Chapter 19: Quantum Physics Flashcards
Photoelectric Effect
The emission of photoelectrons from a metal surface when electromagnetic radiation of a sufficiently high frequency is incident on it
It demonstrates particulate nature of electromagnetic radiation
Photon
a quantum of energy of electromagnetic radiation
where the energy of a single photon is directly proportional to its frequency, E= hf, where h is the Planck Constant
Experimental Observations of photoelectric emission
- Photoelectrons are emitted only when frequency of EM radiation exceeds a certain minimum, threshold frequency (emission independent of intensity)
- Photoelectrons are ejected almost instantaneously
- Stopping potential (Max KE of photoelectrons ejected) is independent of intensity of incident radiation
- current proportional to intensity of incident light
Work function energy
The minimum energy required to remove an electron from the surface of a metal
electrons are held in a metal lattice by electrostatic force, hence minimum amt of energy is required to remove electrons
Threshold freq.
Minimum frequency of incident photons for photo electrons to be emitted from a metal surface
Energy of a photon is proportional to the frequency of the radiation
Stopping potential
Minimum potential difference between the metal and the collector that will prevent an ejected photoelectron from reaching the collector
same Vs, same freq, dependent on freq
Electron Energy levels in isolated atoms
The quantized energy corresponding to an allowed state (or orbital) of the electron within the atom
Excitation by particle collisions vs Photo Absorption/Emission
- Excitation through transfer of energy from a colliding particle to the atom: colliding particle must have kinetic energy greater or equal to energy diff between 2 discrete energy levels
- Excitation through Photon Absorption/Emission : Photon must have exact amt of energy that corresponds to energy diff btw 2 discrete energy levels
Emission spectrum
A series of separate, differently coloured lines on a dark background. These lines are specific photon energies emitted by atoms when electrons in the excited atoms return to lower energy levels
Fast electrons cause excitation followed by deexcitation causing release of characteristic photons
Absorption spectrum
A series of separate, dark lines on a continuous spectrum (coloured bg)
These lines are specific photon energies absorbed by atoms of cold gas causing electrons to move to higher energy levels
Cool gas absorbs photons which cause excitation to higher states. The light emerging from cool gas has characteristic photons missing.
Electrons within the cool cloud of gas, in their ground state, absorbs the photon energy from the incident white light,
where photon energy corresponds to the energy difference between two discrete energy levels, causing electrons to become excited and transit to a higher energy level.
The excited electrons, being unstable, de-excite to a lower energy level and emit photons, but in all directions.
Hence, those frequencies of the continuous spectrum which were absorbed and re-emitted in all directions appear as dark lines.
Xrays
EM waves with wavelenghts 0.001nm to 10nm
Continuous spectrum
When fast bombarding electrons collides with metal atoms in the target, they can lose any fraction or all their KE in a single interaction. With varying KE lost, Xray photons of various wavelengths are emitted, resulting in continuous spectrum
Minimum Wavelength
When all of the KE of a very fast bombarding electron is lost in a single collision to produce one photon, the most energetic photon with the highest freq. & lowest wavelength is produced
Characteristic Wavelengths
Bombarding electrons cause the target atoms to be ionized by removing the inner electrons from target atoms, so the outer electrons will decay to the empty level and emit x-ray photons characteristic of the target atoms
Heisenberg Uncertainty Principle
it is impossible to measure the exact position and momentum at the same time
How does electron show wave nature? ( act like particles )
Electrons are diffracted, forming a diffraction pattern of concentric circles on fluorescent screen.
De broglie wavelength
A particle/electron has a wavelength which is dependent on its momentum.
Why electron diffraction will b observed on the screen? (concentric circles)
separation of ___ is in the same order of magnitude as the de broglie wavelength of the electron, hence effect of diffraction is significant.
Electrons exhibit wave properties and spread/diffract when they pass through the slits by carbon firm. The electron waves interfere constructively to form bright concentric circles on the screen
Line spectra
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 of electrons within the atom are discrete as well
Explain an evidence provided by the photoelectric effect experiment for the failure of the wave theory of light.
- Wave theory predicts that the photoelectric effect should occur for any frequency of the monochromatic incident light. If frequency of light is too low, its intensity can be increased to cause photoelectron emission. However, no electrons were emitted at all unless the frequency of the monochromatic incident light was greater than a minimum value (i.e. threshold frequency).
- Wave theory predicts that at very low intensities, there will be a time delay in Emission of Photoelectrons as the electrons would need time to accumulate sufficient energy in order to escape from the metal surface.
- Wave theory predicts that if the incident light intensity is increased, a greater amount of energy will be incident on the surface and hence electrons will escape with more energy from the surface of the metal. However, maximum kinetic energy of the photoelectrons and hence stopping potential is independent of the incident light intensity.
Threshold wavelength
the longest wavelength of electromagnetic radiation that can eject electron from a particular metal surface
Einstein’s Particle theory
- Photon energy is proportional to the frequency of radiation
- One photon interacts with one electron with entire energy of photon transferred to the electron during the interaction
energy levels shown as negative
implying electron is bounded by attractive force and that energy must be provided in order for electron to move to higher state
describe how you wld produce an emission line spectrum of hydrogen in the laboratory
-Connect a voltage across the hydrogen discharge tube
-Clamp a diffraction grating to a retort stand and place it infront of the discharge tube
-Switch off background lights
-Look through the diffraction grating and slowly move to one side
-Distinct coloured lines will be seen
explain how emission line spectrum demonstrate existence of discrete energy levels in isolated atoms
- Each coloured line is a single frequency emitted by the atom
- Energy of a photon is proportional to frequency
- So atoms emit discrete energies when electrons in the atom undergo energy change, implying there must exist discrete energy levels within atoms
explain how absorption line spectrum demonstrate existence of discrete energy levels in isolated atoms
- Each coloured line is a single frequency absorbed by the atom
- Energy of a photon is proportional to frequency
- this implied atom absorb discrete energies when electrons in the atom undergo energy change, implying there must exist discrete energy levels within atoms
