quantum physics Flashcards
what is the photoelectric effect?
process in which electrons are emitted from a metal surface when electromagnetic radiation of sufficiently high frequency is incident on the surface
what is a photoelectron?
electron emitted from the surface of a material due to the incident electromagnetic radiation
what happens when EM radiation is incident on emitter such that the collector is sufficiently positive?
- every single photoelectron is attracted to C
- rate of emission of photoelectrons = rate at which photoelectrons reach C
- ammeter reads a constant saturated/max photocurrent
what happens when EM radiation is incident on emitter such that the collector is sufficiently negative?
- most energetic photoelectrons do not have sufficient energy to reach C
- photocurrent is zero
define stopping potential
magnitude of the negative potential of collector wrt emitter which prevents the most energetic photoelectrons from reaching the collector, results in zero photocurrent
what are the observations of photoelectric effect?
- saturated photocurrent is proportional to the intensity of the monochromatic EM radiation at a constant frequency
- existence of the threshold frequency (below which no emission of photoelectrons occurs irrespective of the intensity of the EM radiation
- max KE independent of intensity (energies emitted increases linearly with frequency)
- insignificant time lag (almost immediate emissions)
formula for the quantum theory of light
E = hf = hc/lumda
where
E = energy carried in EACH quantum emitted
h = 6.33 * 10^-34
f = frequency
c = 3.0*10^8
define a photon
quantum of electromagnetic energy
define the work function energy of a metal
the minimum amount of energy necessary for an electron to escape from the surface of a material
what is einstein’s photoelectric equation?
photon energy = work function energy + maximum KE of a photoelectron
relate max KE with stopping potential
EK max = eVs
what are the formulas for the particulate nature of light?
lumda = h/p
p = hf/c
how to prove light behaves as a wave?
interference / diffraction of light
how to prove light behaves as particles?
photoelectric effect
electrons behave as particles
electrons undergo collision, have mass and charge
electrons behave as a wave
electron diffraction
key features of the energy level diagram
- ground state = lowest energy level (n=1)
- higher energy levels –> excited state
- n = infinity (0 eV) –> electron escaped from atom
- lower energy –> more negative energy value = more stable states
- ionisation energy from n = 1 –> n = infinity
what is ionisation
process of creating charged particles
what is excitation
process whereby atoms absorb energy without ionisation
what are the two main sources of energy that can cause the electron to be ionised or excited to a higher energy state
- particle collision (high speed particle collides and imparts energy to an atom –> excited electron at a higher energy level **energy of electrons need not match)
- photons (for an orbital electron to gain energy, photon energy must be exactyl equal to energy difference btwn 2 levels in the atom)
explain emission line spectrum
gases (H,Ne) in a discharge tube at low pressure with high voltage applied btwn ends of a tube, gas glows, light emitted from tube.
light examined through a diffraction grating with a spectrometer –> distinct lines observed –> emission line spectrum
gas heated/bombarded by electrons –> electrons in gas excited to higher energy levels
de-excitation –> photons emitted with energy corresponding to energy difference btwn 2 energy levels
energy levels & differences are discrete –> corresponds to series of lines on spectrum
discrete bright lines of definite wavelength on a dark backrground
