Quantum Flashcards
(28 cards)
What is the photoelectric effect?
If you shine light of a high enough frequency onto the surface of a metal, it will instantly emit electrons, once these electrons are emitted they’re called photoelectrons
What are the 4 conclusions that came from the photoelectric effect?
1) That no photoelectrons are emitted if the frequency of light is below a certain value (a threshold frequency)
2) The photoelectrons are emitted with a variety of kinetic energies ranging from zero to some maximum value (which increases with the frequency)
3) The maximum kinetic energy of the photoelectrons is unaffected by the varying intensity of the radiation
4) The number of photoelectrons emitted per second is proportional to the intensity of the radiation
What is a photon?
A packet of electromagnetic radiation that has a discrete (fixed) energy
What happens as the intensity of light is increased in the photoelectric effect?
More electrons (of the same energy) are emitted per second
What happens as the frequency of light is increased in the photoelectric effect?
The energy of each photon increases and they transfer all of that energy to one electron (therefore the number of electrons emitted stays the same but they have more energy)
What is meant by the work function of a metal?
The minimum energy that an electron needs to gain in order to break the bonds holding it to the surface of the metal (to escape the surface)
What is meant by threshold frequency?
The minimum frequency a photon needs to have for electrons to be ejected from the surface of a metal
Explain why photoelectrons are emitted with ranging kinetic energies in the photoelectric effect?
Because some electrons are already at the surface of the metal whereas some need more energy to bring them to the surface, therefore the electrons that need less energy to escape (those at the surface) will have more kinetic energy after they leave compared to those that need more energy (remembering that each electron only gets a certain amount of energy from ONE photon)
What is meant by the stopping potential?
The potential difference required to stop the fastest moving electrons, travelling with Ek(max)
What is meant by excitation?
It is when an electron in an atom gains/absorbs a discrete (specific) amount of energy which allows it to move to a higher energy level without being removed from the atom completely
What is meant by de-excitation?
It is when an excited electron in an atom falls back to a lower energy level and in the process it releases the energy in the form of a photon
What is meant by the ground state?
The lowest energy level that an electron can occupy in an atom, where it is closest to the nucleus (hence it is in the first shell, n=1)
What does it mean for an atom to be in its ground state?
All of the electrons in the atom are in their lowest possible energy level and none of them have been excited to a higher energy level
What is ionisation?
When an electron gain enough energy to be completely removed from an atom, leaving behind a positively charged ion
What is ionisation energy?
The minimum amount of energy that an electron in its ground state needs to gain in order to be removed from the atom completely
Describe the two ways that an electron can be excited
By absorbing a photon (with the exact amount of energy required to move between energy levels) or by colliding with another particle (an electron can gain energy by colliding with another electron or particle, and it can gain kinetic energy this way causing excitation)
How does a fluorescent tube work?
A high voltage is applied across the tube (which contains Mercury gas), which causes free electrons in the tube to be accelerated. Some of these electron ionise Mercury atoms creating more free electrons. All of the free electrons collide with the Mercury atoms causing them to excite. These Mercury atoms emit photons in the UV range as they de-excite. These UV photons are then absorbed by the phosphor coating on the inside of the tube, causing the phosphor atoms to excite. When the Phosphor atoms de-excite they re-emit these photons as visible (fluorescent) light
What happens when light from a fluorescent tube is passed through a prism or diffraction grating?
A line emission spectrum is produced
What is a line emission spectrum?
A series of discrete bright lines of specific colours (wavelengths) are produced against a dark background, each line corresponds to a photon emitted when an electron in an atom drops to a lower energy level
What happens if you pass white light through a prism or diffraction grating?
You see a continuous spectrum of colours that all merge into eachother (there aren’t any gaps in the spectrum)
What is a line absorption spectrum?
When light with a continuous spectrum of energy (white light) passes through a cool gas, photons of the correct wavelength are absorbed by the electrons to excite them to higher energy levels. These wavelengths are the missing from the spectrum when it comes out of the other side of the gas. This creates a series of black lines (gaps) against a continuous spectrum
What is diffraction in terms of waves?
The bending or spreading of waves as they pass through a gap or around an obstacle
What condition is needed for significant diffraction to happen?
The size of the gap or obstacle that the wave is passing through or around must be similar to or smaller than the wavelength of the wave
What is the significance of diffraction?
Because we had observed the photoelectric effect we thought that light could only act as a particle but diffraction shows that it can also at as a wave (provides evidence for wave-particle duality)
