EM Radiation and 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.
What is the range of kinetic energies of the emitted photoelectrons?
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 (fo).
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 is greater than or equal to the work function and e = hf
- φ = hfo
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)
How does increasing frequency affect the photoelectric effect?
Photon energy (E) increases, Work function (φ) is constant, Maximum kinetic energy of electrons increases as Ek(max) = hf – φ.
How does increasing intensity of EM radiation affect the photoelectric effect?
Number of photons incident per second increases, Number of photoelectrons emitted per second increases. As one photon transfers all it’s energy to one electron.
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.
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.
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.
What happens to the electrons in the mercury vapour?
Free electrons collide with electrons in mercury atoms and transfer energy. Mercury electrons excite then immediately de-excite, emitting UV photons.
What happens to the electrons in the phosphor coating?
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?
lambda = h (planck’s constant)/p (momentum) = h (planck’s constant)/ m (mass) x v (velocity)
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.
