2. Electromagnetic Radiation and Quantum Phenomena Flashcards
What device was used to achieve the ‘photoelectric’ effect?
gold leaf electroscope
What does a gold leaf electroscope consist of?
a metal plate attached to a rigid metal pole with a flexible piece of gold foil attached
What is the charge of a gold leaf electroscope?
negatively charged
How is a gold leaf electroscope made to be negatively charged?
adding extra electrons
Why is the flexible gold foil repelled from the metal pole?
both are negatively charged
When does the gold foil fall back down?
when light of a certain frequency is shone onto the metal plate
Why does the gold foil fall back down when light of a certain frequency is shone onto the metal plate?
because the light was causing electrons to be released from the metal causing the apparatus to become less negatively charged meaning the force of repulsion weakened
Why doesn’t the experiment work if charged positively?
- positive charges are fixed in the metal and can’t be released like the electrons
- if the metal is positively charged, then the negative electrons are attracted back to the plate and so it is harder for them to escape
What three things did wave theory predict?
- any frequency (colour) of light should cause the photoelectric effect
- increasing the intensity (brightness) of the light should increase the energy of the electrons emitted
- it should take longer for electrons to be emitted when using low intensity light compared to high intensity light
Regarding theory 1, what actually happened?
only above a certain ‘threshold’ frequency were electrons emitted
What was the conclusion made about the 1st theory?
- energy must be delivered in packets (particles), and must be proportional to the frequency of the wave
- electrons can’t store energy
- must be delivered all in one go
Regarding theory 2, what actually happened?
as long as you are above a certain threshold frequency, increasing intensity increases the amount of electrons but their individual energy stayed the same
What was the conclusion made the 2nd theory?
- there is a one-to-one interaction between a photon and an electron
- the greater the intensity the more photons produced, therefore the greater the number of electrons emitted
Regarding theory 3, what actually happened?
the electrons were emitted instantly regardless of the intensity of the light
What was the conclusion made about the 3rd theory?
- energy must be delivered in packets in one go, rather than continuously
- electrons can’t store energy
What is required for an electron to be released from the surface of a metal?
energy
What is the work function?
the minimum amount of energy needed for an electron to escape the surface of a metal
Is the work function the same or different for different metals?
different
What supplies the energy needed for electrons to escape?
photons
What happens when photons have less energy than the work function?
- nothing will happen
- increasing the intensity of the light (producing more photons) will also have no effect
- each electron can only interact with one photon
- they can’t store energy
What happens when photons have the same energy as the work function?
- electrons is released
- but with no kinetic energy
- the frequency of this photon is known as the ‘threshold frequency’
What is the threshold frequency?
the minimum frequency needed for an electron to escape the surface of a metal
What is the equation for threshold frequency?
threshold frequency = work function/Planck’s constant
What happens when photons have more than the energy of the work function?
- electron is released
- it leaves faster as the extra energy is transformed into kinetic energy
- we don’t release extra electrons as it is still only a 1 to 1 interaction
What is the equation for the photoelectric effect?
hf = work function + max kinetic energy
Why are electrons emitted with a range of speeds?
those at deeper levels require more energy to escape
What is intensity?
the amount of energy arriving every second per unit area
How can you cause a gas to glow?
heat it up/excite it
How can you heat up a gas?
by passing a very high current through it
What is this high current made up of?
fast-moving electrons
What is the single colour we see made up of?
multiple photons
How can we see the multiple photons?
by splitting the light either by using a prism or a diffraction grating
What is an emission spectrum?
the photons that get emitted
What does a continuous spectrum consist of?
all the visible wavelengths
What happens when you pass white light through the same gas, but cold (de-excited)?
it blocks certain photons
What are the photons that the gas blocks?
photons of the same wavelength as the ones it emitted
What causes elements to both emit AND absorb very specific photons?
electrons
What can the electrons around an atom only exist at?
certain levels of energy
How do electrons move up and down energy levels?
- they can absorb energy and move up
- they can emit energy and move down
Can electrons exist between energy levels?
no - it is the forbidden zone
What is level 1 known as?
the ground state (n=1)
Where do electrons need to receive the most energy to escape the atom?
level 1/ground state
Where are the electrons most stable?
ground state/level 1
What is (n=∞)
ionisation level
What does the ionisation level represent?
when an electron has left the atom entirely, turning the atom into an ion
How can electrons move up and down energy levels?
by gaining and losing energy
How can an electron gain energy?
