5.4 - Quantum Physics

Question 1

What is Huygens principle

Answer 1

Scientist Christiaan Huygens came up with a principle for predicting the future movement of waves if we know the current position of the wavefront

Basic idea is to consider that at any and every point on the wavefront is a new source of circular waves travelling forward from that point

When the movement of these myriad (lots of) circular waves are plotted, and superstition is considered, the resultant wave will be the position of the new wave front

Question 2

What does Huygens principle explain

Answer 2

Huygens geometrical system exaclty explains all the basic phenomena we see light undergoing

Question 3

How can we use Huygens principle

Answer 3

We can make drawing to correctly predict the movement of wavefronts in reflection, retraction, diffraction, interference and straight line propagation (spreading out) of light

Question 4

How is the interference pattern produced by diffraction and Young’s two slit experiment /standing wave interference patterns Evidence for light as a wave

Answer 4

Diffraction and Young’s 2 slut experiment both require the superposition of wave displacements to generate the standing wave pattern seen

This is only possible if light is behaving as a wave and has the appropriate repeating cycles of displacement that cause this on going superposition to maintain the constructive and destructive interference that produces nodes and antinodes - particles can not superpose in this way

Question 5

How is polarisation evidence that light is a wave

Answer 5

Polarisation (the action of restricting the vibrations of a transverse wave, especially light, wholly or partially to one direction.) is another phenomenon exhibited by EM waves, including light that can only be explained in classical physics by using the ideas of waves

Question 6

What did max planck say

Answer 6

Made a suggestion in 1901 that light could exhaust as quantised packets of energy called photons

Question 7

What’s an argument for light is a particle

Answer 7

The photoelectric efffect, Einstein said the photoelectric effect cannot be explained using a wave theory for light, however the idea that light travels as particles or photons, whose energy is proportional to the frequency it would have when considered as a wave, fits all observations perfectly

Question 8

What’s plancks constant

Answer 8

Often symbol “h”

This constant has an extremely small value as it represents the fundamental minimum step in energy, h = 6.63 x 10^-34 Js

On very small scales, photons can’t have energy values that differ by less than the Planck constant

This means there are some energy values impossible in our universe !! Such a system of minimum sized steps is called quantisation

Question 9

What’s quantisation ???

Answer 9

A system of minimum sized steps is called quantisation

Eg plancks constant - there cannot be changes in energy of less than this amount !

Question 10

How do we calculate the energy of a photon (packet of light)

Answer 10

Photon energy (J) = plancks constant (J s) x frequency (Hz)

E = hf

Question 11

How were electrons proven as particles

Answer 11

Experiments that produce ions can demonstrate electrons behaving as particles Becuase a fixed lump of mass and charge is removed from the atom In order to change it to an ion

Robert Millikan did an oil drop experiment, to find the charge of the electron itself

The fact that electrons hold a fixed amount of charge and a fixed mass indicate they are localised particles

Question 12

What does the charge to mass ratio show

Answer 12

It’s a uniquely identifying property of particles, and was first demonstrated for the electron by J.J Thompson in 1897

Question 13

What’s the evidence electrons are waves

Answer 13

If electrons are made to travel at high speeds, they will pass through gaps and produce a diffraction pattern, they will similarly interact with a double-slit apparatus to produce the interference pattern as seen when waves pass through two slits. Diffraction and interference are not expected by classical particles, as they should simply travel straight through the slits. Observation of these experimental results proves that electrons can behave as waves

Question 14

Do we have a complete and perfect single theory that explains both correctly, for either behaviour of electrons or EM radiation

Answer 14

No ! Not yet broski

Wait up ❤️❤️❤️🤛🏼

Question 15

What’s the evidence for light as waves

Answer 15

Diffraction, interference, polarisation

Question 16

What’s the evidence for electrons as waves

Answer 16

Diffraction and interference

Question 17

What’s the evidence for light being particles

Answer 17

Photoelectric effect

Question 18

What’s evidence for electrons as particles

Answer 18

Ionisation

Question 19

What’s wave-particle duality

Answer 19

The principle that these things behave as waves under certain circumstances and as particles under other circumstances is known as wave-particle duality

Question 20

Define Photons

Answer 20

They are packets of electromagnetic radiation energy where the amount of energy E = hf, which is plancks constant multiplied by the frequency of the radiation : the quantum unit that is being considered when electromagnetic radiation is understood using a particle model

Question 21

Define quantisation

Answer 21

It’s the concept that there is a minimum smallest amount by which a quantity can change: infinitesimal changes are not permitted in a quantum universe . The quantisation of a quantity is analogous to the idea of the precision of an instrument measuring it

Question 22

Define wave particle duality

Answer 22

It’s the principle that the behaviour of em radiation can be described in terms of both waves and photons

Question 23

What are the photoelectrons in the photoelectric effect

Answer 23

If uv light is shone onto a negatively charged zinc plate, the plate loses its charge

The explanation for this is that the light causes electrons to leave the metal, removing the negative charge

The electrons that are released are called photoelectrons

Question 24

What’s the work function

Answer 24

It’s symbol is phi like a circle with line in the middle

There is a certain minimum amount of energy that an electron needs in order to escape the surface of the metal it is in

