2. Electromagnetic Radiation And Quantum Phenomena Flashcards
(21 cards)
What is the photoelectric effect?
If you shine radiation of a high enough frequency onto the surface of a metal, it will instantly emit electrons. For most metals, it is frequency is in the UV range.
How does the photoelectric effect work?
Free electrons near the surface of the metal vibrate, and with enough energy, they break the bonds holding them to the metal and are released. Electrons emitted are called photoelectrons.
What are the conclusions drawn from experiments on the photoelectric effect? (4)
- No photoelectrons are emitted if the radiation is below their threshold frequency.
- The photoelectrons are emitted with kinetic energies (0-max), where the max value increases with frequency.
- The intensity is the amount of radiation hitting an area per second. Photoelectrons max kinetic energy is unaffected by the intensity.
- The number of photoelectrons emitted per second is proportional to the intensity of the radiation.
Why is wave theory unable to explain threshold frequency and the kinetic energy of photoelectrons?
Threshold frequency:
- Wave theory says that the energy carried should be proportional to the intensity of the beam, and that energy should be spread out evenly over the wavefront.
- This would mean that electrons gain energy over time (proportionally to frequency) and this is not the case. Instead there is a threshold frequency.
Kinetic energy:
- More energy should be transferred to each photoelectron with increasing intensity, but it is actually dependent on frequency.
How is the photoelectric effect demonstrated?
- A zinc plate is negatively charged and so repels a piece of gold leaf.
- When UV light is shone on it, electrons are emitted so the plate is less negatively charged the gold is no longer repelled and so falls back down.
How much of a photons energy is transferred to each free electron in the photoelectric effect?
All of it.
What must be overcome in order for a free electron to be emitted, and what is the equation for this?
f(0) = φ / h
This is because each electron is bonded to the metal, and so the energy used to overcome these bonds is called the work function.
What is the equation for the maximum kinetic energy of a photoelectron?
hf = φ + Ek(max)
Or
hf = φ + 0.5mv^2
What is stopping potential, and what is its equation?
- It is the potential difference needed to stop the fastest moving electrons with Ek(max). The work done to stop them is equal to the energy that they were carrying.
eV(s) = Ek(max)
What is an electron volt?
The kinetic energy carried by an electron after is has been accelerated from rest through a potential difference of one volt.
1eV = 1.6x10^-19 J
What is the concept of discrete energy levels in atoms?
- Electrons can only exist in certain well defined energy levels, with the ground state being n=1.
- When electrons move down an energy level they emit a photon with energy equal to the difference in energy levels that the electron has moved between.
Can electrons move up energy levels?
Yes, if they absorb a photon with the exact energy required. This is called excitation.
What is ionisation and ionisation energy?
When an electron is emitted from an atom. Ionisation energy is the energy needed to remove an electron that is in its ground state.
How do fluorescent tubes work?
- A high voltage is applied to mercury vapour.
- The high voltage accelerates free electrons so they ionise mercury atoms
- Mercury is ionised, so more free electrons are produced.
- Excited electrons go back to their ground state, emitting photons in the UV range.
- A phosphor coating absorbs these, which excited electrons that then cascade down the energy levels, emitting many wavelengths of visible light.
What is a line emission spectrum?
The light emitted from various de-excitations is represented on a line with a black background. Clearer when done with a diffraction grating than a prism. Each line corresponds with a particular wavelength emitted.
What is a continuous line spectrum?
When there aren’t any gaps due to the electrons not having fixed energy levels. Found with hot things.
What are line absorption spectra?
When white light passes through a cool gas (cool as most electrons will be in their ground states), photons of the correct wavelength will be absorbed when the excite electrons. On a spectrum, the black lines correspond to those that have been absorbed.
How can photons be evidenced to be a particle or a wave?
Particle: the photoelectric effect can only happen is they are a particle.
Wave: they can only be diffracted if they are a wave.
This is called wave-particle duality.
What is the de Broglie equation?
λ = h / mv
Where λ is the de Broglie wavelength, h is the Planck constant (Js) and mv is the momentum.
How can a de Broglie wave be interpreted?
- A probability wave.
What is electron diffraction?
- Electrons are passed through a graphite crystal sheet and so a re diffracted due to the gaps between the atoms. A higher voltage means a higher velocity so it is diffracted more. More massive particles have a lower de Broglie wavelength.
