Unit 1: Quantum Phenomena Flashcards
When does photoelectric emission not take place?
When the frequency of the incident electromagnetic radiation is below a certain value known as the threshold frequency and the wavelength of the incident light must be less than a maximum value equal to the speed of light divided by the threshold frequency.
What is known as the photoelectric effect?
When electrons are emitted from the surface of a metal as electromagnetic radiation above a certain frequency is directed at the metal.
What does threshold frequency depend on?
The type of metal.
What doesn’t matter if the frequency of the incident radiation is smaller than the threshold frequency?
The intensity of the radiation.
What is proportional to the intensity of the incident radiation?
The number of electrons emitted per second.
How fast does the photoelectric prevail?
As soon as the incident radiation is directed at the metal (without delay) no matter how weak the intensity.
What can the wave theory of light not explain?
The existence of the threshold frequency or why the photoelectric effect occurs without delay.
What did wave theory incorrectly predict about the photoelectric effect?
Emission should take place with waves at any frequency and emission would take longer when using low intensity waves.
What is light composed of?
Photons
What is the formula for the energy of a photon?
Hf
What is the equation for wavelength?
C/f
What happens when light shines onto a metal surface?
An electron at the surface absorbs a single photon from the incident light and therefore gains energy equal to hf.
When can an electron leave a metal surface?
If the energy gained from a single photon exceeds the work function of the metal.
What is the work function?
The minimum energy needed by an electron to escape from the metal surface.
What is the formula for the maximum kinetic energy of an emitted electron?
Ekmax = hf - work function
What is the formula for the threshold frequency of a metal?
Fmin = work function/h
The energy of each vibrating atom is…
Quantised meaning only certain levels of energy are allowed.
What happens when the energy of the photon exceeds the work function?
The conduction electron can leave the metal.
What happens if the electron does not leave the metal?
It collided repeatedly with other electrons and positive ions and it quickly loses its extra kinetic energy.
What happens when light is directed at the photocathode?
Electrons are emitted from the cathode to the anode.
What is proportional to the number of electrons per second that transfer from the cathode to the anode?
The reading on the ammeter.
What is the equation for the number of photoelectrons that transfer from the cathode to the anode?
I/e where I is the photoelectric current and e is the charge of an electron.
What is the photoelectric current proportional to?
The intensity of the light incident on the cathode.
What is light intensity a measure of?
The energy per second carried by the incident light.
What doesn’t the intensity of the incident light affect?
The maximum kinetic energy of a photoelectron.
How is Ekmax = hf - work function similar to y=mx+c
Y=Ekmax, h=gradient, x-intercept=threshold frequency and y intercept=-work function
What is the process of creating ions (charged atoms) called?
Ionisation
What is the equation for the ionisation energy of a gas atom?
eV
What is the electron volt?
A unit of energy equal to the work done when an electron is moved through a pd of 1 volt.
What is excitation?
This is when atoms can absorb energy from colliding electrons without being ionised.
What do the certain energies that excitation can occur at link to?
The atoms of the gas.
What happens if a colliding electron loses all its kinetic energy when it causes excitation?
The current due to the flow of electrons through the gas is reduced.
What happens if the colliding electron does not have enough kinetic energy to cause excitation?
It is deflected by the atom with no overall loss in kinetic energy.
What are excitation energies?
The energy values at which an atom absorbs energy.
What happens when excitation occurs?
The colliding electron makes an electron inside the atom move from an inner shell to an outer shell.
What is the excitation energy always less than?
The ionisation energy
Is the energy of an electron in a shell constant?
Yes
Which electrons have the most energy?
Those that are furthest from the nucleus.
What is the ground state?
The lowest energy state of an atom.
What is the excited state?
When an atom in the ground state absorbs energy and one of its electrons moves to a shell of higher energy.
Why is the electron configuration in an excited atom unstable?
An electron that moves to an outer shell leaves a vacancy in the shell it moved from.
What happens when an electron emits a photon?
An electron moves to an inner shell to fill in a vacancy. The atom therefore moves to a lower energy level.
What is the energy of a photon equal to in excitation?
The energy lost by the electron and therefore the atom.
What is the equation for the energy of an emitted photon?
E1-E2 (E1=higher energy level)
What is the condition for an electron to absorb a photon and move to an outer shell?
The energy of the photon must exactly equal the gain in the electron’s energy.
Why does a fluorescent tube emit visible light?
Ionisation and excitation of the Mercury atoms occurs as they collide with each other and with electrons in the tube. The Mercury atoms emit UV photons when they de-excite, the UV atoms are absorbed by atoms of the fluorescent coating which causes excitation of the atoms and then the coating atoms de-excite and emit visible photons.
Is a fluorescent tube more efficient than a filament lamp?
Yes
Are wavelengths of the lines of a line spectrum of an element characteristic to the atoms of that element?
Yes
Why do elements not produce the same pattern of light wavelengths?
The energy levels of each type of atom are unique to that atom.
What is proof for the wave-like nature of light?
When diffraction of light takes place.
What causes greater diffraction?
A narrower gap or a longer wavelength.
When is the particle-like nature of light observed?
The photoelectric effect.
Do matter particles have a dual wave-particle nature?
Yes
What evidence is there that matter has particle-like properties?
Electrons in a beam can be deflected by a magnetic field.
What evidence is there for matter having wave-like behaviour?
The de Broglie wavelength and the diffraction of electrons.
What is the equation for the de Broglie wavelength?
H/p where p is momentum
