitter off of a flat metal sheet.
This produced a standing wave.
When passing a detector, a large signal was detected at antinodes and no signal at the nodes.
This allowed him to find the wavelength.
Frequency was already known using properties of the transmitter circuit.
He used v = f x λ to determine speed
How did Fizeau measure the speed of light
He shone a beam of light at a mirror several km away.
In the path of the light he placed a toothed wheel which spun at a very high speed.
Teeth of wheel and gaps inbetween periodically passed over the beam of light
This created regular pulses of light travelling towards distant mirror
Derivation for how Fizeau was able to calculate speed of light
total time for light to pass and return through toothed wheel = 2d/c
t = T / 2n (wheel has n gaps and n teeth both of same width)
t = 1 / 2nf
2d/c = 1/2nf
c = 4dnf
Perfect black body
A theoretical object that absorbs all of the radiation incident on it and does not reflect or transmit any radiation
What wavelengths are emitted when an objects temperature increases
Shorter wavelengths
Ultra-violet catastrophe
The disagreement with experimentally measured black body spectra and the spectra predicted by physics
What did the spectra when treating EM radiation as a wave predict
It emits an infinite amount of ultra-violet as temperature of object increased
What was Planck’s interpretation
Oscillators were responsible for emitting EM radiation and he assumed energy emitted was quantised, which meant it could only be emitted in integer multiples of these quanta of energy
Describe the photoelectric effect
Incident radiation on a metal’s surface causes it to emit electrons.
This only happens for radiation above a certain frequency.
If radiation is below the threshold frequency, no matter the intensity, no photoelectrons will be emitted
How did wave theory contradict the photoelectric effect
It predicted if low frequency radiations was aimed at the metal at a high enough intensity, enough energy would be transferred to remove photoelectrons.
What did Einstein propose about EM radiation
It was made from discrete quanta/ packets of energy of size:
E =hf
How did Einsteins theory explain why radiation below threshold frequency did not cause photoelectric emission
Only one photon was able to transfer its energy to one electron
If hf was not large enough to energise an electron, the photons could not combine to energise the electron
How did Einsteins theory explain why the energy of emitted photoelectrons increased with the frequency of incident light
Photon transferred all their energy, hf, to electrons
If this was greater than energy needed to emit electrons, rest of energy was transferred to the KE store of the electrons.
If hf was larger, more energy was left over for KE of electrons.
What was DeBroglie’s hypothesis
All particles can behave as both waves and particles
Derivation to find the wavelength of electrons
eV = 1/2 x mv^2
v = sqrt (2eV / m)
momentum = mass x velocity
momentum = m x sqrt (2eV / m)
momentum = sqrt(m^2) x sqrt (2eV / m)
momentum = sqrt (m^2 x 2eV / m)
momentum = sqrt (2meV)
wavelength = h / sqrt (2meV)
What happens to the DeBroglie wavelength of an electron as speed increases
Wavelength decreases
What happens to electron interference pattern if pd increases
The diffraction rings move closer to the centre of the screen
What does changing the voltage across the anode allow scientists to do
Manipulate the wavelength of the electron
What does the resolving power of a microscope depend on
The wavelength of the radiation being used
Shorter wavelength = see finer details in an object
Size of an atom
1x10^-10m
How do scientists find wavelengths the size of an atom
They estimate the anode voltage/accelerating voltage needed to make electrons wavelength the same size as an atom
How do electrons travel in a transmission electron microscope (TEM)
Electron gun emits electrons through thermionic emission.
The e- are accelerated to high speeds by a large potential difference.
Function of condenser lens in TEM
Produces parallel beam of electron waves to illuminate the sample
Purpose of objective lens in TEM
Produces an image of the sample
Purpose of the projector lens in TEM
Creates magnified image
Drawbacks of TEM
Level of detail depends on resolving power (fast electrons and short wavelength)
Electrons must pass through sample
Not all electrons emitted by thermionic emission have same speed so not all electrons slowed by sample same amt
Electrons in beam have range of speeds so blurrier image
Explain the process of quantum tunnelling
Waves have a probability of travelling through potential barrier
What size gap can electrons tunnel across
A barrier the same order as the DeBroglie wavelength
Explain how a STM can map the shape of a sample when the height of the probe is kept constant
Tip moved across sample.
Current is measured.
When the current is larger, gap is smaller and sample closer to the tip
How is a constant current STM different
Height of top is varied so current constant.
Height of tip is measured and corresponds to profile of sample
