quantum phenomena

describe the photoelectric effect

when UV light is shone on a metal, electrons are emitted

condition for photoelectric effect

frequency of light must exceed threshold frequency

is there a time-lag for the photoelectric effect?

no, electrons are immediately emitted

effect of increasing intensity on the photoelectric effect

more photoelectrons are emitted as increased intensity means there are more photons

effect of increasing frequency on the photoelectric effect

higher max KE

define threshold frequency

minimum frequency of photons required for photoelectrons to be emitted from the surface of a metal plate through the photoelectric effect

equal to the metal’s work function divided by Planck’s constant

define the work function of a metal

minimum energy required to remove an electron from a metal’s surface

define stopping potential

minimum potential difference required to stop the highest kinetic energy electrons from leaving the metal plate in the photoelectric effect

what can we do after measuring stopping potential

use it to find the max KE of the released photoelectrons

max KE = eVs

describe excitation

electrons in atoms can only exist in discrete energy levels
these electrons can gain energy from collisions with free electrons
this causes them to move up an energy level

describe ionisation

when the energy of the free electrons is more than the ionisation energy, electrons gain enough energy to be removed from the atom entirely

define excitation

the process of an electron taking in exactly the right quantity of energy to move to a higher energy level

define ionisation

the process of an atom losing an orbital electron and becoming charged

what happens when an electron becomes excited

it quickly returns to its original energy level (ground state) and releases the energy it gained, in the form of a photon

use of excitation

fluorescent tubes

explain how fluorescent tubes work

filled with mercury vapour, across which a high voltage is applied

this voltage accelerates free electrons through the tube and they collide with mercury atoms and become ionised so release more free electrons

free electrons collide with mercury atoms and become excited

when they de-excite they release photons, mostly within the UV range

fluorescent coating on the inside of the tube absorbs the photons

therefore the electrons in the atoms of the coating become excited and de-excite, releasing photons of visible light

define electron volt

energy gained by (work done on) 1 electron when passing through a potential difference of 1V

1 eV =

1.6 × 10^-19 J

wave property of electrons

electron diffraction

De Broglie

if light has particle properties, particles also have wave properties

effect of increased momentum on diffraction

wavelength decreases

diffraction decreases

concentric rings of the interference pattern are closer together

Explain how line spectra are produced.

In your answer you should describe:

• how the collisions of charged particles with gas atoms can cause the atoms to emit photons.

• how spectral lines are explained by the concept of discrete energy levels.

• Energy from collision of charged particles transfers to electrons in gas molecules.

• Electrons excited to higher energy levels

• The more energy the electrons absorb the higher the energy levels reached.

• Electrons are unstable at higher energy levels so will fall back down.

• When it falls down it will emit a photon

Formation of spectral lines:

• Photon energy = hf/ or photon energy proportional to frequency.

• Spectral lines are at specific wavelengths.

• Each spectral line corresponds to an electron falling down to a lower energy state.

• Energy gap, ΔE = hc/λ

• Larger energy gap means higher energy photon is emitted so shorter wavelength or vice versa.