Section 2: Electromagnetic Radiation and Quantum Phenomena Flashcards
What is the photoelectric effect?
With a high enough frequency shone onto a surface of a metal, it can cause the emission of electrons. Free electrons contained in the metal near the surface absorb the energy from the radiation and vibrate. With enough energy, the bonds in the metal can break and the electron can be released. This is the photoelectric effect.
What are the electrons emitted in the photoelectric effect called?
Photoelectrons
What is the threshold frequency?
The minimum frequency needed for a photoelectron to be emitted from the metal
Why are photoelectrons emitted with a variety of kinetic energies?
Because the value of maximum kinetic energy increases with the frequency of the radiation
What is the intensity of radiation?
The amount of energy per second hitting an area of the metal. - doesn’t effect the kinetic energy
What is the number of photoelectrons emitted per second proportional to?
The intensity of the radiation
What does the wave theory state?
That for a particular frequency of EM wave, the energy carried should be proportional to the intensity of the beam and should also spread evenly over the wave front
Why can’t threshold frequency be explained by wave theory?
Because wave theory suggests that each electron would gain a bit of energy from each wave front and gradually gain enough energy to leave the metal however, electrons are never emitted unless the wave is above the threshold frequency
What is the kinetic energy like of photoelectrons?
The higher the intensity of the wave the more energy it should transfer however this doesn’t happen kinetic energy is only affected by frequency not the intensity
What did Max Planck suggest?
That em waves can only be released in packets or quanta. The E of these wave packets is hf
What is the equation for the energy of one wave packet?
E = hf = hc/ lambda
What did Einstein suggest about em wave packets?
That EM waves can only exist in discrete packets that he named Photons
What do photons do?
Each photon would transfer all of its energy to one specific electron an helped to explain the photoelectric effect
How can you demonstrate the photo electric effect?
A zinc plate is attached to the top of an electroscope. The zinc plate is negatively charged and the zinc then repels the gold leaf causing it to rise up. UV light is shone onto the zinc plate. This energy causes the electrons to be lost from the zinc plate and loses its negative charge and the gold leaf falls back down
How can you explain the photoelectric effect?
When EM radiation hits the metal, it is bombarded with photons. If one of these photons collides with a free electron it will gain energy to equal hf. Before it can leave the surface of the metal it needs enough energy to to break the bonds (the work function). If the energy gained from the photon is greater than the work function it is able to escape
What is the equation that shows how much energy an electron needs to be emitted?
hf >_ work function
Fo = work function / h
Fo = threshold frequency
How can you calculate maximum kinetic energy of an electron?
Ek = hf - work function
Work function is the minimum amount of energy an electron can lose
What is the photoelectric equation?
Hf = work function - Ek
What is the maximum energy a photoelectron can have?
Ek(max) = 1/2 m x (v max)^2
M = 9.11 x 10^-31 kg
What is the photoelectric equation in terms of 1/2 m v max^2?
Hf = work function + 1/2m x v max^2
How can maximum kinetic energy be measured using the idea of stopping potential?
eVs = Ek(max)
Charge on the electron x stopping potential (volts) = maximum kinetic energy (j)
What is an electron volt?
The kinetic energy carried by an electron after it has been accelerated from rest through a pd of 1 volt
How can you convert between joules and electron volts?
1 ev = 1.6 x 10^-19 j
What are discrete energy levels in atoms?
Electrons in an atom can only exist in certain well defined energy levels
What does n=1 represent in terms of energy levels?
N=1 represents the lowest energy level an electron can be in - the ground state
How can electrons move down the energy levels?
By emitting a photon
What does it mean for an electron to be excited?
When it is in a higher energy level than the ground state
What is the energy carried by a photon emitted after a transition equal to?
The difference in energies between the two levels of a transition
When can electrons move up an energy level?
When they absorb a photon with the exact energy difference between the two levels
Which equation shows a transition between two energy levels?
Delta E = E1 - E2 = hf = hc/lambda
What happens when an electron is removed from an atom?
The atom is ionised.
What is the ionisation energy of an atom?
The amount of energy needed to remove an electron from the ground state atom
How do fluorescent tubes work?
They use the excitation of electrons and photon emissions to produce visible light. They use a high voltage to accelerate fast moving free electrons in a mercury vapor which ionises the mercury atoms. Photons are created by the excited electrons returning to their ground state. A phosphorus coating on the inside of the tube absorbs the photons exciting electrons to much higher energy levels. These electrons lose energy by emitting many lower energy photons of visible light
What does the wave theory state?
That for a particular frequency of EM wave, the energy carried should be proportional to the intensity of the beam and should also spread evenly over the wave front
Why can’t threshold frequency be explained by wave theory?
Because wave theory suggests that each electron would gain a bit of energy from each wave front and gradually gain enough energy to leave the metal however, electrons are never emitted unless the wave is above the threshold frequency
What is the kinetic energy like of photoelectrons?
The higher the intensity of the wave the more energy it should transfer however this doesn’t happen kinetic energy is only affected by frequency not the intensity
What did Max Planck suggest?
