3. Photons and Lasers Flashcards
Photon
The smallest possible amount of light energy
Absorption
When electrons on lower energy levels receive photons of an energy that matches the change in the energy levels
Emission
When the electron releases a photon back into space
Ionisation
When an electron is moved up or down an energy level depending on how much energy it has
Ionisation energy
The energy needed to knock an electron out of its energy level
The energy needed to move an electron from its ground state
Stimulated emission
The emission of a photon from an excited atom which is triggered by an incident photon of energy equal to the gap between the excited state and lower energy level.
Population inversion
When more electrons (more populated) are on a higher energy level than the one below it
Pumping
When an electron receives a photon of the same energy difference and then moves up to a higher energy level
Photoelectric effect
The release of electrons from the surface of a metal when electromagnetic radiation is incident on its surface. This then reduced the overall charge of the object.
Work function (WF)
The minimum amount of energy needed by an electron to allow it to move off the surface of a metal
Max kinetic energy of electrons
Ekmax = hf - WF
Threshold frequency
The minimum frequency for which photoelectrons are emitted
The Ekmax would be 0
Electron volt (eV)
The amount of kinetic energy an electron will receive after being accelerated through 1V
Stopping potential
The amount of volts needed to show the ammeter reading as 0
V stop equation
Ekmax = e x Vstop
Change in energy equation
E2 - E1
Photon energy equation
E = hf (- WF)
Alternate photon energy equation
E = hc / wavelength (-WF)
What are the quantities gathered from a Graph of KE against Frequency
Gradient = Plancks constant
Y-intercept = The work function x -1
Milikans experiment
The experiment to find the Vstop of an electron by using a circuit.
Measured the frequency of light and also the KE of the photons that are emitted off the metal surface.
Wave particle duality
The ability of waves to behave as particles and as waves
Conductor
A material through which electrons can flow easily
Insulator
A material in which electrons cannot flow easily
Electroscope
A device used to detect the existence of electrical charge
Absorption spectra
A rainbow spectrum with black lines, this shows the certain frequencies of light radiated.
Emission spectra
A black background with coloured lines, it represents the electrons moving through different energy levels. They loose energy as they produce a photon which then creates emission lines.
Monochromatic
Photons or waves of a single wavelength
Coherence between incident and emitted photons
The relationship between the incident photon and the emitted photon is coherent as they are the dame frequency and in phase
Gas laser pros and cons
Large
Expensive
Inefficient
However, used when the laser needs to produce a specific wavelength
Semiconductor / laser diodes pros and cons
Small
Cheap
Very Efficient
Most common and used for eye surgery / image scanning
cant produce high power
Metastable
When an energy level in a laser is almost stable to hold electrons without spontaneous emission
Ionisation energy
The energy needed for an electron to escape the atom, the energy needed is just above the highest energy level
(In an exam question if it asks for the ionisation energy then write the highest energy level)
Light Amplification
The stimulation causes double the amount of photons to be emitted. (exponentially increases)
Different types of lasers
2 level energy system
3 level
4 level
Excitation
When an electron gains more energy and moves to a higher energy level
Spontaneous emission
When an electron drops to a lower metastable level and emits a photon
What is the photo electric effect
When photons are incident upon a metal surface with the same wavelength as the electrons. this causes the electrons to eject from the surface
What is the de broglie wavelength
wave length = h / momentum
we use this equation as the size of the gap needed to diffract them cannot be achieved easily
What is the radiation pressure
The change in momentum of electromagnetic radiation, this then causes pressure to be exerted upon a surface
What is electron diffraction?
When electrons are accelerated towards graphite crystals they create a diffraction pattern.
This proves they have wave like properties
Structure of a typical laser
There is an amplifying medium between two mirrors, 99% is reflected and the other 1% is transmitted as the laser light