- absorb a photon (1 to 1 interaction)
- get hit with an external electron
What is ionisation?
if an electron gets enough energy it can leave the atom entirely
When is an electron excited?
- when it is in an energy level higher than the ground state
- the atom is now an ‘excited’ atom
What is the ionisation energy?
the amount of energy needed to leave the atom from the ground state
When can an electron only absorb a photon?
when the photon gives the electron exactly the right amount of energy to move up one or more whole levels
Technically, how much energy does an electron have when it has left the atom?
zero energy (used it all up to escape)
Do we mark down the energy of each level as positive or negative?
negative
How much energy do we mark down (n=∞) as having?
zero
Is energy gained or lost moving down energy levels?
moving down energy levels loses you energy so you get further from 0 (more negative)
Which element is most used for energy level diagrams in exams?
hydrogen
Why do elections not like being outside of the ground state?
the ground state is where they have the least amount of energy and are the most stable?
How do the electrons return to the ground state?
by emitting energy
How is this energy emitted?
as photons
How can electrons return to the ground state?
either in a single ‘jump’ or they will cascade
What happens when excited electrons ‘de-excite’?
- they will release photons with energy equal to the energy different between levels
- meaning that the exact same photons are emitted that can be absorbed
What do the absorption and emission spectrum equal when combined?
a continuous spectrum
What do fluorescent tubes use energy levels to produce?
white light
How do fluorescent tubes work?
- fluorescent tubes are filled with mercury vapour
- a high p.d. is applied across the tube
- this causes free electrons to rapidly accelerate from one side to the other
- the free electrons collide with the ground state electrons in the mercury, transferring energy
- this excites them to a higher energy level
- the excited electron will return to the ground state, releasing a UV photon, which is not visible to the naked eye
- the reason a high energy UV photon is released is because the energy gaps in mercury are large
- the tube is coated with a fluorescent coating
- this turns the UV photons into visible light
- the UV photons will excite the electrons in the ground state of the coating to a higher energy level
- the electron cascades back down to the ground state, releasing lower energy photons which are in the visible part of the spectrum, appeasing white to our eyes
- the energy gaps are smaller so the photons emitted have a lower energy
What is the photoelectric effect evidence of?
light behaving as a particle
What are the particles of light called?
photons (packets of waves)
What is diffraction an example of?
light acting like a wave
What happens when light passes by an object?
the light ‘bends’ around the object
What is wave particle duality?
the fact that light shows particles like behaviour in certain scenarios and wave like behaviour in others
What did scientist Louis De Broglie suggest?
that if waves can behave like particles, then surely particles can behave like waves
What is the equation De Broglie developed to allow you to work out the wavelength of a particle?
λ = h/mv = h/p
How can you prove that De Broglie is correct?
by observing a particle doing something that only a wave can do - diffraction
When do waves show the greatest amount of diffraction?
when the wavelength is similar in size to the gap/object they diffract around
When can an electron be shown to diffract?
when going at a great enough speed
How is an electron made to travel at great enough speeds to diffract?
- accelerating electrons using an electron gun
- then passing them through a thin graphite screen
- graphite has very regular spacings between atoms - these are similar in size to the De Broglie wavelength of the electrons
- the electrons impact a fluorescent screen; where they shit, the screen will glow
What would you expect to see if electrons behaved only like particles?
a single point of green light on the screen
What do you see instead?
a series of rings
What are these rings caused by?
electrons diffracting through the graphite
What are the bright spots and dark rings caused by?
the electrons constructively and destructively interfering
What happens when the electrons are slow?
- slow electrons = bigger wavelength
- bigger wavelength = more diffraction
- more diffraction = more spaced out rings
What happens when the electrons are fast?
- fast electrons = smaller diffraction
- smaller wavelength = less diffraction
- less diffraction = less spaced out rings
When does light display particle-like behaviour? What is this evidence of?
- photoelectric effect
- existence of threshold frequency
When does light display wave-like behaviour? What is this evidence of?
- diffraction
- interference pattern seen
When do electrons display particle-like behaviour? What is this evidence of?
- deflection in a magnetic or electric field
- curved track of electron
When do electrons display wave-like behaviour? What is this evidence of?
- electron diffraction
- interference pattern seen
