EM Radiation & Quantum Phenomena Flashcards
Describe the photoelectric effect.
Photons of EM radiation are incident on a metal surface.
1 photon transfers all its energy to 1 electron near the surface.
Photoelectrons emitted.
Give details about the kinetic energy of the photoelectrons emitted.
Photoelectrons have a range of kinetic energies,
From 0 up to a maximum value (determined by frequency of radiation).
Define work function (φ).
The minimum photon energy required,
to cause the emission of an electron from the surface of a metal.
Define threshold frequency (f0).
The minimum frequency of EM radiation required,
to cause the emission of an electron from the surface of a metal.
Give the equation linking work function and threshold frequency.
As energy of the photon ≥
work function and E= hf:
φ = hf0
Give the equation linking frequency, work function and max kinetic energy of electrons.
As photon energy = work function + max KE of electrons:
hf = φ + Ek(max)
Explain how increasing frequency (or decreasing wavelength) of EM radiation affects the photoelectric effect.
If frequency (f) increases or wavelength (ʎ) decreases,
Photon energy (E) increases (as E=hf or E = hc/ʎ),
Work function (φ) is constant,
Maximum kinetic energy of electrons increases as Ek(max) = hf – φ.
Explain how increasing intensity (power per unit area) of EM radiation affects the photoelectric effect.
If intensity increases, the number of photons incident per second increases.
As 1 photon transfers all its energy to 1 electron.
Number of photoelectrons emitted per second increases.
Define stopping potential
The potential difference required,
to stop the emission of electrons from the surface of a metal.
Give the equation for stopping potential.
As work done stopping fastest electrons = max KE of electrons,
Vs = Ek(max) / e
Define ground state.
Lowest energy level (n=1).
Closest to the nucleus.
Define excitation in terms of energy levels and energy.
An electron moves to a higher energy level,
when it gains energy,
equal to the difference between the two levels.
Describe two ways that electrons can gain energy to become excited.
1 photon transfers all its energy to 1 electron.
A free electron collides with the electron and transfers some energy from its kinetic store.
Define de-excitation in terms of energy levels, energy and photons.
An electron moves to a lower energy level,
when it loses energy,
by emitting a photon,
with energy equal to the difference between the two levels.
Can happen directly or in stages/cascading.
Define ionisation.
An electron gains enough energy,
to be removed from an atom.
Define ionisation energy.
The amount of energy required,
to completely remove an electron from an atom,
from the ground state.
Why are energy levels defined as having negative energy?
A free electron removed from an atom is defined as 0 J.
All energy levels are negative relative to this, as energy needs to be supplied to remove an electron.
Describe the structure of the discharge tube used for fluorescent lights.
Glass tube filled with mercury vapour.
Beam of free electrons accelerated by a p.d..
Phosphor coating.
Describe what happens to the electrons in the mercury vapour in the flourescent light.
Free electrons collide with electrons in mercury atoms and transfer energy.
Mercury electrons excite then immediately de-excite,
emitting UV photons.
Describe what happens to the electrons in the phosphor coating in flourescent lights.
UV photons are absorbed by electrons in phosphor coating.
Phosphor electrons are excited,
then de-excite in stages, emitting visible light photons.
What do line emission spectra look like?
Black background.
Coloured lines at certain wavelengths.
How are line emission spectra formed?
Discharge tube filled with gas.
Electrons in the gas are excited when free electrons collide and transfer energy.
De-excitation of electrons causes photons to be emitted with certain energies equal to difference in energy levels,
which correspond to certain wavelength / frequencies,
as E=hf or E = hc/ʎ.
What do line absorption spectra look like?
Colour spectrum background.
Black lines at certain wavelengths.
How are line absorption spectra formed?
White light is shone through a cold gas.
Electrons are excited when they absorb photons with certain energies,
equal to the difference in energy levels,
which correspond to certain wavelength / frequencies,
as E=hf or E = hc/ʎ.
What is meant by the duality of particles?
Particles can behave like particles and like waves.
What is the equation for de Broglie wavelength? (Data sheet)
ʎ = h/p = h/mv
What is diffraction?
Spreading out of waves,
as they pass through a gap /go round an obstacle.
When does diffraction happen?
When the wavelength is the same order of magnitude as gap size / obstacle size.
What factors affect diffraction?
Smaller gap -> greater diffraction effect.
Longer wavelength -> greater diffraction effect.
In electron diffraction, how do electrons reach high speeds? Is this particle or wave behaviour?
Accelerated by a potential difference.
Particle behaviour.
In electron diffraction, how are electrons diffracted? Is this particle or wave behaviour?
Diffracted through the gaps between atoms in graphite
Wave behaviour.
In electron diffraction, how do the electrons produce the pattern on the screen? Is this particle or wave behaviour?
Electrons collide with electrons in the screen causing fluorescence.
Particle behaviour.
