Quantum Physics Flashcards
What is the photoelectric effect?
The photoelectric effect is the emission of photoelectrons from a metal surface when electromagnetic radiation of a sufficiently high frequency is incident on it.
What is work function and threshold frequncy?
The work function of a metal is the minimum amount of energy that will remove an electron from the surface of the metal.
The threshold frequency is the minimum frequency of an incident photon that will remove an electron from the surface of the metal.
What is stopping potential?
Stopping potential V is the minimum potential difference between the metal and the collector that will prevent an ejected photoelectron from reaching the collector.
What does line spectra prove?
The existence of line spectra provides convincing evidence for the existence of discrete electron energy levels in isolated atoms.
As each line on the line spectrum corresponds to a single wavelength, and hence a single frequency of light, this implies that the energies of the photons emitted are discrete. This is only possible if the energy levels within the atom are discrete as well.
What is an energy level?
An energy level is the quantized energy corresponding to an allowed state of the electron within the atom.
What do the characteristic lines show?
Bombarding electrons cause the target atoms to be ionized by removing the inner electrons from the target atoms, so the outer electrons will decay to the empty level and emit x-ray photons characteristic of the target atoms.
What does the continuous spectrum show?
Bombarding electrons are slowed down by the target, their KE is progressively lost as a series of X-ray photons which form a continuous spectrum.
What does minimum wavelength show?
Bombaring electrons are stopped by the target atoms; all the KE of each electron produces one photon.
State what is meant by a photon.
A photon is a small discrete quantum of electromagnetic energy when the energy of a single photon is directly proportional to its frequency. E = hf
Describe existence of threshold frequency proof
No electrons were emitted at all unless the frequency of the monochromatic incident light is higher than a minimum value.
Wave theory predicts that photoelectric emission should occur at any frequency of incident light since energy transfer is continuous and electron can accumulate energy until sufficient to escape.
Quantum theory states that if the frequency of the incident photon is less than the threshold frequency of the target metal, the energy of the incident photon is insufficient to cause emission, and increasing intensity would not help, but only increase number of photons per unit time on metal surface.
Describe no time delay in emission of photoelectrons proof
Photoelectrons are emitted almost immediately with no appreciable time lag when the metal is illuminated.
Wave theory predicts that at very low intensities, there will be time lag between emissions of photoelectrons and when the light is first incident, as electrons need time to accumulate energy.
Quantum theory explains that as long as frequency of incident photon is higher than threshold frequency, the energy is sufficient, so photoelectrons are emitted as soon as the light is incident on the metal surface.
Describe stopping potential is independent on intensity of incident light proof
The stopping potential is independent of the incident light intensity at a constant frequency.
Wave theory predict incident light intensity increase, greater amount of energy incident, hence emitted photoelectrons posses more energy. Thus, max KE increase and stopping potential increase.
Quantum theory explains that incrasing intensity causes a higher photon rate arrival, but energy of incident photon remains the same. Since each photonelectron is liberated by one photon, max KE and stopping potential remains unchanged.
Describe emission line specrtrum
When a high potential difference is applied across a discharge tube containing the gas at low pressure, the atoms of the gas are excited through inelastic collisions. The excited atoms are highly unstable and will quickly de-excite to low energy states, emitting one or more photons in the process, with energies corresponding to the energy difference between any two discrete energy levels. Hence, an emission spectrum consists of coloured lines on a dark background.
Describe absorption line spectrum
White light, consisting of photons of a continuous range of frequencies, passes through a discharge tube containing gas at low pressure and temperature. Photons having the energies that correspond to the energy difference between discrete energy levels are absorbed, causing the electrons to be excited from the ground state to higher energy levels. Light of these frequencies appear to be mising from the continuous spectrum, hence an absorption spectrum consists of dark lines and a coloured spectrum.
Heisenberg
The Heisenberg Uncertainty Principle states that it is impossible to measure the exact position and momentum at the same time.