That em waves can only be released in packets or quanta. The E of these wave packets is hf
What is the equation for the energy of one wave packet?
E = hf = hc/ lambda
What did Einstein suggest about em wave packets?
That EM waves can only exist in discrete packets that he named Photons
What do photons do?
Each photon would transfer all of its energy to one specific electron an helped to explain the photoelectric effect
How can you demonstrate the photo electric effect?
A zinc plate is attached to the top of an electroscope. The zinc plate is negatively charged and the zinc then repels the gold leaf causing it to rise up. UV light is shone onto the zinc plate. This energy causes the electrons to be lost from the zinc plate and loses its negative charge and the gold leaf falls back down
How can you explain the photoelectric effect?
When EM radiation hits the metal, it is bombarded with photons. If one of these photons collides with a free electron it will gain energy to equal hf. Before it can leave the surface of the metal it needs enough energy to to break the bonds (the work function). If the energy gained from the photon is greater than the work function it is able to escape
What is the equation that shows how much energy an electron needs to be emitted?
hf >_ work function
Fo = work function / h
Fo = threshold frequency
How can you calculate the maximum kinetic energy of an electron and therefore the photoelectric equation?
Ek = hf - work function
Work function = the minimum amount of energy an electron can lose
hf = work function + Ek(max)
What is the maximum energy a photoelectron can have?
Ek(max) = 1/2mvmax^2
V max = the maximum velocity of an emitted electron
What is the photoelectric equation?
hf = work function + 1/2mvmax^2
What is the intensity?
More photons per second on an area
How can the maximum kinetic energy be measured using stopping potential?
eVs = Ek(Max)
Charge on the electron x the stopping potential (v) = eVs
What is an electron volt?
The kinetic energy carried by an electron after it has been accelerated from rest through a potential difference of 1 volt.
How can you convert between eV and joules?
1 eV = 1.6 x 10^-19 J
What does n=1 represent in terms of energy levels in atoms?
N=1 is the lowest energy level an electron can be in - the ground state
When is an electron excited?
When it is in an energy level higher than the ground state
How can electrons move between the energy levels?
- move down by emitting a photon
* move up if the electron absorbs a photon with the exact energy difference between two levels
What is the equation that shows the transition between two energy levels?
Delta E = E1 - E2 = hf = hc/lambda
When is an atom ionised?
When an electron has been removed from an atom
What does the ionisation energy show?
The amount of energy needed to remove an electron from the ground state atom
How do fluorescent tubes work?
Fluorescent tubes use the excitation of electrons and photon emission to produce visible light. They contain mercury vapour, across which a high voltage is applied. This voltage accelerates fast moving free electrons that, via collisions, ionise some of the mercury atoms and subsequently produce more free electrons. When this flow of free electrons collides with the flow of electrons in the mercury atoms, the atomic mercury electrons are excited to a higher energy level.
When these excited electrons return to their ground states, they lose energy by emitting high energy photons in the UV range. The photons emitted have a range of energies and wavelengths that correspond to the different transitions of the electrons.
A phosphorous coating on the inside of the tube absorbs these photons, resulting in an excitation of its electrons to much higher energy levels. These electrons then decrease down the energy levels to maintain a constant energy level, and lose energy by emitting many lower energy photons of visible light. The light from the fluorescent tube is then split with a diffraction grating, a line spectrum is produced as the light given off consists of many different wavelengths, and diffraction grating diffracts light of differing wavelengths at different angles, producing clear and defined spectral lines.
What happens if you split the light from a fluorescent tube?
You get a line spectrum. A diffraction grating diffracts light of different wave lengths at different angles. It is seen as a series of bright lines against a black background
What does line spectra provide evidence for?
That the electrons in atoms exist in discrete energy levels. Atoms can only emit photon with a certain energy and so you can only see certain lines
What can you see in a continuous spectra?
If you split light up with a prism the colours all merge into each other.
What are line absorption spectra?
You get it when light with a continuous spectra passes through a cool gas. At low temperatures most of the electrons will be in their ground states. Photons of the correct wavelength are then absorbed and then are missing from the continuous spectra
What is diffraction?
When a beam of light passes through a narrow gap it spreads out. It can only be explained using waves
What is the only way results from the photoelectric effect can be explained by?
Light is a series of particle like photons
What is wave-particle duality?
The photoelectric effect and diffraction show light to be both a wave and a particle
What did Louis de Broglie suggest?
That if a wave like light showed particle properties (photons) particles like electrons should be expected to show wave like properties
What is the equation of the de Broglie wavelength?
Lambda = h/mv
How do Electron diffraction tubes work?
Electrons are accelerated to high velocities in a vacuum and passed through a graphite crystal, they diffract like waves passing through a narrow slot and produce a pattern of rings
What does a greater wavelength show in Electron diffraction?
The spread of lines increases.
If the velocity is higher, the wavelength is shorter and the spread of lines is smaller
How do electron microscopes work?
A shorter wavelength gives smaller diffraction effects. An electron microscope can resolve finer detail than a light microscope. This allows you to see something as small as a single strand of DNA
