electromagnetic radiation and quantum Flashcards
what is the electron volt?
the kinetic energy carried by an electron after its been accelerated from rest through a pd of 1 volt
what is one eV in Joules?
1.6x10-19 J
what’s the lowest energy level an electron can be in?
the ground state
when an electron is excited, what is its energy level?
higher than the ground state
how do we represent the energy level an electron is in?
n=1 (ground state), n=2, n=3…
can electrons move energy levels?
yes, they can move down energy systems by emitting a photon
and can move up by absorbing a photon with the exact same energy difference as the two levels
what is excitation?
the movement of an electron to a higher energy level.
what is the ionisation energy of an atom?
the amount of energy needed to remove and electron from the ground state atom
how is ionisation energy and energy to change energy levels linked?
they are both the exact same
fluorescent tubes use excitation of electrons and photo emission to produce visible light, how does this occur
they contain mercury vapour, high pd is applied across this
this high pd accelerates fast-moving free electrons that ionise some of the mercury atoms, producing more free electrons
when this flow of free electrons collides with the electrons in mercury atoms, the atomic mercury electrons are excited to the highest energy level
when these excited electrons return to their ground state, they lose energy by emitting high-energy photons in the UV range
the photons emitted have a range of energies and wavelengths that correspond to different transitions of the electrons
a phosphor coating on the inside of the tube absorbs these photons, exciting the electrons to much higher energy levels
electrons then cascade down energy levels and lose energy by emitting many lower energy photons of visible light
what do fluorescent use in order to produce visible light?
excitation of electrons and photon emission
if you split light from a fluorescent tube with a prism or a diffraction grating what will you get?
a line spectrum
what is a line emission spectrum?
a series of bright lines against a black background, each line corresponds to a particular wavelength of light emitted by a source
what does the line emission spectrum provide evidence for?
that electrons in atoms exist in discrete energy levels
why do you only see certain wavelengths in the line emission spectrum?
because only certain photon energies are allowed so you can only see the corresponding wavelengths as atoms can only emit photons with energies the same as the difference between two energy level
what causes a continuous spectra with a line absorption spectra?
white light, if you split the light with a prism, the colours all merge into each other
hot things cause a continuous spectrum in the visible and infrared
why are all wavelengths allowed in the continuous spectra?
because electrons are not confined to energy levels in the object producing the continuous spectrum. the electrons are not bound to atoms and are free
when will you get a line absorption spectra?
when light with a continuous spectrum of energy (white light) passes through a cool gas
what does a line absorption spectra look like and how does this happen?
multicoloured with black lines corresponding to absorbed wavelengths
at low temperatures, most electrons are at ground state
photons of correct wavelength are absorbed by the electrons to excite them to higher energy levels
these wavelengths then become missing from the continuous spectrum when it comes out the other side of the gas
whats an example of light acting as a wave?
the photoelectric effect
what is the photoelectric effect?
if an electron absorbs enough energy from UV radiation then the bonds holding it to the metal can break and the electron can get released (these are called photoelectrons)
what conclusions do you get from the experiment of the photoelectric effect?
- for a certain metal, no photoelectrons are released if the radiation has a frequency below a certain value (threshold frequency)
- the photoelectrons are emitted with a range of kinetic energies. max value increases with frequency
- the intensity of radiation is the energy per second hitting an area of the metal
- the number of photoelectrons emitted per second is proportional to the intensity of the radiation
what model can be used to to explain the photoelectric effect?
photon model- seen that photons of light had one-on-one particle interactions with an electron in a metal surface. each photon would transfer all its energy to one specific electron
experiment to demonstrate the photoelectric effect
attach a zinc plate to the top of an electroscope. the zinc plate is negatively charged so the metal in the box is also negatively charged. metal then repels the gold leaf causing it to move up.
UV light is then shone onto the zinc plate
the energy from the light then causes electrons to be lost from the zinc plate. the zinc plate and metal then lose their negative charge so gold leaf is no longer repelled and goes back down
what is the work function?
the amount of energy needed to break the bond between an electron and surface of a metal
what will happen if the energy is less than the work function?
the electron will shake about a bit then release the energy as another photon. the metal will heat up a bit but no electrons will be emitted
how can you work out threshold frequency?
work function/ Planck constant
ø/h
whats the reason electrons emitted from a metal have a range of kinetic energies?
energy losses
what is stopping potential?
the potential difference needed to stop fast moving electrons travelling with maximum KE
how do you calculate stopping potential?
max KE/ charge of an electron
how has light been shown to act like a wave?
diffraction- when a light beam passes through a narrow gap, it spreads out
how has light been shown to act like a particle?
the results of photoelectric effect experiments can only be explained by thinking light as a series of particle-like photons
how can we provide evidence that electrons show wave-like qualities?
electron diffraction
-diffraction patterns can be shown using a electron diffraction tube
-electrons are accelerated to high velocities in a vacuum and then passed through a graphite crystal
-as they pass through spaces between the atoms of the crystal, they diffract like waves passing through a slit
if wavelength of a wave is greater, what happens to the spread of lines in the diffraction pattern?
the spread increases
in general, what is the wavelength for electrons accelerated in a vacuum tube about the same size as?
electromagnetic waves in the X-ray part of the spectrum
when will a electron beam diffract?
when the de Broglie wavelength is around the same size as the gap between the atoms
what happens to the diffraction pattern if you use particles with a larger mass?
more tightly packed
if you are using an electron microscope and want to resolve tiny data on an image, what wavelength do you need?
shorter
are electron microscopes or light microscopes in more detail?
electron microscopes
