Particles and radiation chapter 3: Quantum phenomena Flashcards
What is the photoelectric effect?
The emission of electrons from a metal surface when electromagnetic radiation is incident upon it
What is the threshold frequency?
The threshold frequency is the minimum frequency of light that can cause the photoelectric effect.
What explains the threshold frequency?
The photon model of light suggests that electromagnetic waves travel in discreet packets called photons, and that each electron can absorb a single photon.
In the photoelectric effect, what happens if the intensity of light is increased (assuming that the frequency is above threshold)?
More photoelectrons are emitted per second.
Why was the wave theory of light unable to explain the threshold frequency?
It suggested that any frequency of light would be able to cause photoelectric emission, as the energy absorbed by each electron would gradually increase with each incoming wave.
This was proved to be wrong, as there is a min. frequency for different metals in order for photoelectric emission to take place.
What is the work function?
minimum energy required by an electron to escape from a metal surface.
What is the stopping potential?
Stopping potential is the minimum potential that needs to be applied to a metal plate to attract all the photoelectrons emitted from its surface back to the surface.
Electrons in atoms can only exist in….
…Discrete energy levels.
What is excitation?
electrons gain energy from collisions with other free electrons, which can cause them to move up in energy level.
What is ionisation?
where electrons gain enough energy to be removed from the atom entirely. It only occurs if the energy of the free electron is greater than the ionisation energy.
What happens after an electron becomes excited?
It will quickly return to its original energy level (ground state) and therefore release the energy it gained in the form of a photon.
Give an example of a practical use of excitation.
A practical use of excitation is in a fluorescent tube in order to produce light. They are usually filled with mercury vapour, across which a high voltage is applied.
Explain how fluorescent tubes can produce light by excitation of electrons.
- Ionisation and excitation of the mercury atoms occur as they collide with each other and with electrons in the tube.
- The mercury atoms emit ultraviolet photons, as well as visible photons photons of much less energy, when they de-excite…
- The ultraviolet photons are absorbed by the atoms of the fluorescent coating, causing excitation of the atoms.
- The coating atoms de-excite in steps and emit visible photons.
What unit of energy do we use when describing the difference between energy levels?
The electron volt (eV)
How do we describe electron volts?
An electron volt is the energy gained by one electron when passing through a potential difference of one volt.
Explain why this line emission spectrum looks like this.
- Each line represents a different wavelength
- This spectrum contains only discrete values of wavelength
- The only photon energies emitted correspond to these wavelengths
- Therefore this shows that electrons in an atom only transition between discrete energy levels.
What is the equation for the difference between two energy levels?
