Particles and Radiation (3): Quantum Phenomena Flashcards
Define the photoelectric effect
This is when photoelectrons are emitted from the surface of a metal when electromagnetic radiation above a threshold frequency is incident on it
What was the assumption before people observed the results from the photoelectric effect?
That light was only a wave
What is the definition of the threshold frequency?
An intrinsic numerical property of metals where incident EM radiation/photons above which will cause photoelectrons to be emitted
Describe 4 assumptions about how people thought that the photoelectric effect worked vs what they found after
- Electrons absorb energy continuously from the EM waves
-> Electrons actually absorb energy in discrete ‘all or nothing’ packets called photons - All frequencies of light should be able to eject electrons from the surface of a material given enough time and intensity
-> Electrons are only ejected from a metal when the incident frequency is above the threshold frequency of that metal - Photoelectric emission occurs with delay
-> Actually occurs without delay provided the f is greater than or equal to the f(min) - Wave energy is determined by E=I=A^2 therefore increasing the intensity means that the emitted photoelectrons should have more kinetic energy
-> No relationship between intensity and Ek of photoelectrons emitted, instead as the frequency above the threshold frequency increases the Ek of the photoelectrons will increase
Does intensity of the EM radiation effect the Ek of the emitted photoelectrons?
no
Does intensity of the EM radiation effect the number of the emitted photoelectrons?
yes - but only once the frequency is above the threshold frequency of the metal
How many photons can each electron absorb?
1
What was the discovery that occurred due to the photoelectric effect?
That light can behave as a particle:
-> has both wave-like and particle-like properties
What did Einstein theorise light was made of?
Discrete ‘all or nothing’ packets called photons
How can you change the frequency of light?
Change its colour
Which colour of light has the highest frequency?
Violet
Which colour of light has the lowest frequency?
Red
What is the equation for the frequency of an electromagnetic wave?
f=c/(lambda)
What is the equation for the energy of a photon?
E = hf = hc/(lambda)
What is the energy gained by an electron on the surface of a metal?
hf - as the electron gains the energy from 1 photon
Work function defintion
The minimum energy required for an electron to escape from the surface of a metal
Will all emitted photoelectrons be from the surface of the metal?
no
If the value of the work function, theta, is smaller than the value of E gained by the electron from the photon, where does the extra energy go?
Into the kinetic energy of the emitted photoelectron
What is the equation for the Ek (max) of an emitted electron?
Ek (max) = hf - (theta)
hf = Ek (max) + theta
What is required for an electron emission to take place from a metal surface?
Ek (max) > 0 or hf > (theta)
What is the equation for the threshold frequency of a metal material?
f (min) = (theta)/h
What is the definition of the stopping potential?
The potential you would need to apply across the metal to stop the photoelectrons with the maximum kinetic energy
By applying a potential to your metal, what are you physically doing?
Giving the plate a sufficiently positive charge for the photoelectrons with the maximum kinetic energies to be attracted back
What is the equation for Ek (max) of a photoelectron when a stopping voltage has been applied?
Ek (max) = eVs
Where:
Vs - stopping voltage
- Derived from: energy = charge x voltage
What is the value of Ek (max) when the stopping voltage is applied?
0
What did Plank suggest?
That the energy of each vibrating atom is quantised -> only certain levels of energy (energy levels) are allows corresponding to E = hf
What does the word quantised mean?
Discrete and distinct amounts
What happens if an electron does not gain enough energy from an incident photon in order to escape the metal surface?
It will collide repeatedly with other electrons and positive metal ions, quickly losing its extra kinetic energy
Describe the structure of a vacuum photocell
A glass tube containing a metal plate known as the photocathode and a smaller metal electrode opposite it known as the anode. The anode and cathode are connected to a microammeter in a series circuit.
Describe how a vacuum photocell works
When light with a frequency greater than the threshold frequency of the photocathode metal is incident, photoelectrons are emitted. These electrons are attracted to the anode and the microammeter is able to measure the photoelectric current.
What is the photoelectric current produced in a vacuum photocell proportional to?
The number of electrons per second that transfer from the cathode to the anode, hence the intensity of the incident light on the photocathode
What is the equation for the number of photoelectrons per second that transfer from the cathode to the anode?
I/e
What is the equipment used to measure the Ek (max) of photoelectrons emitted for different frequencies of incident light?
A photocell
Describe what graph you plot and the features of the graph when you are investigating how Ek (max) changes with different frequencies of light
Using equation Ek (max) = hf - (theta)
Plot Ek (max) against f
- The gradient of your line = plancks constant, h
- The y-intercept of your line = work function
- The x-intercept of your line = threshold frequency
What is the shape of the line of best fit on an Ek(max) - frequency graph? Why?
- Straight line through the threshold frequency and work function
- Gradient of the line of best fit always equals planks constant, h
What is the process of creating ions called?
Ionisation
What is an ion?
A charged atom - an atom which has lost or gained a certain number of electrons
What are the two methods of ionisation that you need to know?
- Ionising radiation, Alpha, beta or gamma, passing through substances and colliding with the atoms -> this may knock off an electron
- Electrons passing through a fluorescent tube create ions when they collide with atoms of the gas or vapour in the tube
What is the definition of an electron-volt?
A unit of energy equal to the work done when an electron is accelerated through a pd of 1V
1 eV = …?
1.6 x 10^-19J
Energy levels in atoms are…?
