What is photon energy equal to?
E = HF, or E = HC / wavelength
What happens when light is incident on a metal surface?
An electron absorbs a single photon from the incident light, equal to hf.
What is required for photoelectric effect to occur? What is the threshold frequency and what does it depend on?
Incident light frequency has to be greater than the threshold frequency. This is minimum frequency at which the photon has energy equal to the work function of the metal. This depends on type of metal being used, not the light source.
What is the threshold frequency equation?
fmin = work function / planks constant. Equate e = hf to work function to work it out.
When can an electron leave the surface of a metal?
If the energy it gains from a single photon exceeds the work function.
Define work function
The minimum energy needed by an electron to escape from a metal surface.
What happens to excess energy gained by the photoelectron? What is the equation used to find this value?
It becomes the photon’s kinetic energy. Ekmax = hf - work function. Equate e = hf to work function plus KE.
What is proportional to the intensity of the light?
The number of electrons emitted per second.
What is proportional to the frequency of the light?
When the frequency of the light increases, the maximum K.E of the photo-electrons increases.
Why is it a max kinetic energy?
Energy gains energy equal to hf. Some electrons deeper in metal need more energy to leave the metal, and therefore will have less kinetic energy when out of the metal. Electrons closer to surface require less energy to get out, and therefore have more kinetic energy in the end.
What is stopping potential?
The minimum potential required to stop photoelectric emission. Equate E = 1/2mv^2 to QV.
How do electrons get attracted back to the plate?
By giving the plate a sufficient positive charge.
What occurs at the stopping potential?
The max kinetic energy of the emitted electron s reduced to zero.
What is ionisation?
The process of creating ions
What is excitation?
Gas atoms absorbing energy from colliding electrons or by absorbing photons without being ionised.
What are excitation energies?
The energy values at which an atom absorbs energy.
What occurs during excitation?
A colliding electron makes an electron inside the atom move from an inner shell to an outer shell.
Why is energy needed for the process of excitation?
The atomic electron moves away from the nucleus of the atom
Why is excitation energy always less than the ionisation energy?
Because atomic electron isn’t completely removed from the atom when excitation occurs.
What is the ground state?
The lowest energy state of an atom.
What is the excited state?
When an atom in its ground state absorbs energy, one electron moves to a shell of higher energy, so atom is now in its excited state.
Why do atoms de-excite?
Atoms don’t retain the absorbed energy permanently. The electronic configuration is unstable in an excited atom, as there is a vacancy in the shell it leaves from.
What happens during de-excitation?
An electron from an outer shell transfers to a lower energy level and the electron emits a photon
What is the energy of the photon equal to when released?
The energy lost by the electron and therefore by the atom.
When can an atom de-excite via several energy levels?
If there are intermediate energy levels present.
What is the energy of the emitted photon equal to?
HF = E1 - E2, when an electron moves from energy level E1 to a lower energy level E2
When can excitation by photons occur?
Electron in an atom can absorb a photon and move to an outer shell where vacancy exists, only if the energy of the photon is exactly equal to the gain in the electron’s energy.
What is fluorescent tube?
A glass tube with fluorescent coating on the inner surface. The tube contains mercury vapour at low pressure. When the tube is on it emits visible light.
Explain how the fluorescent tube emits visible light?
Mercury atoms collide with each other and with electrons, so ionisation and excitation occurs. Mercury atoms emit UV photons, and visible photons when they de-excite. The UV photons are absorbed by atoms of the fluorescent coating, causing excitation of atoms. The coating atoms then de-excite in steps and emit visible photons.
How is the wave like nature of light observed?
During the diffraction of light. Light passes through a narrow slit and spreads out.
How is the particle like nature of light observed?
During the photoelectric effect.
What is evidence for the particle like nature of light?
Electrons are deflected in an electric field.
What is the de broglie wavelength?
wavelength = planck’s constant / momentum (MV)
What is evidence for De Broglie’s theory that matter particles also have a wave - like nature?
It’s demonstrated by a beam of electrons being diffracted. A beam of electrons is aimed at a sheet of metal foil. the rows of atoms on metal foil cause the electrons in the beam to be diffracted. Electrons then form a pattern of rings on a fluorescent screen, and diffraction pattern is seen. Constructive interference occurs where rings are seen, and destructive interference at the gaps.
How is the beam of electrons produced in electron diffraction ?
By attracting electrons from a heated filament wire to a positively charged metal plate, which has a small hole at its centre. Electrons which pass through the hole form a beam.
How can the speed of the electrons be increased in the electron diffraction process?
By increasing the p.d between the filament and the metal plate.
How does increasing the speed of the electrons in electron diffraction affect the size of the diffraction rings?
It makes the rings smaller, because the increase in speed makes the De broglie’s wavelength smaller, so less diffraction occurs.
What is the photoelectric effect?
The process by which a metal with photons incident on it emits electrons (photoelectrons).
What do emission and absorption spectra tell us?
They are evidence for discrete energy levels within an atom. In an emission spectra you can see the frequency of photons that certain elements emit. In absoprtion spectra you can see what frequency of photons certain elements absorb. These correlate to energy levels within the atom. These frequencies proportional to energy since E = HF for a single photon. Emission spectra appears as bright lines whereas absorption spectra appear as black lines.