Quantum phenomena Flashcards
what is the photoelectric effect
where electrons are emitted from the surface of a metal when photons are incident on it
how do you achieve the photoelectric effect
you could use a gold leaf electroscope, a metal plate attached to a metal pole and a flexible gold leaf
how does a gold leaf electroscope work
the metal in the electroscope is set up to be negatively charged causing the leaf to repel away. when light of a certain frequency was incident, electrons were released causing less repulsion.
when would an electroscope not work
the electroscope is positively charged, (protons are fixed in the metal so they cant be released), harder for the electrons the escape (attracted back). gold leaf will repel slightly but won’t fall back when photons are incident.
what did wave theory predicted would happen when photoelectric effect was replicated
- any frequency should cause the photoelectric effect
-increasing intensity (brightness) increases energy - it should take longer for electrons to be emitted with lower intensity light
what happened in the photoelectric effect (wave vs actually)
-only above a certain threshold frequency were electrons were emitted
- as long as frequency was above threshold the electrons had a consistent energy (more got released)
- electrons were emitted instantly regardless of intensity
conclusions of the photoelectric effect
- energy is delivered in packets, and is proportional to the frequency of the photon (electrons cant store energy)
-one to one interaction between photon and electron
-energy is delivered in one packet in one go
what is the work function
the minimum energy required to release an electron from the metals surface
how to calculate threshold frequency
f0=work function/h
why are electrons released with different speeds
not all electrons are at the metals surface so more energy is needed to release it. that’s why when electrons are emitted with different kinetic energies.
what is the photoelectric graph
x-axis kinetic energy
y-axis frequency
gradient plancks constant
y-intercept work function
what is intensity
the amount of energy arriving per second per unit area
what is a vacuum photocell
measures the amount of energy to stop an electron
photons release photoelectrons (from the smile)
they travel to the ‘eye’
completing the circuit
(happy cyclops)
how does a vacuum photocell work
as voltage increases the ‘eye’ becomes more negative, making it harder for electrons escape
eventually the voltage would be high enough that none cross the gap so the circuit is no longer complete
stopping potential equation
eVs=Ek(max)
what happens when you excite a gas
if you excite a gas they glow
different elements glow with different colours
this could be done by passing a high current through them
what happens with emission of photons from a excited gas
even though we see one colour, its made up of multiple photons which can be split up with a prism
what is an emission spectra
the visible light colour that can be seen (on a black strip)
a continuous spectra is all the visible light (white light)
how to create and absorption spectra
if you take the same gas (de-excited) and shine white light through it then it will create an absorption spectra (opposite of emission spectra)
it blocks the photons that are emitted by the gas
what are energy levels
around atoms, electrons only exist in energy levels (not the same as shells)
they can absorb and emit photons to move up and down energy levels
what is ground state
where electrons have the least amount of energy/ need to receive the most amount to be ionised (leave the atom). they are most stable in ground state
how can electrons gain energy
they can absorb a photon (one to one) of he exact same energy needed to move up a level.
hit with an electron with kinetic energy- only absorbs what it needs
what is ionisation energy
the amount of energy needed to get an electron released from the tom turning it into an ion
what’s the energy levels required in an atom
there are discrete energy levels that are different for each element, the lower down the atom is, the more energy needed to escape
what happens when an electron in ground state is hit with a slow electron
nothing happens as there isn’t enough energy to move up energy levels
what happens when an electron in ground state is hit with fast electron
the exact energy needed to move up a level is transferred to the ground state electron and the incoming electron moves away with the left over energy
what happens when an electron in ground state is hit with a low energy photon
nothing happens, not enough energy to move up and you cant absorb part of a photon
what happens when an electron in ground state is hit with a high energy photon
if its the exact right energy of the energy level it will move up to that level
why do electrons jump down energy levels
they are most stable in ground state
how do electrons jump down energy levels
electrons will emit photons of the exact energy of the energy jump in photons to move down.
they can cascade down levels or move straight to ground state in one jump
why are only certain photons are absorbed in energy levels
absorption spectra are dependant of the photons emitted by the electrons cascading down of certain energies. these being different for different elements
how do fluorescent tubes use energy levels
-Fluorescent tubes are filled with mercury vapour
-a high pd is put through the tube causing fast electrons to pass through.
-free electrons collide with ground state electrons in the mercury atoms, moving them up
-excited electrons cascade down the energy levels releasing UV photons
-tube has fluorescent coating turning UV photons into visible light, using ground state electrons in the coating.
-cascading electrons back down release lower energy photons (visible light) of all colours (white light)
when does light behave like waves
in diffraction- light bends around objects
when does light behave like particles
in the photoelectric effect- packets of light arrive (photons) one to one interaction
what is wave particle duality
light can show wave like behaviour and particle like behaviour depending on the conditions and circumstances
what is the de broglie theory
If waves can behave like particles then particles should be able to behave like waves
how can you prove de Broglie theory
particle diffraction- the greatest diffraction is when the wavelength is similar in size to the gap its being diffracted through
what is the electron diffraction experiment
accelerated electrons are fired past a thin graphite screen causing them to diffract and hit a fluorescent screen.
if they behaved like particles there would be a single spot on the screen however there are rings that come out from the centre spots (constructive/destructive interference)
why is graphite used to diffract the electrons in electron diffraction
the graphite lattice are in similar size to the de broglie wavelength of the electrons
why does electron speed effect the diffraction rings
slow electrons- bigger wavelength- more diffraction- more spaced out rings
