G482 - Photons Flashcards
Wave Particle Duality
The theory which states that all objects can exhibit both wave and particle properties
Photon Definition
Quantum packets of energy
Energy of a Photon
E = hf
E = photon energy h = planck's constant f = frequency of the radiation the photon is associated with
electron volt
eV
A unit of energy
One electron volt is the energy required to accelerate an electron through a p.d. of one volt
Electronvolts to Joules
x 1.6x10^-19
Joules to Electronvolts
÷ 1.6x10^-19
Experiment to Estimate h
When an LED emits light each electron that passes through it loses energy
The f zed amount of energy lost corresponds to the photon energy of the emitted light (assuming no heating effect)
Energy Lost by Electron = e x p.d. across LED
e = elementary charge
hf = eV
The frequency is labelled on the LED so this equation can be rearrange to find h
eV and Kinetic Energy
When energy is lost by an electron to give something else kinetic energy:
eV = 0.5mv²
The Photoelectric Effect
The emission of electrons from a metal surface when radiation is incident on the surface
Light
Evidence of Wave Behaviour
Diffraction
Interference
Light
Evidence of Particle Behaviour
photoelectric effect
Threshold Frequency
Definition
The lowest frequency if electromagnetic radiation that will result in the emission of a photoelectron from a specific metal surface
Work Function
Definition
ϕ
The minimum energy required to release a photoelectron from the surface of a metal
Photoelectric Effect
Kinetic Energy
Energy is conserved when a photon interacts with an electron i.e. it gives all of its energy to the electron
If this results in more energy than the work function of the metal then the extra energy will give the electron kinetic energy
Electrons that are closer to the nucleus of an atom will use more energy escaping the metal so will have less kinetic energy
Maximum Kinetic Energy
Formula
hf = ϕ + KEmax
Photoelectric Effect
Intensity
Kinetic energy is independent of intensity as each photon gives all of its energy to one electron
Current increases with intensity as more photoelectrons are emitted so more charges hit the collector plate per unit time
The number of electrons emitted is proportional to the intensity of the incident radiation
Planck’s Constant
Value
h = 6.63 x 10^-34
Energy of Photoelectrons
Positive Applied P.D.
All the photoelectrons emitted are collected
Negative electrons are emitted for the negative plate which repels them and they are attracted to the positive plate so a current flows
Energy of Photoelectrons
Negative Applied P.D.
Negative electrons are emitted when radiation is applied to the positive plate
They are emitted with kinetic energy but are repelled by the negative charge on the negative plate
Only electrons with sufficient kinetic energy can reach the negative plate and record a current
If the p.d. is high enough no current will be recorded
Stopping Potential
Definition
The potential difference required to completely stop electrons from reaching the anode
Electrons
Evidence of Wave Behaviour
Electron diffraction
Electrons
Evidence of Particle Behaviour
Deflection in magnetic / electric fields
Electron Diffraction
Electrons from an electron gun are accelerate through a vacuum towards a layer of polycrystalline graphite
The atomic spacing in graphite is close enough to the wavelength of an electron that it diffracts it
However the graphite atoms are not lined up in the same direction so a circular electron diffraction pattern is produced
The de Broglie Equation
λ = h / mv
λ = wavelength of particle m h = planck's constant m = mass of particle kg v = velocity of particle m/s
Spacing of Atoms in a Solid
10^-10m
Applications of Electron Diffraction
Can be used to determine at spacing as diffraction will only occur if the wavelength of the electron is similar to the spacing between the atoms
High speed electrons can be used to determine the arrangement of atoms within crystalline structures or to measure the diameter of a nucleus
Emission Line Spectrum
A spectrum produced by a material that contains only certain frequencies due to electron transitions between energy levels
Absorption Line Spectrum
A spectrum of dark line over a continuous colour spectrum produced when light passes through a gas and the gas absorbs certain frequencies depending on the elements present
The Bohr Atom
…is an atom model
An electron can revolve around the nucleus in certain allowed orbits in which it doesn’t emit radiation and each orbit is associated with a definite amount of energy.
The Bohr Atom
Quantum Jumps - Emitting Energy
An electron can jump from an orbit of higher energy, E2, to an orbit of lower energy, E1,
The energy lost is emitted as a photon the frequency of which is given by
f = (E2-E1)/ h
Each downward jump produces the emission of a photon of a definite energy, frequency and wavelength which is visible as a coloured line on an emission spectra
The combination of jumps that can take place inside an atom are unique to that element
The Bohr Atom
Quantum Jumps - Absorbing Energy
Electrons in an atom will absorb energy equal to the gap between the energy level it is at and a higher energy level
The frequency of photons of this energy correspond to the dark lines on a absorption spectra for that element
