electromagnetic radiation and quantum phenomena Flashcards
What is the photoelectric effect
if radiation of high enough frequency is shone onto the surface of a metal
free electrons absorb energy and vibrate
if enough energy is absorbed bonds holding it to the metal are broken and the electron is released
what are the electrons emitted called
photoelectrons
explain threshold frequency
no photoelectrons are emitted if radiation incident on a certain metal is below a certain frequency
what effect does increasing the frequency of incident radiation have
photoelectrons are emitted with kinetic energies varying up to the maximum
the maximum increases with the frequency
what effect does changing the intensity of incident radiation have on the max kinetic energy of photoelectrons
unaffected.
increasing intensity increases the rate at which photoelectrons are emitted proportionally
what is the intensity of radiation
amount of energy hitting an area of metal per second
why are wave theory and threshold frequency incompatible
wave theory suggest energy supplied by the EM wave should be proportional to the intensity
over time electrons would gain enough energy to be released regardless of the frequency of incident radiation
however, electrons are never released unless the wave is above the threshold frequency
what is a photon
discrete wave packet of EM radiation
how can we demonstrate the photoelectric effect
electroscope
thin metal zinc plate
gold leaf
plate is negatively charges, therefore so is the box
UV light shone onto the plate
electrons are lost from the zinc plate, so negative charge lost. gold leaf falls
energy of a photon
how does this correlate to the electron which collides with this photon
E = hf
electron absorbs this energy
if E > work function, it can be released
any extra energy becomes kinetic energy correlating to the speed at which it is released
what is work function
minimum energy an electron needs tp break the bonds holding it to the metal
dependent on the metal
work function {}
work function = threshold frequency * h
how do you calculate the maximum kinetic energy of a photoelectron
E = hf
hf - work function = ke max
how do you calculate the speed of a photoelectron
calculate the maximum kinetic energy
k.e = 1/2 * m * v^2
m= 9.11 * 10^-31 ( mass of an electron )
rearrange for v
what is stopping potential
potential difference needed to stop the fastest moving electrons ( k.e max)
E = QV
E = k.e max
QV = k.e max
Q = 1.6 * 10^-19 ( 1ev )
v = stopping potential
define electron volt
kinetic energy carried carried by an electron after it has been accelerated from rest through a pd of 1volt
what does it mean to say an electron is excited
when it is in an energy level higher than ground state
what is ground state
the lowest energy level an electron can be in
how can electron move down an energy level
by emitting a photon of specific energy
must be specific since energy levels are fixed
what is excitation
the movement of an electron to a higher energy level
what is ionisation energy
the amount of energy needed to remove an electron from ground state
how do fluorescent tubes work
excitation of electrons to higher energy levels
when they return to their ground state they lose energy by emitting (uv range) high energy photons
phosphor absorbs these photons , more excitation.
they then lose energy again by emitting lower energy photons of visible light
what is line spectra evidence for
evidence for transitions between discrete energy levels in atoms.
atoms can only emit photon with specific energy ( difference between energy levels )
specific energy means specific wavelengths
what is the spectrum of white light like
continuous
all wavelengths appear
since electrons aren’t confined to energy levels and are free
how are line absorption spectra formed
light with continuous spectrum
passes through a cool gas
photons of specific wavelength are absorbed by electrons which are then excited
these wavelengths are then missing (black lines)
what are two experiments which show wave-particle duality of light
diffraction
photoelectric effect
de Broccoli (Broglie) {}
λ = h/mv
describe electron diffraction
electrons accelerated to high velocities in a vacuum then passed through a graphite crystal
they diffract like waves would
produces a pattern of rings
when will an electron beam diffract
when it’s de broglie wavelength is roughly the same size (order) as the gap it’s being sent through
how would the diffraction pattern vary with speed of the electrons
slower electrons -> widely spaced rings
larger wavelength
faster electrons ( higher velocity ) -> shorter wavelength -> closely spaced rings
de broglie wavelength
how would the diffraction pattern vary with the mass of the particle
particles with greater mass they would show a more tightly packed pattern
greater mass -> shorter de broglie wavelength
when is an atom ionised
when an electron is removed
the ionisation energy is equal to the energy required to remove an electron from an atom when it is in ground state
