Quantum Flashcards
Photoelectric effect definition
Phenomenon where electrons are emitted from a metal surface where electromagnetic radiation of sufficiently high frequency is incident on
Particulate nature of EM
Em radiation exists as discrete quanta of energy known as photons.
(Photons are discrete bundles of EM energy)
Energy of single photon
E=hf
Photon energy only dependent on frequency
Intensity just gives no. of photons per unit time
I=P/A = Etotal/tA = NE/tA = Nhf/tA
Energy of single photon
E=hf
Photon energy onl y
Photoelectric equation
hf = φ + Ekmax
What does intensity increase
Rate of incident photons thus rate of electrons emitted
Describe how the photoelectric effect provide evidence for the particulate nature of electromagnetic radiation.
- The existence of a threshold frequency below which no photoelectrons are emitted proves that EM radiation consists of discrete quanta energy given by hf.
- The instantaneous emission of photoelectrons when all the photon energy is transferred immediately to the electron gives evidence of particulate nature of EM.
- The maximum kinetic energy of photoelectrons being dependent only on frequency of radiation f, which relates to the discrete energy of photon, and independent on the intensity of radiation gives evidence for the particulate nature of EM.
Stopping potential setup
When metal E is illuminated with an EM radiation of sufficiently high frequency, electrons are emitted from E.
Adjust the potential difference between emitter E and colector C such that potential of C is held negative with respect to E, by reversing the polarity of the voltage source.
Adjust the variable voltage source slowly such that the negative potential is made more negative just until no electron can reach C indicated by 0 photocurrrent on ammeter
This is stopping potential where even the minimum negative potential will stop the most energetic electron from reaching C
All the KE of fastest electrons will be converted to electric PE just before reaching C
Stopping potential
Minimum retarding potential to stop all the emitted electrons from reaching the collector plate
Ekmax =eVs
Threshold frequency
Minimum frequency of the incident radiation for electron to escape
f = φ/h
Work function energy
Minimum amount of energy required to remove an electron from surface of material
Criteria for emission of electrons
Frequency greater of equal to threshold frequency
For observable wave nature..
The de broglie wavelength should not be too small ( ~10^-15). Given value of h is small, mass of particle must also be very small for wavelength to be noticable.
Evidence that light behaves like a wave
Interference/ diffraction of light
eg
youngs double slit
single slit: diffraction grating
Polarisation of light
Evidence that light behave as particles
Photoelectric effect
Electrons behave as particles
Electrons undergo collision, has mass and charge
Electrons behave as waves
Electron diffraction
Evidence for existence of discrete energy levels in atom
Line spectra
Why do energy levels have negative values?
The energy level of an electron in the orbit of an atom is negative as the electrons require energy in order to be removed from the attraction of the nucleus. Showing that an electron is bound to nucleus.
When the electron gain energy and transit to an higher orbit, the energy level of the electron tend to zero when it reaches highest energy level of n= ∞. The position correspond to not being bound to nucleus.
Or
electrons and nucleus forms a bound system, where force between electrons and nucleus is attractive.
Given potential at infinity is negative, energy levels closer to nucleus will be lower and negative.
What is an emission line spectrum
Emission line spectrum consist of discrete bright coloured lines in a dark background
Observed when a gas at low pressure is made to glow by heating or an electric discharge(applying high voltage)
Describe how the line spectra can be explained using the idea of discrete electron energy levels in isolated atoms
- Each line correspond to a specific photon energy
- Photon is emitted when electron makes a transition from high to low E level
- Since photon energy is of a specific value, it implies thst energy change between E levels are of a discrete amount.
- Discrete energy level changes implies that energy levels are discrete.
How is an emission line spectrum produced?
- Gas such as H or Ne can be placed in a discharge tube at low pressure.
- A voltage is applied between the metal electrodes in the tube large enough to produce an electric current in the gas
- The gas atoms become excited by the collsions with the electrons passing through the tube from cathode to anode of tube.
- The excited gas atoms are unstable and transits to a lower energy level.
- Since atoms have deicrete energy levels, inly specific high to low energy level transitions are possible, thus photons emitted are of specific energies and specific frequencies
- Coloured lines present in tube correspond to these photons of specific frequencies
Absorption line spectrum
Dark lines against a continuous spectrum of white light
How is an absorption line spectrum produced?
- Produced when white light passes through cold gas
- Gas atoms are excited to higher energy levels when they absorb photons from white light
- Since atoms have discrete energy levels, only specific low to high energy level transitions are possible. Thus photons absorbed are of specific energies and specific frequencies.
- When atoms transit back to ground state, photons of same frquencies are reradiated in all directions
- The part of absorption spectrum that correspond to these frequencies appear as dark lines in comparison to the non absorbed frequencies.
