Electromagnetic Radiation and Quantum Phenomena Flashcards
What happens if you shine radiation of high enough frequency onto the surface of a metal?
It will emit electrons
For most metals, what range is the frequency of radiation necessary for the metal to emit electrons?
In the UV range
What is the photoelectric effect?
The emission of electrons from a metal when light of high enough frequency is shone on it
What are the electrons that are emitted from a metal called?
Photoelectrons
Why does shining radiation of high enough frequency onto a metal surface make electrons be released?
Metals contain free electrons on the inside, which absorbs energy from the radiation, making it vibrate. If an electron absorbs enough energy, the bonds holding it to the metal will break and the electron is released
What is the minimum frequency of radiation needed for a given metal to emit electrons called?
Threshold frequency
The photoelectrons are emitted with a variety of…. This value of maximum…. increases with….?
The photoelectrons are emitted with a variety of kinetic energies. This value of maximum kinetic energy increases with the frequency of the radiation
What is the intensity of radiation on a metal?
The amount of energy per second hitting an area of the metal
For the photoelectric effect, what is unaffected by varying the intensity of radiation?
The maximum kinetic energies of the photoelectrons
What is proportional to the intensity of the radiation for the photoelectric effect?
The number of photoelectrons emitted per second
Why can’t wave theory explain the photoelectric effect?
The energy of a wave is proportional to its intensity. Increasing the intensity of the waves should increase the energy of the electrons that are emitted from the metal, but this doesn’t happen
What does the photoelectric effect show about light?
That light can’t just act as a wave
What are the discrete packets of EM waves also known as?
Quanta
Who discovered the discrete packets of EM waves?
Max Planck
What did Einstein suggest about EM waves?
He suggested that waves can only exist in discrete packets that he called photons
What is the photon model, developed by Einstein, which can explain the photoelectric effect?
He saw these photons of light as having a particle-like interaction with an electron in the metal surface, where each photon transferred all its energy to one specific electron
Why can’t visible light be used to demonstrate the photoelectric effect?
It’s not got a high enough frequency
What is the work function?
The minimum amount of energy needed for an electron to escape a metals surface
What does the energy have to be compared to the work function for an electron to be emitted?
Energy has to be greater than the work function
What happens if the energy of a photon is less than the work function of the metal?
The electron will shake a bit then release it’s energy as a photon
What is the equation for the threshold frequency?
Threshold frequency = Work function / Planck’s constant
How do solar cells use the photoelectric effect?
They convert light energy into electricity
What is the energy transferred from EM radiation to an electron equal to?
The energy the electron absorbs from one photon, hf
What is the kinetic energy as the electron leaves the metal equal to?
hf (energy of photon) minus any other energy losses
Why do electrons emitted from metals have a range of different kinetic energies?
Because they all have different energy losses
What is the minimum amount of energy an electron can lose equal to?
The work function
What is the photoelectric equation?
hf = work function + max kinetic energy
hf = Φ + E k max hf = Φ + 0.5 m (Vmax)^2
What is the intensity of photons?
The number of photons per second on an area
What effect on the kinetic energy of the electrons does increasing the intensity of photons?
Doesn’t have an effect
Why does increasing the intensity not effect the kinetic energy of the electrons?
An electron can only absorb one photon at a time. Increasing the number hitting an area per second doesn’t change the energy of the photons, so each electron absorbs the same amount of energy
What is the stopping potential?
The P.D needed to stop the fastest moving photoelectrons in the photoelectric effect
What is the equation for stopping potential?
eVs = Ekmax
e = charge on electron Vs = stopping potential Ekmax = maximum kinetic energy
What is the electron volt defined as?
The kinetic energy carried by an electron after it has been accelerated from rest through a potential difference of 1V
What is 1eV equal to in joules?
1.6x10^-19 J
What is the ground state?
The lowest energy level an electron can be in, given the value n=1
What is meant by ‘an electron is excited’?
It is in a higher energy level than the ground state
How do electrons move down an energy level?
