Unit 2.7 - Photons Flashcards
Draw and label the set up for the gold leaf experiment
(See notes)
What are the exact conditions required that caused the golf leaf to fall?
Negative charge, not positive
Zinc, not chromium
UV light, not white light
No sheet of glass between the UV and zinc
What happens when all the conditions were correct with the gold leaf experiment?
The leaf fell
What does placing a sheet of glass between the UV and the zinc do to the photoelectric effect in the gold leaf experiment? Why?
Stops the effect
Glass is opaque to UV light
How do we set up the gold leaf experiment?
Make the top charges first by attaching it to the negative site of the power pack
What does the gold leaf experiment test for?
The photoelectric effect
What happens when you touch the zinc on the top of the gold leaf experiment?
Pushes additional electrons onto the plate, which are pushed down the metal stem and leaf
Why is the leaf lifted in the first place in the gold leaf experiment?
Negative charges in the metal stem and leaf = repelling each other
The leaf is free to move so it lifts up
What does white light contain?
All visible colours
Which effect causes the gold leaf to fall during the experiment?
The photoelectric effect
The photoelectric effect
The emission of electrons when electromagnetic radiation, such as light, hits a material
What happens to the apparatus of the gold leaf that causes it to fall when the photoelectric effect occurs?
The apparatus discharged, so the gold leaf fell
Would the gold leaf fall without the photoelectric effect eventually? Why?
Yes
Electrons would jump onto air particles
Why does UV light allow electrons to escape, but visible light doesn’t?
When you shine a light on metal, energy isn’t arriving in a continuous stream - it arrives in packets known as photons
Energy of a photon
E = hf
UV light has a higher frequency
Photons
Packets of energy
What are photons in terms of light?
Complete units
How do photons interact with matter?
In a quantised way
Energy of a photon equation
E = hf
What does the energy of a photon increase with?
Frequency
Photoelectrons
The rest of the photons energy after dislodging the electron
Free negative charge
What are the number of photoelectrons in direct proportion with?
The intensity of the incident light
Relationship between the number of photoelectrons and the intensity of incident light
Directly proportional
What does the kinetic energy of photoelectrons vary from?
Zero to maximum
What does the maximum kinetic energy of photoelectrons depend on?
The frequency of the incident radiation
What produces photoelectrons of higher kinetic energy - weak blue light or bright red light? Why?
Weak blue light
Higher frequency
What produces the most photoelectrons - weak blue light or bright red light? Why?
Bright red light
Red radiation - intensity
What is required for any emission to occur?
A minimum frequency, regardless of intensity
What is the name for the minimum frequency required for emission to occur?
Threshold frequency
What does a threshold frequency depend on?
The electronegativity of the material itself
How long does it take between the absorption of radiation and the emission of electrons?
Happens instantly
What does 1 photon of light striking a surface release according to Einstein?
1 electron
Who’s equation was E = hf?
Einstein’s
What happens to a photon as it strikes a surface?
It is absorbed (ceases to be a photon)
It’s energy is given to the metal surface and the electron
Describe what happens to an electron right at the surface of a metal when hit by a photon
Is at the surface and requires the least possible energy to liberate it
= escapes with the maximum kinetic energy
Describe what happens during the emission of an electron that’s deep inside the metal
Is deep within the metal and it has lost so much kinetic energy by the time it reaches the surface that it is attracted back and doesn’t escape
What happens to an electron when hit by a photon relatively near th surface?
Is slightly deeper than at the surface so escapes with slightly less kinetic energy
What happens to an electron facing away from a photon when hit at the surface?
Gains enough energy to escape but it is moving in the wrong direction and so it absorbed by the metal
Draw and label a vacuum photocell
(See notes)
What is done on the plate on the right of a vacuum photocell?
Light of a certain frequency is shone onto the metal plate
What condition must be true for the electrons to be released from the surface on a vacuum photocell?
If the photon energy is high enough (E = hf)
What do electrons do in a photon microcell and why?
Since the capsule contains a vacuum, the electrons will cross the gap without colliding with anything and reach the metal plate on the left
What will the electrons hitting the metal plate on the left on a vacuum photometer give us?
A reading of a current on a milli-ammeter
Will electrons reach the metal plate on a vacuum photocell without a power supply? Why?
