Electromagnetic radiation and quantum phenomena Flashcards
what is the photoelectric effect
is where the photoelctrons are emitted from the surface of a metal after light above a certain frequency is shone on it
what is threshold frequency
the minimum amount of energy needed to emit electrons from the surface if a metal
why cant threshold frequency be explained by wave theory
as it suggests that any frequency of light should be able to cause photoelectric emission as the energy absorbed by each electron will gradually increase with each incoming wave
what could threshold frequency be explained by
by the photon model of light
what did the photon model of light suggest
- that EM waves travel in discrete packets called photons which have an energy which is directly proportional to the frequency
- each electron can be absorbed by a single photon therefore a photoelectron is only emitted if the frequency is above the threshold frequency
- if the intensity of light is increased, if the frequency is above the threshold frequency, more photoelectrons are emitted per second
what is the work function
is the minimum energy required for electrons to be emitted from the surface of a metal
what is the stopping potential
is the potential difference you would need to apply across the metal to stop the photoelectrons with the max kinetic energy
what can you find by measuring the stopping potential
you can find the max kinetic energy
where can electrons only exist in an atom
in discrete energy levels
what is excitation
when an electron gains enough energy (from a collision with an electron) to move to a higher energy level
what is ionisation
when an electron gain enough energy to leave the atom all together
when does ionisation occur
when the energy of the free electron is greater than the ionisation energy
what is the original energy level of an atom called
the ground state
when an electron becomes excited it will quickly return to its original energy level what does it release the energy that it gained in the form of
a photon
what is an example of a practical use of excitation
a fluorescent lamp in order to produce light
how does a fluorescent lamp produce light
- a voltage is applied across it which accelerates the free electrons through the tube, which collide with the mercury atoms causing them to become ionized, releasing more free electrons
- the free electrons collide with the mercury atoms, causing them to become excited
- when they deexcite they release photons, most of which are in the UV range
-the (phosphorous) fluorescent coating on the inside of the tube, absorbs these UV photons and therefore electrons in the atoms of the coating become excited and de-excite releasing photons of visible light
what unit is used to describe the energy difference between energy levels
eV (electron volts)
what is an electron volt
is defined as the energy gained by one electron when passing through a potential difference of 1 volt
how many joules in 1 eV equal too
1.6x10^-19
what do you get by passing the light from a fluorescent tube through a diffraction grating or prism
a line spectrum
what will each line in the spectrum represent
a different wavelength of light emitted by the tube
as this spectrum is not continuous what does it contain
only discrete values of wavelength
what will correspond to the wavelengths shown on the spectrum and what does this show evidence for
the photon energies emitted, which show evidence that electrons in atoms can only transition between discrete energy levels
what type of spectrum do you get by passing white light through a cooled gas
a line absorption spectrum
what does a line absorption spectrum look like
looks like a continuous spectrum of all possible wavelengths with black lines at certain wavelengths
what do the black lines represent on a line absorption spectrum
the possible differences in energy levels as the atoms in the gas can only absorb photons of an energy equal to the exact difference between two energy levels
what is the difference between two energy levels equal to
the photon energy emitted by a fluorescent tube or absorbed in a line absorption spectrum
what properties can light be shown as having
wave and particle properties
what are the examples of light acting as wave
diffraction and interference
what is the example of light acting as a particle
the photoelectric effect
what else csan be observed as having wave and particle properties
an electron
how can the wave nature of electrons be observed
through electron diffraction
what was De Broglies hypothesis
that if light was shown to have particle properties then particles should have wave like properties
what is De Broglies equation
lambda = h/(mv)
what can you tell from using De Broglies equation
the amount if diffraction changes as a particles momentum changes
if the momentum is increased what happens to the wavelength and diffraction
wavelength will decrease and diffraction will decrease so the concentric rings of the interference pattern become closer
why does knowledge and understanding of any scientific concept change overtime
because more experimental evidence is gathered by the scientific community
how can experimental evidence become accepted
after it is published and peer reviewed by the community to become validated and then it is eventually accepted