Discrete
Do all atoms that collide with electrons get ionised?
no
What is the definition of excitation?
When electrons in atoms gain energy from colliding with free electrons causing them to move up energy levels
If an electron in an atom gains enough energy from a collision to leave the atom, this process is known as…?
Ionisation
If you were given energy levels of an atom what value would correspond to the ionisation energy an electron at n=1?
The value at n=1
What happens after an electron has been excited?
- It will de-excite going back to a lower energy
- The electron will release the energy difference in the form of a photon
In what value are energy levels described by?
eV
What is the lowest energy state of an atom also known as?
The ground state
Is an excited atom stable or unstable?
Unstable
What is the equation for the energy of an emitted photon during a de-excitation?
hf = E1 - E2
What is the only case when a photon can be absorbed by an electron?
When the energy of the photon is exactly equal to the gain in the electrons energy (the difference between the final and initial energy levels of the atom)
How do you go from eV -> J?
V = E/Q
eV x (1.6 x 10^-19)
How do you go from eV -> V?
divide by (1.6 x 10^-19)
Can electrons de-excite in multiple step?
Yes
Why do certain substances fluoresce?
- When they absorb UV radiation atoms in the substance absorb the UV photons and become excited
- When the atoms de-excite them emit visible photons
Describe what the set up of a fluorescent tube is?
- A glass tube with fluorescent coating on its inner surface
- Tube contains mercury gas at low pressure
- Tube has filament electrodes connected to a mains supply at either end - they are heated to a hot enough temperature so that ionisation of the gas can occur
Describe what happens when a fluorescent tube is on?
- Ionisation and excitation of mercury atoms occur as they collide with each other and electrons in the tube (electrons produced from thermionic emission)
- The mercury atoms emit UV photons, Visible light photons and lower energy photons when they de-excite
- The UV photons are absorbed by some of the atoms in the fluorescent coating causing them to excite
- The coating atoms de-excite via other energy levels emitting photons with a range of lower energies to be emitted, usually visible light photons
Why do energy levels have more negative values as you get closer to the nucleus?
As the electrons in those energy levels have a greater potential energy and potential is always negative?
What are the 3 series for the hydrogen atom energy levels that you need to know?
- Lymen series
- Balmer series
- Paschen series
What is the spectral line observed when there is an electron de-excitation in the Lymen series (Hydrogen atom)?
UV
What energy level do electrons de-excite to in the Lymen series (Hydrogen atom)?
n = 1
What is the spectral line observed when there is an electron de-excitation in the Balmer series (Hydrogen atom)?
Visible light
What energy level do electrons de-excite to in the Balmer series (Hydrogen atom)?
n = 2
What is the spectral line observed when there is an electron de-excitation in the Paschen series (Hydrogen atom)?
IR
What energy level do electrons de-excite to in the Paschen series (Hydrogen atom)?
n = 3
What is the range of wavelengths for the visible light section of EM radiation?
400nm -> 650nm
What happens when you shine white light through a prism?
You see a continuous spectrum of colours
What happens when you use a tube of glowing gas/fluorescent tube as a light source and shine it through a prism?
Discrete lines of different colours show up
How can we identify the element from a line spectrum?
- The wavelengths of the lines of a line spectrum of an element are characteristic of the atoms of that element
- By measuring the wavelengths of a line spectrum we can therefore identify the the element producing that light as each line spectrum is unique
-> The energy levels of each type of atom in an element are unique to that atom
A line on a line spectrum is produced from photons all of the same…?
energy
How would you get a line absorption spectrum?
Passing white light through a cooled gas
What do the blacked out lines on a line absorption spectrum correspond to?
Possible differences in energy levels as the atoms in the gas can only absorb photons of an energy exactly equal to the energy difference between 2 energy levels
What does a line absorption spectrum look like?
Continuous spectrum of all the possible colours with some blacked out lines
What are some examples of light acting as a wave?
Interference and diffraction
What are some examples of light acting as a particle
photoelectric effect
Can electrons also have wave or particle properties?
Yes
When does diffraction of light occur?
When light is passed through a narrow slit and the light emerging spreads out (in the same way as water does after passing through a gap)
What did De Broglie hypothesise?
That if light could have particle properties then particles can have wave properties
How can the wave-like nature of electrons be observed?
Electron diffraction
What is the equation for the De Broglie wavelength?
lambda = h/mv
What shows that electrons can behave like particles?
That they are deflected by a magnetic field
What does the de Broglie wavelength of a particle depend on?
- Velocity
- Mass
Describe how electron diffraction works
- A narrow beam of electrons is passed through a thin layer of metal foil
- As the positive ions are in rows in a regular pattern they cause the electron beam to be diffracted
- The beam of electrons is produced by attracting electrons from a heated filament wire to a positively charged metal plate
What shape is formed form electron diffraction?
Pattern of concentric rings
What happens to the shape of the electron diffraction pattern as you increase the velocity of the electrons?
- The De Broglie wavelength decreases when v increases
- Using w = (lambda)D/s, decreasing the wavelength decreases the fringe spacing OR less diffraction occurs
- Therefore, spacing of the concentric rings decreases
Describe how a cathode ray tube works to produce a energy beam of fast electrons?
- Thermionic emission of electrons occur on the wire
- This is because the heating done by the electric current overcomes the work function of the material
- The electrons are then accelerated by the electric field between the positive anode and negative cathode
What was used as a proof of discrete energy levels being present in atoms?
Line spectra of atomic hydrogen
