Chapter 3 - Quantum Phenomena Flashcards
What is threshold frequency?
The minimum frequency to overcome work function
Why is the photoelectric effect not observed below the threshold frequency?
If the frequency is below the threshold frequency, it doesn’t supply the electrons on the surface of the metal enough energy to break their bonds and emit a photon
What is meant by the wave-particle duality of electromagnetic radiation
The electromagnetic spectrum can be waves or particles, shown by the way they can refract but also b radiation
Which aspect of the wave-particle duality of electromagnetic radiation is shown by the photoelectric effect?-
Particle
During the photoelectric effect, if the frequency is maintained but the intensity is doubled, what happens to the number of electrons released per second and maximum kinetic energy?
Electrons released per second = doubles
Maximum kinetic energy = stays the same
Wave particle duality
- Sometimes matter will behave as a wave and sometimes as a particle
- We can calculate the warelength of any particle using the de Broglie wavelength
= h/mv
v = velocity
m = mass
h = Planck’s constant
Examples of wave properties in wave particle duality
Refraction
Reflection
Diffraction
Interference
Examples of particle properties in wave particle duality
Photoelectric effect
Momentum
Mass
Energy levels (absorption, emission)
Alpha, beta radiation
What is the photoelectric effect?
The photoelectric effect is the emission of electrons from a metal surface when light is incident on the surface. Emission only takes place if the frequency of the incident light is greater than a minimum value, which is known as the threshold frequency.
Light is composed in packers of energy called photons. Einstein explained this by assuming a single electron in the metal, or near its surface, absorbs a photon and gains sufficient energy to escape from the metal.
Energy of a photon = hf
There is a minimum amount of energy, the work function, of the metal needed by an electron to escape from the metal surface
Equation for the maximum kinetic energy of a photoelectron
Maximum kinetic energy = (planck’s constant x frequency) - work function
What happens during photoelectric emission?
The photon arrives at the metal surface
The photon is absorbed
The electron gains energy
The electron uses energy to reach the surface
The electron does work to leave the metal surface
Ionisation
An ion is a charged atom. This means the numbers of electrons and protons is not equal
A positively charged ion is created when electrons are removed and the opposite for a negatively charged ion
Any process creating ions is called ionisation
For example:
- Alpha, beta, and gamma radiation create ions when they pass through substances and collide with the atoms of the substance
- Electrons passing through a fluorescent tube create ions when they collide with the atoms of the gas or vapour in the table
- Absorption of electromagnetic radiation
- Heating them
Excitation
Gas atoms can absorb energy from colliding electrons without being ionised. This process is called excitation and happens a certain energies relating to the atoms of the gas.
When excitation occurs, the colliding electron makes an electron inside the atom move from an inner shell to an outer shell. Energy is needed for this process (moving an atomic electron to a higher energy level around the atom). The excitation energy is always less than the ionisation energy of the atom, because the atomic electron is not removed completely from the atom when excitation occurs
An electron will only move to an outer electron shell if it absorbs the exact energy
Energy is needed to move the electron to an outer shell because it is moving away from the nucleus
Can happen by:
- absorbing some or all of the kinetic energy of another electron (collision)
- absorbing all the energy from an incident photon
Work function
Minimum energy of the incident photon to cause a photoelectron to be released from the metal surface
Threshold frequency
Minimum frequency of the incident photon to cause a photoelectron to be released from the metal surface
Photoelectric effect - conservation of energy
One photon can release one photoelectron
The energy from the incident photon will be used to release the photoelectron from the surface, any extra energy will become the kinetic energy of the photoelectron
Kinetic energy equation
Kinetic energy (max) = 1/2 x mv^2
hf = work function + kinetic energy
Energy of incident photon = energy needed to release an electron from the metal + kinetic energy of the released electron
More about photoelectric effect
Increasing the intensity (brightness)
- increases number of photons emitted by the source each second
- increases the number of photoelectrons emitted from the surface (if above threshold frequency)
Kinetic energy is proportional to the frequency of the incident radiation
- above threshold frequency
- conservation of energy
Stopping potential
- positive potential difference (voltage) of the surface, at which photoelectrons are no longer released
- depends on the surface and frequency of incident radiation
What are the differences in the photoelectric effect depending on if the incident radiation is a wave or particle?
Wave:
- photo electrons released at all frequencies
- minimum intensity needed
Particle:
- photoelectrons released when the frequency is above a minimum value
- number of photoelectrons released is proportional to the intensity
Energy levels in atoms
The lowest energy level is called the ground state, and is labelled n = 1
All atoms of the same element have the same energy level diagram
De-excitation
- An excited atom is unstable
- there is now a vacancy in a lower energy electron shell
- this will be “filled” by an electron moving down from an outer shell
- a photon with energy exactly matching the energy difference between levels will be emitted
Fluoresence
- contains mercury vapour
- mercury coms are excited by collisions with electrons
- absorb some of their kinetic energy
- equal to energy difference between levels
- energy level spacing leads to emission of ultraviolet photons as mercury de excites
- these uv photons are absorbed by the phosphor coating
- this excites the phosphor atoms
- phosphor emits visible light photons as it de excites
