Photoelectricity is the emissions of electrons from a metal surface when it is illuminated by a certain frequency which is greater than the minimum threshold frequency
1) Photoelectric emission of electrons from a metal surface does not take place if the frequency of the incident electromagnetic radiation is below a certain value known as the threshold frequency. This minimum frequency depends on the type of metal.
2) The wavelength of the incident light must be less than a maximum value equal to the speed of light divided by the threshold frequency
3) The number of electrons emitted per second is proportional to the intensity of the incident radiation.
4) Photoelectric emission occurs as soon as the incident radiation is directed at the surface.
Einstein's theory of photoelectricity.
The Photon Theory
Einstein assumed that light is composed of photons
energy of a photon = hf
When light is incident on a metal surface, an electron at the surface absorbs a single photon from the incident light and therefore gains energy equal to hf.
An electron can leave the metal surface if the energy gained from a single photon exceeds the work function of the metal.
Equation for the threshold frequency of the metal
More about photoelectricity
Quantum - means a quantity of energy proportional in magnitude to the frequency of the radiation it represents
The work function oios characteristic of the metal
The threshold frequency related the work function to the incident radiation.
An ion is a charged atom. The number of electrons is not equal to the number of protons. An ion if formed from an uncharged atom by adding or removing electrons to make it positive or negative ions.
Any process of creating ions is called ionisation.
To measure the energy needed to ionise a gas atom we make the electrons collide at increasing speed with the gas atoms. The electrons are emitted from a heated filament in the tube and are attracted to a positive metal plate at the other end of the tube. The gas needs to be at low pressure otherwise there are too many atoms in the tube.
The Electron Volt
The electron volt is a unit of energy equal to the work done when an electron is moved through a pd of 1 volt. The work done when an electron moves through a potential difference of 1 volt is equal to 1.6 x10^-19 J. This amount of energy is defined as 1 electron volt eV.
The work done on an electron when it moves through a pd of 1000V= 1000eV
The work done on an ion of charge +2e when it moves through a pd of 10 V= 20 eV
Excitation by Collision
The excitation of an atom is the process in which an atom absorbs energy without being ionised by colliding with an electron. This happens at certain energies which are characteristic of the atoms of the gas.
If a colliding electron loses all its kinetic energy when it causes excitation, the current through the gas is reduced. If the colliding electron does not have enough kinetic energy to cause excitation, it is deflected by the atom.
Inside an excited atom, an electron moves from an inner shell to an outer shell.
When excitation occurs, the colliding electron makes an electron inside the atom move from an inner shell to an outer shell. 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
Electrons in atoms
Each type of atom has a certain number of electrons.
The lowest energy state of an atom is called its ground state. When an atom in the ground state absorbs energy, one of its electrons moves to a shell of higher energy so that the atom is now in an excited state.
The excitation energy measurements can be used to construct an energy level diagram for the atom.
De-excitation and excitation using photons
De-excitation: this is the process when an atom loses energy by photon emission, as a result of an electron inside the shell moving from an outer shell to an inner shell.
An atom in an excited state may de-excite to the ground state indirectly.
In general when an electron moves from energy level E1 to a lower energy level E2, the energy of the emitted photon hf = E1 - E2.
An electron in an atom can absorb a photon and move to an outer shell where a vacancy exists but only if the energy of the photon is exactly equal to the gain in the electron's energy.
So the photon energy must be equal to the difference between the final and initial energy levels of the atom.
The overall process of de-excitation can explain why certain substances fluoresce with visible light when they absorb UV radiation. Atoms in the substance absorb UV photons and become excited. When they de-excite they emit visible photons.
The fluorescent tube is a glass tube with a fluorescent coating on its inner surface. The tube contains mercury vapour at low pressure. When the tube is on it emits visible light because:
1) ionasation and excitation of the mercury atoms occur as they collide with each other and the electrons
2) the mercury atoms emit uv photons
3) the uv photons are absorbed by the atoms of the fluorescent coating causing excitation
4) the coating atoms de-excite and emit visible photons
A line spectrum is produced by using a tube of glowing gas as a light source. It produces a spectrum of discrete lines of different colours. The lines in the spectrum are due to energy levels.
The energy of the emitted photon hf = E1- E2
Wave Particle Duality
Photons have dual nature as they have a wave like nature and a particle like nature as well.
The wave like nature is observed when diffraction of light takes place. This happens when light passes through a narrow slit. The narrower the gap the greater the diffraction
The particle like nature is observed eg in the photoelectric effect.
De Broglie's Equations
The wave like behaviour of a matter particle is characterised by a wavelength, the de broglie wavelength, which is related to the momentum of the particle by means of
Since the momentum of a particle is defined by mass x velocity