1. Quantum phenomena
AS Physics pp 30-45

2. Photoelectricity
Metals contain conduction electrons free to move around the metal
Hertz discovered that his radio spark gap detector was more efficient if the metal were illuminated with UV
It was discovered that metals emit electrons if the frequency of the incident radiation is high enough.
This the photoelectric effect

3. The puzzle of the photoelectric effect
The photoelectric effect could not be explained by wave theory
There is a threshold frequency below which no photoelectrons are emitted
The number of electrons emitted is proportional to the intensity (provided the frequency is above threshold)
Photoelectric effect is immediate even in the lowest intensity conditions

4. Einstein’s explanation
This was Einstein’ third paper of the year, 1905
He recognised that light was better described as a particle of energy E=hf rather than as a wave
The photon could interact and release all its energy or not and release none.
No wave amplitudes here for energy...
A photoelectron was released if the incident energy of the photon exceeded the work function ( φ ) the minimum energy required to emit the electron
The remainder of any energy from the photon is given to the electron as kinetic energy
Hence

5. The equations
As long as the frequency is above the threshold frequency,

6. More on the photoelectric effect
Energy only available in fixed amounts of energy – the energy of the photon is quantised (in units or quanta of energy)
Waves do not work – this is the new world of Quantum Physics
The photon can only give all or none of its energy
Since there is no option for the energy to be split – the interaction is instantaneous and the electron is either bound or released – no intermediate steps

7. The Photocell
The photocell emits electrons from its cathode when light is incident upon it.
They flow towards to anode creating a current
The presence of a current indicates that photons are incident on the cathode and the rate of photon arrival is proportional to the current

8. Electron interactions with atoms
If an atom loses or gains an electron it becomes an ion – it is charged.
Ionisation is the process by which atoms become ions
Electrons can create ions in a gas when a current flows though it
This is how a fluorescent tube works

9. Measuring ionisation energy
The energy needed to ionise a gas is called the ionisation energy or ionisation potential
We can measure this by increasing the energy of electrons moving in a gas until the electrons are emitted and reach the anode creating a small current
The gas needs to be a low pressure to ensure that the electrons can reach the anode
The pd between the anode and cathode is increased until the current suddenly increases
The electrons in the small current are now moving fast enough and when they collide with the bound electrons they are removed – ionisation occurs
The Ionisation energy = eV

10. The electron volt Unit of energy
Energy = Charge x Voltage (potential difference)
1 Joule = 1 Coulomb x 1 Volt
1 eV = 1 electronic charge x 1 Volt
e = 1.6 x C
Therefore 1eV = 1.6 x J
1J = 1/(1.6 x 10-19)eV = 6.25 x 1018 eV

11. Excitation
If electron energies are not sufficient to release electron – cause ionisation – it is possible for energy to be absorbed – this excites the gas atoms
The energies which are absorbed are distinct energies and are called excitation energies
E.g. For Mercury (Hg) two important excitation energies are 4.9eV and 5.7eV
The electron collides with a confined electron which moves to a higher energy shell
The energy of the colliding electron is reduced

12. Energy levels in atoms
Electrons in atoms are considered to inhabit energy levels with atoms
These can be thought of a bit like a ladder
The bottom rung is called ground state (n=1)
Other rungs represent higher energy states, or excited states (n=2,3,4,...)

13. De-excitation
When an electron in a higher energy state jumps to a lower state it reduces the excitation of the atom and a photon is emitted
The energy of the photon is related to the frequency, E=hf
Since the photon can only have a fixed value of energy, this interaction must be instantaneous
The initial collision can also be considered to be a photon transfer (photon is the exchange particle for the EM interaction)
Energy of emitted photon = E2-E1 where the electron is jumping from energy level E1 to another energy level E2

14. Energy of light...

15. Fluorescence
An excited atom can de-excite directly to ground state or de-excite to another ower excited state and them to ground
A fluorescent tube emits UV light
This excites a phosphor which can de-excite by emitting some visible photons as part of the de-excitation – giving us more visible light
This process is called fluorescence
A fluorescent lamp emits about 10-15% visible light compared to 2-3% for a filament lamp

16. Starting fluorescent lamps
To start a fluorescent tube we need to use a starter
The main voltage is too small so we heat a wire to heat the gas until it will ionise and conduct electricity
Uses a bimetallic strip which shorts the mains and heats to gas until it gets hot enough and the circuit breaks

17. Energy levels and spectra
Continuous spectrum – hot objects
Line spectrum
Emission spectrum
Excited gases
Absorption spectrum – cooler gases

18. Energy and spectra
Energy jumps to n=1 are called Lyman lines – usually are UV
Energy jumps to n=2 are called Balmer lines and are usually visible light
Energy jumps to n=3 are called Paschen lines and are usually IR
With n levels there are n-1 possible jumps
Energy gaps in series are the same – patterns can be matched and from spectra find the energy levels

19. Bohr Atom

20. Discovery of Helium We can use telescopes to collect light from stars
The spectra from stars tell us about the elements in the stars – as well as the pressure and the temperature
Helium (named after the Greek name for the Sun, Helios) was first discovered in the Sun from its spectrum, before it was found on Earth
Pierre Janssen was a French astronomer who discovered helium in He was observing a solar eclipse in India when he noticed the yellow spectral emission lines of the element. An English astronomer by the name of Norman Lockyer observed the same spectra and proposed the name helium after the Greek name for the sun, Helios. Helium can be observed at nanometres in the spectrum of the chromosphere of the Sun.

21. Wave Particle Duality Wave like nature – diffraction
Light travels through a small gap about the same as the wavelength

22. Wave Particle Duality Particle properties Photoelectric effect
Electron beams

23. Wave Particle Duality
Louis de Broglie 1929
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24. Quantum technology PET Scanner Scanning Tunnelling Microscope (STM)
Transmission Electron Microscope (TEM)
Magnetic Resonance Imaging (MRI) also Nuclear magnetic Resonance (NMR)
Superconducting Quantum Interference Devices (SQUIDs)