Section 2: Electromagnetic Radiation and Quantum Phenomena Flashcards
What is the photoelectric effect?
If you shine radiation of high enough frequency onto the surface of a metal, it will instantly emit electrons.
What is wave-particle duality?
The idea that electromagnetic waves can behave as particles, and particles can show wave-like properties.
Explain how and why diffraction changes when the momentum of a particle changes.
Since the wavelength of a particle is inversely proportional to it’s momentum, the faster it is moving the less diffraction it experiences as its wavelength is smaller. Slower moving particles experience larger diffraction as their wavelength is larger.
Deifne excitation and ionisation.
The movement of an electron to a higher energy level is called excitation.
If an electron can absorb enough energy to be removed from the atom, the atom is ionised.
What is the work function and how does it dictate threshold frequency?
The work function (ϕ) is the minimum amount of energy an electron needs to absorb before it breaks the bonds holding it there and is emitted from a metal.
The energy gained from a photon is hf (as E = hf). Since, for electrons to be released hf ≥ ϕ , the threshold frequency, f0, must be:
How does the photoelectric effect work?
Metals contain free electrons that are able to move throughout the metal. If an electron on the surface of the metal can absorb enough energy to break the bonds holding it there the electron can be released.
What two ways can wave theory not explain the photoelectric effect?
Threshold Frequency: Wave theory says that for a panicular frequency of EM wave, the energy carried should be proportional to the intensity of the beam. However, electrons are never emitted unless the wave is above a certain threshold frequency which wave theory cannot explain.
Kinetic Energy of Photoelectrons: The higher the intensity of the wave, the more energy it should transfer to each electron — the kinetic energy of the electrons should increase with intensity. Wave theory can’t explain the fact that the kinetic energy depends only on the frequency
Why are line spectra evidence of transitions between discrete energy levels in atoms?
Line absorption spectrum: White light passes through a cool gas. Photons of the correct wavelength are absorbed by the electrons to excite them to higher energy levels. You see a continous spectrum with black lines in it corresponding to the absorbed wavelengths.
Line emission spectrum: Light emitted from a source is passed through a prism or diffraction grating. A line emission spectrum is seen as a series of bright lines against a black background. Each line corresponds to a particular wavelength of light dictated by the photons emitted and their electron transitions.
Describe electron transitions.
Electrons can move up levels by absorbing a photon, or colliding with a high velocity electron. Electrons can then cascade down an energy level by emitting a photon. Since these transistions are between definite energy levels, the energy of the photon emitted can only take a certain value.
What is the stopping potential?
A way of measuring the maximum kinetic energy of photoelectrons.
Photoelectrons can be made to lose their energy by doing work against an applied potential difference. The stopping potential Vs is the potential difference needed to stop the fastest moving electron with kinetic energy EK(max).
The work done by the potential difference in stopping the electron is equal to the energy it was carrying:
Explain why the photoelectrons have a range of kinetic energies up to a maximum.
The energy transferred from EM radiation to an electron is the energy it absorbs from one photon, hf. The kinetic energy it will be carrying after it leaves the metal will be hf minus any other energy losses. Electrons in the metal exist in different places, each with their own repsective amount of energy needed to liberate them from said place. This is the cause for the range of kinetic energies. In places where an electron only needs the work function to be liberated, this is when the photoelectrons kinetic energy is at a maximum as the energy loss is at a minimum.
How can the wave property of particles be seen?
Electron diffraction. Electrons are accelerated to high velocities in a vacuum and then passed through a graphite crystal. As they pass through the spaces between the atoms of the crystal, they diffract just like waves passing through a narrow slit and produce a pattern of rings - a diffraction pattern.
What is the electron volt?
The energies in an atom are usually so tiny that it makes sens to use a more appropriate unit than the joule. The electron volt (eV) is defined as the kinetic energy carried by an electron after it has been accelerated from rest through a potential difference of 1 volt.
1 eV = 1.6x10-19 J
How does a fluorescent tube work?
They contain mercury vapour, across which a high voltage is applied. This high voltage accelerates fast-moving free electrons that ionise some mercury atoms, producing more free electrons. When this flow of free electrons collides with the 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 emit photons in the UV range. A phosphor coating on the inside of the tube absorbs these photons, exciting its electrons to higher energy levels. These electrons then cascade down the energy levels and emit photons of visible light.
Explain how the photoelectric effect suggests EM waves have a particle like nature.
It suggests that EM waves exist in discrete packets called photons. The photons of light have a one-on-one, particle-like interaction transfering all its energy to one specific electron on a metal surface.
