2.5 Quantum Physics Flashcards
(42 cards)
What is a photon?
A photon is an indivisible packet of electromagnetic energy.
Einstein theorised that light was made up of these little packets.
What is the photoelectric effect?
When electromagnetic radiation above a certain frequency was directed at the metal. This effect is known as the photoelectric effect.
What is work function?
Minimum energy required to release an electron from the surface of a metal.
What is threshold frequency?
The minimum frequency a photon must have to cause an electron to be emitted.
What observations could NOT be explained using the wave theory of light?
1 – Emission of electrons from a metal surface does NOT take place if the frequency of the EM radiation is below the threshold frequency f0.
2 – If the frequency is < f0, no matter how intense the radiation, no emissions will take place.
3 – Photoemission occurs without delay (provided f > threshold frequency, f0).
How did Einstein prove light as a particle?
When light is incident on a metal surface, an electron at the surface absorbs a single photon from the incident light, and therefore gains energy equal to hf, where hf is the energy of a light photon.
How can the photoelectric effect be done?
An electron can leave the metal surface if the energy gained from a single photon exceeds the work function, Ф, of the metal. This is the minimum energy needed by an electron to escape from the metal surface.
How can the number of photoelectrons released be increased (assuming f > f0)?
- Increase the intensity of the radiation.
- Increase the frequency of incident radiation
How can the increase the intensity of the radiation be explained?
If we visualise more photons landing per unit area, clearly more electrons will be emitted.
How can the increase the frequency of incident radiation be explained?
Work function, Ф is the energy required to free electron from the surface and when the radiation goes deeper into the metal the work function increases and the threshold frequency increases.
What are the 2 conditions required for photoelectrons to be emitted from the surface of metal?
- The photons must be absorbed by the electrons.
- Photons must have energy greater than or equal to the work function of the metal.
How can energy levels be described?
- We say electrons exist in energy levels.
- Energy levels are said to be discrete. This means they have exact values.
- Typically, electrons exist in the ground state, this is the lowest quantum level ‘n=1’.
- The energy of an electron in a shell is constant.
Why are electron energy levels negative?
This is due to a ‘free electron’ , i.e. one that has been removed from the atom to infinity having zero energy. So all closer to the nucleus have negative energy and get smaller.
What is the ground state?
Lowest possible energy level or most stable level.
What is an energy level?
An orbit with an energy where an electron can remain.
How can excitation occur?
- When the atom collides with another particle.
- When the electron absorbs a photon with the exact amount of energy it needs to jump a level.
What is relaxation (de-excitation)?
- The electron configuration in an excited atom is unstable because an electron that moved to an outer shell leaves a vacancy in the shell it moved from.
- Sooner or later, the vacancy is filled by an electron from an outer shell transferring to it.
- When an electron falls, it loses energy and the atom emits a photon.
- The energy of the photon is the exact energy that is lost by the electron moving to its lower energy level.
What happens in the absorption spectra?
White light is a mixture of photons of different colour (frequency).
When the light is viewed after passing through the gas we can see black absorbed lines. These lines represent the frequency of photons which have been absorbed to excite electrons.
What happens in the emission spectra?
Each possible drop between atomic energy levels in an atom corresponds to the emission of one specific frequency of photon (e.g. colour of light).
This results in one line in an element’s emission spectrum.
What is line emission?
The emission of light from elements proves that there are discrete energy levels within that atom.
Why does the hydrogen emission spectrum consists of a limited number of characteristic wavelengths only?
- Fixed discrete energy levels.
- Downward transition between levels releases photons of specific energies, hence characteristic lines.
Why does a fluorescent tube emit visible light?
- Ionisation and excitation of the mercury atoms occurs as they collide with each other and with electrons in the tube.
- The mercury atoms emit UV photons, as well as visible photons and photons of much less energy when they de-excite.
- The UV photons are absorbed by the atoms of the fluorescent coating, causing excitation of the atoms.
- The coating atoms de-excite and emit visible photons.
What does laser stand for?
Light
Amplification by the
Stimulated
Emission of
Radiation
What type of light do lasers produce?
- Coherent
- Monochromatic
- Collimated