Yes
The electron released will have a kinetic energy which causes them to cross the gap
What other type of graph does Ekmax against frequency have the same shape as and why?
Stopping voltage against frequency
Since stopping voltage acts as a measure of maximum electron kinetic energy
Draw and label 2 graphs of stopping velocity and then Ekmax against frequency of light
(See notes)
How can we measure the kinetic energy of the electrons in a vacuum photocell?
Use the power supply to try and stop them from crossing the gap
A is made negative so that the electrons start to be repelled
Measuring on the milli-ammeter, as soon as the current reaches zero, it means that the voltage across the gap has stopped the most energetic photons (those with maximum kinetic energy)
The energy is calculated by multiplying the charge of an electron with the voltage (called the stopping potential)
Ekmax = e x V
What current do we want on a vacuum photocell and why?
Zero
This means that the voltage across the gap has stopped the most energetic electrons (those with maximum kinetic energy)
Stopping potential
the voltage across the gap in a vacuum photocell has stopped the most energetic electrons (those with maximum kinetic energy)
The voltage across the gap in a vacuum photocell has stopped the most energetic electrons (those with maximum kinetic energy)
Stopping potential
Maximum kinetic energy equation in a vacuum photocell
Ekmax = e x V
What do the electrons do at stopping potential in a vacuum photocell?
The fastest electrons almost reach the negatively charges ended of the battery before turning back around
What do the electrons do at stopping potential in a vacuum photocell?
The fastest electrons almost reach the negatively charged end of the battery before turning back around
What is the vacuum photocell experiment repeated for?
Various frequencies of light
What actually causes an electron to be released from a surface?
Potential energy gain
What does the energy of an electron after gaining potential energy depend upon?
How tight the electron is tied to the metal surface
What would the potential energy gain of an electron also give it?
Any excess energy, which is converted into kinetic energy
What is the excess energy of an electron converted into?
Kinetic energy
What is converted into kinetic energy for an electron?
The excess energy left over from that required to escape the metal
What happens to the energy of a photon when given to an electron?
Some - transferred to allow it to escape
Left over - available as kinetic energy
What is photon energy equivalent to in terms of electrons?
Energy needed for electrons to escape the surface + kinetic energy of electrons
Give the following expression in equation form
Photon energy = Energy needed for electrons to escape the surface + kinetic energy of electrons
hf = Φ + Ek
What will an electron that only requires a small amount of energy to escape the surface have?
Plenty of energy left over front the photon as kinetic energy
When Φ is at a minimum…
….Ek is at a maximum
What is the equation for the maximum kinetic energy of an electron and where does this come from?
Ekmax = hf - Φ
Photon energy = the energy needed for electrons to escape the surface + kinetic energy of electrons
hf = Φ + Ek
An electron that needs only a small amount of energy to escape t he surface will have plenty of energy left over from the photon as kinetic energy
That is, when Φ is at a minimum, Ek will be at a maximum
hf = Φ + Ekmax
(Rearranged…)
Ekmax = hf - Φ
Work function symbol
Φ
Φ meaning
Work function
Work function
The minimum energy required by an electron in the surface of a metal that allows it to escape
Work function unit
Joules
What type of graph can we draw from the equation Ekmax = hf - Φ?
Ekmax against frequency
What do the different symbols represent on an Ekmax against frequency graph of the following equation?
Ekmax = hf - Φ
h = gradient
Φ = y - intercept
x-axis = frequency
y-axis = Ekmax
Why does the intercept of y = mx + c have a negative value?
Crosses the y-axis under the x-axis
What’s the same about every single Ekmax against frequency graph no matter that the metal surface is?
The gradient (h)
What’s different about different Ekmax against frequency graphs for different materials?
Different intercepts (work function)
Different threshold frequencies (touch the x-axis)
Why do different maximum kinetic energy against frequency graphs have different work functions?
It’s a property of the metal itself
Where is the threshold frequency on an Ekmax against frequency graph?
Where it hits the x-axis
Draw and label a graph of Ekmax against frequency
(See notes)
How would a line for a material of greater work function be different on a Ekmax against frequency graph be different to another?
Greater work function = shifts to the right
(Same gradient, different work function and threshold frequency)
Frequency threshold
The minimum frequency of the light than can begin emission of electrons from the surface
The minimum frequency of the light than can begin emission of electrons from the surface
Frequency threshold
What is frequency threshold related to?
