Year 12 Physics - Wave Particle Duality and Quantum Theory

describe Young’s Double Slit Experiment

monochomatic light shone through a double slit shows diffraction

bright lines are constructive interfernce and dark lines are destructive interference

diffraction is evidence for the wave nature of photons

describe the photoelectric effect

shining light on a metal plate releases electrons

increasing light intensity will increase electrons emitted

there is a threshold frequency of the incident light below which, no electrons are emitted despite increasing intensity and time applied

the existence of a threshold frequency is evidence for the particle nature of photons

if light was a wave, how would it behave in the photoelectric effect

increasing the intensity would increase the light energy, and given time, the electrons would gain enough energy to be removed

what is the work function representative of

the amount of energy needed to only break the bonds hodling an electron so it can be removed

what is threshold frequency

the minimum frequency of the light, to remove an electron

if the photon has less energy than the work function then the electron won’t be released

when do electrons transfer energy

when transferring between energy levels

what is excitation

when an electrons gains an exact amount of energy to transition up a level

what is de-excitation

when an electron emits an exact amount of energy as a photon to transition down an energy level

what is ionisation

when an electron gains enough energy to be removed from orbit around the atom

any excess energy is kinetic

describe line spectra production

when an electron transitions down, it releases energy as a photon which corresponds to a wavelength of light which can sometimes fall in the visible spectrum

if we diffract the light emitted by an atom, we get a spectrum of bright spectral lines unique to that element

it complements an absorption spectrum

describe the stopping voltage

a positive plate attracts electrons regardless of kinetic energy, but a negative plate will repel

therefore only electrons with sufficent energy will reach a negative plate

the minimum voltage at which no electrons reach a negative plate is the stopping voltage

descrieb what the features on a voltage/frequency graph represent

y intercept → work function

x intercept → threshold frequency

gradient → plancks constant

what did DeBroglie suggest

that matter can have wave like properties if it has momentum

when are DeBroglie wavelengths significant

at atomic level

at a larger scale mass becomes larger and hence the wavelength becomes nearly negligible

what can absorption spectra reveal

star density → thickness of lines

motion of star → shift of lines to lower or higher frequency

what evidence is there that electrons behave like waves

electron diffraction

The double slit experiment fired electrons at two slits and a diffraction pattern was observed, which is only possible if they are waves

how can the wave nature of electrons be described using quantised energy levels

electrons act as a wave

hence orbitals are standing waves around the nucleus, based on factors of wavelengths

therefore energy levels of orbitals are quantised

why are absorption spectra formed

becuase when a continuous spectrum is shone through an atmosphere, the elements will absorb specific wavelengths to excite electrons and thus these wavelengths are removed from the spectrum

describe a blackbody

an ideal absorber which absorbs all incident radiation

it is in equilibrium with its surroundings, radiating and absorbing energy at the same rate, hence maintaining a constant temperature

how do incandescent globes produce light

a filament is heated to a very high temperature and the thermal motion of free electrons produces electromagnetic radiation

a continuous spectrum results

what does LED stand for

light emitting diode

describe an LED

a semiconductive device which uses the excitation of electrons to produce light

electrons move from the valence band to the conductive band when supplied with a voltage, across the band and move as an electric current before dropping back down and releasing photons

what does laser stand for

light amplification by stimulated emission of radiation

describe how a laser works

gas atoms need to be in an excited state, and then there must be mirrors on each side of the gas chamber

one mirror should be partially silvered to allow light to escape

some of the light emitted during de-excitation will reflect back into the gase chamber and stimulate further emission of the same wavelength. this process then repeats

all the waves are also in phase

describe a synchotron

a machine which uses electric fields and magents to accelerate charged particles

when electrons are accelerated they release electromagnetic radiation

how can you find maximum kinetic energy when given voltage

W = qV =Ek (max)

how do you find the work function

subtract maximum kinetic energy from photon energy

how do you find threshold frequency

equate the work function to the energy of the photon and solve for the frequency