Quantum Physics Flashcards
Photon
A photon is a discrete packet of energy of electromagnetic radiation.
The energy of one photon is directly proportional to the frequency of EM radiation.
Photoelectric effect
The photoelectric effect refers to the emission of electrons from a cold metal surface when electromagnetic radiation of sufficiently high frequency falls on it.
Work function
The work function of a metal is the MINIMUM ENERGY of a photon to cause emission of electrons from the surface of a metal.
Threshold frequency
The lowest frequency of electromagnetic radiation that gives rise to the ejection of electrons from the metal surface.
de Broglie wavelength
The wavelength associated with a particle that is moving
Emission line spectra
Emission line spectra consists of discrete bright lines of different colours on a dark background.
(has more lines than absorption line spectra as only transitions from ground state give rise to absorption, whereas transitions that give rise to emission occurs between all energy levels)
Absorption line spectra
Absorption line spectra are continuous spectrum crossed by dark lines.
Heisenberg Uncertainty Principle
The Heisenberg uncertainty principle states that if a measurement of POSITION of a particle is made with precision uncertainty x and a simultaneous measurement of its MOMENTUM in the x-direction made with precision uncertainty p, the product of these 2 uncertainties can never be smaller than h.
The uncertainties arise from the quantum fuzziness of matter.
How does the Emission Line Spectra work? (just for understanding, don’t need know in great detail)
1) A high potential difference is applied to the low pressure gas to bring the gas atoms to an excited state.
2) When the electron transits down from a higher excited state to lower states, photons of energies corresponding to the difference between the energy levels will be emitted. Due to the existence of discrete energy levels in isolated atoms, these emitted photons have specific energies which correspond to specific frequencies.
3) These photons are directed at the diffraction grating and get diffracted at specific angles according to their wavelengths. -> Emission line spectra
How does the Absorption Line Spectra work?
1) By passing a continuous spectrum of white light through a cool vapour or gas, the electrons of the vapour absorb photons of certain frequencies and get excited from ground state to higher energy levels. The photons absorbed must have energy exactly equal to the energy difference between the two energy levels of the gas atoms. Due to the existence of discrete energy levels in isolated atoms, the absorbed photons have specific energies which correspond to specific frequencies and specific wavelengths.
2) When the electrons are de-excited, the photons are re-emitted in random directions, causing them to be missing from the “straight through” direction -> Absorption line spectra
Intensity of light
The amount of energy the light carries per unit time per unit area.
How do the wave model and the photon model explain instantaneous emission of electrons (even for light of low intensity)
Wave:
Very intense light should be needed for immediate effect.
Photon:
A single photon is enough to release one electron.
How do the wave model and the photon model explain threshold frequency below which there is no emission
Wave:
Any frequency can give rise to emission if exposure time is long enough.
Photon:
A low frequency photon has energy less than work function so there is no release of an electron.
How do the wave model and the photon model explain max KE of electrons being independent of intensity
Wave:
Greater intensity means more energy so the electrons should have more energy
Photon:
Greater intensity does not mean more energetic photons, so electrons cannot have more energy
How do the wave model and the photon model explain max KE being dependent on frequency
Wave:
Increasing intensity and not frequency increases the energy of electrons
Photon:
Higher frequency means more energetic photons so electrons gain more energy