The work function
When will the electron energy be zero at the frequency threshold and why?
When a photon with a frequency high enough to raise an electron to the surface, but no more than Ek(min) = Φ will mean that the electron energy will be zero here (no extra energy)
Value for Ekmax at the treshold frequency
Zero
Symbol for threshold frequency
Fmin
What is threshold frequency equal to? Why?
The work function
Hfmin = Ekmin + Φ
At the threshold frequency, Ekmax = 0
So, Fmin = Φ
What will happen at a frequency lower than the threshold frequency?
No electrons will be emitted
1ev
The kinetic energy gained by an electron accelerated over a potential different of 1V
The kinetic energy gained by an electron accelerated over a potential different of 1V
1eV
At which frequency will no electrons be emitted?
A frequency lower than the threshold frequency
What do we mean when we say that an electron is “accelerated”?
Put in an electric field
Pushed the opposite way to positive charges
This push = accelerates them to the other side
Why do we use the electron volt?
For convenience as the volt is too large
Converting from eV to J
x1.6x10^-19
Converting from J to eV
Divide by 1.6x10^-19
Wavelength of the visible part of the em spectrum, including colours
700nm (red end) to 400nm (violet end)
How do we work out the typical photon energies for em radiations?
E = hc
—
λ
What can we see when looking at the visible light spectrum of a hot gas through a spectrometer?
We can recognise elements present in a sample by seeing lines at their wavelengths
What do we look thought o recognise elements present in a sample by looking at their wavelengths?
Visible light spectrum of a hot gas through a spectrometer
What elements are present in a street lamp?
Sodium vapour
How does a line spectrum give us information about the structure of an atom?
The fact that every sample of a particular element produced the same 2 visible wavelengths suggests tht there is something in the make-up of the atoms that causes it
What is an emission spectrum caused by?
High temperatures
What type of spectrum is caused by high temperatures?
Emission spectra
What does heat travel by?
Radiation
Which part of the spectrum does heat usually travel by as radiation?
Infrared
How does the wavelength of heat reach the visible part of the spectrum?
If the energy is high enough
What happens when the energy of heat that’s travelling by radiation is high enough?
The wavelength will reach the visible part of the em spectrum
What is the emission spectrum NOT and why?
Not a quantum effect
It is continuous
How do we know that the emission spectrum is not a quantum effect?
The wavelengths are not a “required or allowed amount”
(Quantum = discrete packets)
What do we observe on a line absorption spectrum?
Dark bands on a coloured background
What do the lines on a line absorption spectrum correspond to?
The bright bands in the emission spectrum
What do we observe as the emission spectrum?
Coloured lines on a dark background
What does each day line on the line absorption spectrum correspond to?
A photon of specific wavelength being absorbed
What is the energy of a photon being absorbed equal to?
The difference in energy between 2 energy levels
How can we calculate he energy of a photon using wavelength?
E = hc
—-
λ
What were the problems to solve in explaining the hydrogen spectrum?
Why the following equation worked
Frequency = 3.25x10^15 (1/m^2 - 1/2^2)
An electron in an orbit should lose energy continuously
Who solved the hydrogen spectrum?
Nihls Bohr
How do electrons orbit a nucleus?
In allowed orbits only
What happens to electrons in their allowed orbits?
They do not lose energy
They possess a certain energy called the orbit energy
What happens when an electron moves from a higher energy orbit to a lower one?
Higher orbit = E2
Lower orbit = E1
Emits the energy difference between E2 and E1, as radiation with the energy
E2 - E1 = hf
What does E2-E1 represent?
The energy jump of an electron
What does hf represent when electrons fall from a higher energy level to another?
The photon of energy emitted
The orbit energy to an element?
A feature of it
How do electrons move from one orbit to another?
By gaining or losing energy
What can electrons NOT do in terms of the orbits that they can be on and why?
Cannot fall between 2 orbits
Orbit energy is quantised
What does the fact that orbit energy is quantised mean?
Electrons cannot receive or lose energy so that it falls BETWEEN 2 orbits
What’s the energy of an electron ready to leave an atom?
Zero
What do the orbit energy within an atom have?
Negative values
What do the ideas expressed by Nihls Bohr explain and what do they only work for?
