2.5 Quantum Physics Flashcards
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
What does collimated mean?
Collimated means all parts travel in the same direction
What is laser action?
If a photon of exactly the correct energy approaches an excited atom, an electron in a higher energy level may be induced to fall to a lower level and emit another photon.
The remarkable fact is that this photon has the same phase, frequency and direction of travel as the stimulating photon which is itself unaffected. This process was predicted by Einstein and is called stimulated emission.
What is population inversion?
In a laser, it is arranged that light emission by stimulated emission exceeds that by spontaneous emission.
To achieve this, it is necessary to have more electrons in an upper than a lower level, a process referred to as ‘population inversion’.
What is a method of creating population inversion?
One method of creating this population inversion is known as ‘optical pumping’ and consists of illuminating the laser material with light.
Why can’t population inversion happen in a 2 level system?
The pumping radiation induces stimulated emission from level 2 back down to level 1 since it is of the correct energy.
How does population inversion happen in a 3 level system?
- Electrons absorb radiation causing them to move up energy levels into an excited state.
- Electrons fall down into the meta stable state.
- They stay in this state longer than the excited state.
- This allows population inversion to occur (more electrons in upper energy levels than lower).
- Once an electron falls to the ground state it releases a photon.
- This causes stimulated emission of photons which all have the same energy as the incoming photons.
What are the uses of lasers?
- CDs and DVDs
- Shop checkouts
- Laser printers
- Laser pointers
- Car parking assist.
How are x-rays produced?
- The intensity of the X-Ray beam increases when the number of electrons hitting the target increases, and this is controlled by the filament current.
- The quality, or penetrating power of the X-rays is determined by the speed attained by the electrons and increases with the p.d. across the tube.
What are the two methods of X-ray production that you must be aware of?
- When an electron strikes the target, it undergoes rapid deceleration. An accelerating charged particle emits radiation, as does a decelerating one. Hence, photons of EM radiation with various energies are emitted, some of which are X-rays.
- When a bombarding electron strikes an electron on the target material, it can cause an electron to jump to a higher energy level, leaving a vacancy behind. The vacancy is filled by an (outer) electron falling into the vacancy, with the emission of a high energy photon of EM radiation.
How are x-rays used in industry?
- The detector plate (with film inside) is placed below the target area.
- X-ray tube is positioned above the target area (the X-ray tube and detector plate are now stationary).
What are the advantages of using x-rays?
- Cheaper than a CT scan.
- Lower radiation values compared to CT scan.
What are the disadvantages of using x-rays?
- Only gives information about bones.
- Radiation doses (employee safety).
Who is vulnerable to x-rays?
- Children
- Pregnant women
What does a CT scan stand for?
Computed Tomography.
How are CT scans used?
- X-ray tube can be moved, the detector moves in phase.
- CT scans allow flesh and internal organs to be seen clearly in section view of the patient. The data is sent from the scanner to a computer, where complex software builds together the overall picture.
What are the advantages of using CT scans?
- Allows images of organs in high detail.
- Can detect many life-threatening injuries in accidents.
- Can detect many heart diseases.
What are the disadvantages of using CT scans?
- Costs, much more costly than a conventional X-Ray scan.
- Higher radiation dose than X-Rays.
- Higher preparation time for patients (potential allergic reactions).
Who is vulnerable to CT scans?
- Children
- Pregnant women