By emitting a photon
What is it called when an electron moves up an energy level?
Excitation
What is ionisation?
When an electron reaches the top of the energy ladder (where n=∞) can permanently leave the atom
What are the 2 ways an electron can be excited?
By a photon
By a particle
How does an electron get excited by a photon?
A photon with exactly the correct amount of energy gives up all of its energy to the electron, causing the electron to travel up the energy ladder to a higher energy level
How do you work out the photon energy causing excitation?
It’s the difference in energy of the 2 energy levels (E1 - E2)
What happens to the remaining photon energy once an electron has been ionised?
It is transformed into kinetic energy of the escaping electron
How does an electron get excited by a particle?
An incident particle can give up a fraction of its energy to the electron, causing it to travel up the energy ladder to a higher energy level
What must the kinetic energy of the particle be greater than in order for excitation to take place/
Must be greater than or equal to E1-E2 (photon energy)
What is the main difference between a particle causing excitation compared to a photon causing excitation?
A particle can excite more than one electron before losing all its energy, whereas the photon can only excite one electron
What is the ionisation energy of an atom?
The amount of energy needed to remove an electron from the ground state atom
How do fluorescent tubes work?
They contain mercury vapour, across which a high voltage is applied. This voltage accelerates fast-moving free electrons that ionise some of the mercury atoms, releasing more free electrons. When the flow of free electrons collides with the electrons of the mercury atom, the atomic mercury electrons are excited to a higher energy level. When these excited electrons return to ground state, they release energy be emitting photons in the UV range. A phosphorus coating on the inside of the tube absorbs these photons, exciting its electrons to higher energy levels. These electrons then travel down the energy levels, releasing their energy as lower energy photons of visible light
What do you get if you split the light from a fluorescent tube with a prism of diffraction grating?
A line spectrum
What is a line spectrum?
A pattern of lines produced by photons being emitted or absorbed by electrons moving between energy levels in an atom
What is a line emission spectrum?
A spectrum of bright lines on a dark background corresponding to different wavelengths of light that have been emitted from a light source
What do line spectra provide evidence for?
That the electrons in an atom exist in discrete energy levels
Which has the longest wavelength, red or blue light?
Red
What is a line absorption spectrum?
A light spectrum with dark lines corresponding to different wavelengths of light that have been absorbed
What do you get if you split white light up with a prism?
A continuous spectrum
Why do hot objects emit a continuous spectrum in the visible light and UV range?
Because they emit electromagnetic radiation at all wavelengths or colours
What is a blackbody?
An object that absorbs all the light falling on it, reflecting none of it, hence, it appears black
When do you get a line absorption spectrum?
When light with a continuous spectrum of energy passes through a cool gas
Why do you get a line absorption spectrum when white light passes through a cool gas?
At low temperatures, most of the electrons will be in the ground state. Photons of the correct wavelength are absorbed by the electrons to excite them to higher energy levels. These wavelengths are then missing from the continuous spectrum when it comes out the other side of the gas
How can you compare line absorption spectra with emission spectra?
For a particular gas, the black lines in the absorption spectra correspond to the bright lines in the emission spectra
What is wave-particle duality?
All particles have wave and particle properties (and waves have wave and particle properties)
What is the evidence for the electron having wave-like properties?
Electron diffraction and interference through a piece of graphite
What is the evidence for the electron having particle-like features?
The deflection of electron beam when shone through a magnetic or electric field
What is the evidence for light having wave-like properties?
Youngs Double Slit experiment
What is the evidence for light having particle-like properties?
Photoelectric effect
What is the equation for De Broglie wavelength?
Planck’s Constant / Momentum
h/p = h/mv
How can electrons be used to investigate the spacing between atoms in a crystal?
An electron beam will diffract when the De Broglie wavelength of the electrons is roughly the same size as the spaces between the atoms
Why do electron microscopes have a higher resolution than light microscopes?
A shorter wavelength gives smaller diffraction effects (diffraction effects blur detail on an image)