Explain the origin of the visible electromagnetic radiation BUT only work for a hydrogen atom
What do m and n represent in the equation frequency= 3.25x10^15 (1/m^2-1/n^2)?
M = where the electron finishes its jump (E1)
N = where the electron starts its jump (E2)
How do electron move between levels?
By absorbing or emitting energy, usually in the form of photons
How are the orbits of an atom labelled?
n = 1 to n = infinity
Describe the energy of an electron at n=1 and explain this
More negative energy
Closer to the nucleus = the minimum energy
Describe the energy at n=infinity
The orbit at the boundary of an atom, where the orbit energy is zero
What is bigger, a UV or IR “jump”? What does this tell us?
UV is bigger
The emitted photon is more energetic as E2-E1 = hf
What do we talk about to apply the ideas of Nihls Bohr to all atoms?
Talk about energy levels rather than orbit energy
What does an increase in orbit size suggest?
That the atom grows as the orbit increases
How does the energy of the levels change as we move down them?
Becomes more negative
How are the jumps between the higher energy levels different?
Much smaller jumps
What is n=1 also known as?
The ground state
Which energy state do electrons “prefer” to be in?
The lowest energy state
Where would a hydrogen electron “prefer” to be?
In the ground state
What is an electron said to be if it’s excited to another energy level?
Excited
Describe in electron in an excited state
Unstable
What happens to electrons when they’re in an excited state and why?
Unstable state
Stays or about 0.1mm before returning to the ground state
When is an atom ionised?
When one electron recovered enough energy to completely escape the atom (i.e - to reach n = infinity)
What energy level must an electron reach for the atom to become ionised?
n = infinity
Ionisation energy
The energy required to release the least bound electron from the atom
The energy required to release the least bound electron from the atom
Ionisation energy
What is the ionisation energy for hydrogen?
The energy required to move the electron from the ground state to n = infinity
What do electrons have characteristics of?
Both particles and waves
Who applied the wave theory to electrons?
De Broglie
What’s the name for the wave theory being applied to electrons?
The DeBroglie relation
Draw and label an electron diffraction tube
(See notes)
How is an electron diffraction tube set up?
The tube has a cathode (an electron “gun” ) at the base
An anode at the neck to the bulb
Where are electrons attracted to in an electron diffraction tube?
The anode
What do electrons pass through in an electron diffraction tube?
A fine sheet of graphite
What is an electron diffraction tube used to demonstrate?
That electrons have characteristics of waves
Why do the electrons pass through a fine sheet of graphite in an electron diffraction tube?
Crystalline structure
Behaves as a 2D diffraction gratin g
What is the front of the bulb coated in in an electron diffraction tube?
Phosphorus
Why is the front of the bulb coated in phosphorus in an electron diffraction tube?
Glows green when electrons strike it
Draw and label the pattern observed on the bulb in n electron diffraction tube
(See notes)
Why are electrons accelerated in an electron diffraction tube?
For more diffraction
How would we receive more diffraction in an electron diffraction tube?
Accelerate the electrons
How does the pattern produced by an electron diffraction tube support the idea that the electron beam is behaving as a wave?
We see an interference pattern emerging due to the stream of electrons, which is a property of waves
The electron waves diffract in 2 dimensions, producing dark bands where destructive interference happens and bright bands where they constructively interfere
How does the emission of light from the fluorescent screen in an electron diffraction tube show that the electrons incident on it are behaving as particles?
Each electron transfers kinetic energy to 1 electron in the phosphorus
This then gets excited and then falls to to lowest energy level (ground state), in turn releasing 1 photon
This means than 1 electron causes 1 photon emission, which is a particle property
Which part of the electron diffraction tube shows electrons acting as waves?
The pattern produced
Which part of the electron diffraction tube shows electrons acting as particles?
The emission of light from the fluorescent screen
Matter waves
Refer to the wave nature of objects with mass
Do all matter have wave-like properties?
Yes
What does it mean that all matter have wave-like properties?
They will have matter and wave properties
What links the matter and wave properties of electrons and all matter?
p = h
—
λ
What does p represent in p = h ?
—
λ
Momentum
Momentum equation
p = mv
What does h represent in p = h ?
—
λ
Planck constant
What does λ represent in p = h ?
—
λ
Wavelength of the particle/wave
What is a sign to use the p = m/λ equation?
Whenever a question mentions “de Broglie wavelengths”
Which equation do we use if a question mentions “de Broglie wavelengths”?
p = h/λ
What does the p = h/λ equation mean?
Anything that as a wavelength can have momentum
Radiation or “light” can except a pressure on a surface
If all matter have was-like properties, why don’t we see things diffracting on a day-to-day basis?
Wave diffraction is best observed by passing a wave through a gap about as wide as its wavelength
Something like a tennis ball would have a tiny de Broglie wavelength, and to see it diffract, we would need a slit this distance wide
When is wave diffraction best observed?
By passing a wave through a gap about as wide as its wavelength
What does Newton’s second law state?
Force is proportional to the rate of change of momentum
Newton’s second law (in terms of momentum) in an equation
F = ΔP/t
Which law does F = Δp/t express?
Newton’s second law
What do photons carry?
Momentum
What happens to a photons momentum when they’re absorbed or reflected by a surface?
Changes
When does a photon’s momentum change?
When they’re absorbed or reflected by a surface
What does the fact that a photons momentum changes when they’re absorbed or reflected by a surface mean?
They will exert a force on the surface (Newton’s second law)
Pressure equation
Force
———
Area
Why do photons exert radiation pressure on a surface when reflected or absorbed?
Photons carry momentum, which changes when they are absorbed or reflected by a surface
Therefore, they will exert a force on the surface
Pressure is the force per unit area
Pressure = force
———
Area
What do photons exert on a surface when they’re absorbed or reflected?
Radiation pressure
What is the force on a surface if a photon is absorbed?
The rate of transfer of momentum of the photon
What happens to a photon when its absorbed by a surface?
It ceases to exist
Describe the incident and the reflected path when a photon is reflected on a surface
In the same line
How do we subtract vectors?
Flip them over and add them together
What kind of measurement is momentum?
Vector
Show the calculation for working out the change in momentum of a photon when t’s reflected by a surface
Δp = preflected - pincident
Δp = -preflected +-pincident
=-2p
What’s the relationship between the momentum before and after an incident?
Momentum before = momentum after
What’s most effected - to have a photon reflected or absorbed? By how much?
Twice as effective to have a photon reflected than absorbed
What is radiation pressure applied in?
Space propulsion
How do we calculate the number of photons absorbed by a surface each second?
N = power
———
Energy of 1 photon
What is the momentum of a proton after being absorbed?
Zero
What will the change of momentum each second be when photons are absorbed?
The sum of the momentum of each photon
Number of photons absorbed each second x momentum of 1 photon
How do we calculate the force photos exert on a surface?
F = λp
—
t
Equation for measuring the kinetic energy of electrons in a vacuum photocell
Ekmax = e x V
What’s the wavelength at when at the threshold frequency?
At its longest
Why exactly do electrons behave as waves in an electron diffraction tube?
They diffraction as atoms behave as a diffraction grating
The wavelength of the beam is similar to the atom spacing
What happens to the interference pattern produced by electrons in an electron diffraction tube if a different material with smaller spaces between the atoms is used? Why?
The intensity decreases as the radius is greater
What’s essential to ensure when completing the experiment with the vacuum photocell to measure the kinetic energy of electrons?
Correct polarity
An ammeter
Voltmeter connected correctly
A variable supply
What type of emission of a photon does spontaneous emission cause?
Random phase and random direction
How do we alter the pd when trying to achieve a current of zero in the vacuum photocell to determine Ekmax?
Increase it
What can be referenced when discussing the electron diffraction tube?
Wave particle duality
What momentum do photons have if reflected by a surface?
Twice the momentum
When do photons have twice the momentum?
When reflected by a surface
Calculating the number of photons emitted per second
Power
———
Energy of 1 photon
Calculating the number of electrons emitted per second
Current
————
Charge of an electron
What’s the assumption made when calculating the number of electrons emitted per second?
All emitted electrons are captured
Calculating the probability of a photon emitting an electron
Number of electrons emitted per second
————————————————————
Number of photons emitted per second
How many photons are required to eject a single electron from a surface?
1 photon
Where do electrons diffract in the electron diffraction tube?
Between atoms
Which mass do we use if electrons are in a de Broglie wavelength question?
Mass of an electron = 9.11x10^-31 (in data sheet)