Energy is called the work function

Question 25

How is the photoelectric effect eveidence for light as a particle and how we prove threshold frequency

Answer 25

The wave theory of light would allow some of the wave energy to be passed to the electrons to enable them to gain the work function and escape. In the wave theory, the energy carried by a wave depends on its amplitude, which means for light it’s brightness/ intensity. So the wave theory would predict that any colour of light, if made sufficiently bright - should enable the release of photoelectrons. But if waves were of smaller amplitude, it could shine for longer and slowly pass energy to the electrons until they had gained as Much as the work function and escape.
However this is not what is observed, there is a a maximum wavelength for the light, above which no photoelectrons are emitted - can also be thought of as a minimum frequency or threshold frequency

Question 26

Practical: investigating photoelectrons

Answer 26

If you charge a gold leaf electroscope then it will show a deflection on the gold leaf. if you place a zinc plate on top of the electroscope, the photoelectric effect will allow the electroscope to be discharged if it held a negative charge. If you shine UV light onto the zinc plate, the gold leaf will fall immediately. This observation shows that electrons on the zinc plate have escaped. Those electrons remaining on the gold leaf and electroscope stalk spread out more, so that there is no longer a mutual repulsion to hold up the gold leaf.

You may need to clean the surface of the zinc, as it oxidises quite easily in air, so there may be a layer of oxide in top of the pure metal, stoping the light absorption.

If you attempt to discharge the electroscope with an ordinary desk lamp, or sunlight, there will be very little effect as these sources provide little UV light intensity, and longer wavelengths will not cause the photoelectric effect with zinc.

Question 27

Summarise an explanation for observations during the photoelectric effect

Answer 27

Light travels as photons, with a photons energy proportional to the frequency

When a photon encounters an electron, it transfers all its energy to the electron (the photon ceases to exist)

If an electron gains sufficient energy - more than the work function - it can escape the surface of the metal as a photo electron.

Brighter illumination means more photons per second, which will mean a greater number of photoelectrons emitted per second.

If an electron does not gain sufficient energy from an encounter with a photon to escape the metal surface, it will transfer the energy gained from the photon to the metal as whole before it can interact with another photon. Thus, if the photon energy is too low, no photoelectrons are observed.

Question 28

What is the equation linking energy of a photon to frequency

Answer 28

E = hf

E = energy of a photon 
h = plancks constant
f = frequency of light

Question 29

What is the photoelectric effect based on

Answer 29

The conservation of energy

Question 30

What is Einstein’s photoelectric effect equation

Answer 30

1/2mv^2(subscript max) = hf - phi symbol (work function)

Or, in direct conservation of energy terms, the photon energy is used by the electron partly to escape the metal surface and partly as its kinetic energy. So a rearrangement is:

Hf = phi + 1/2mv^2(subscript max)

Question 31

What are the SI units for the terms in the photoelectric effect equation

Answer 31

The SI units are joules (J) but it is often used in units of electron volts (eV)

Remember 1eV = 1.6 x 10^-19 J

Question 32

How can we used stopping voltage(Vs) to give us max kinetic energy

Answer 32

1/2mv^2(subscript max) = e x V(subscript s)

Question 33

If we plot a graph y = mx + c

Equal to 1/2mv^2 = hf - phi - what happens

Answer 33

The y intercept will represent the work function value, when the value of y is zero, (the x intercept) then the photon energy must be equal to the photon - since max KE is on the y axis
- this means that the value of the x intercept will give the threshold frequency for the metal

Question 34

Define photoelectrons

Answer 34

Photoelectrons are electrons released from a metal surface as a result of its exposure to electromagnetic radiation

Question 35

Define the work function

Answer 35

The work function is the minimum energy needed by an electron at the surface of a metal to escape from the metal

Question 36

Define the threshold frequency

Answer 36

The threshold frequency for a given metal is the minimum frequency of electromagnetic radiation that can cause the emission of photoelectrons from the metal.

Question 37

Define the stopping voltage

Answer 37

The stopping voltage in an appropriately illuminated photoelectric cell is the minimum voltage needed to reduce the photoelectric current to zero.

Question 38

What’s the de Broglie equation

Answer 38

A french prince called Louis de Broglie suggested electrons could behave as waves and proposed an equation to calculate their wavelength. Their wavelength is inversely proportional to the momentum they have when considered as particles

Electron wavelength (m) = plancks constant (J s) / momentum (kg m s^-1)

lander = h/p

Question 39

How can we investigate electron diffraction

Answer 39

Using an electron beam diffraction tube, this accelerates a beam of electrons through a high voltage, and then passes the beam through a sliver of graphite. The array of carbon atoms in the graphite act as a diffraction granting in two dimensions, which produces a circular diffraction pattern. The front end of the tube has a phosphorescent screen that will show up the diffraction pattern.

If we make careful measurements of the dimensions of the tube and the diffraction pattern produced with different accelerating voltages, we can carry out an approximate calculation of the atom spacing in graphite.

Question 40

Tell be about two slit electron interference

Answer 40

Richard Feynman suggested that electrons should also be able to produce the two slit interference pattern seen with light, as they can behave with waves. Recently, this has been shown to be the case, giving further evidence for the wave nature of electrons.

However, largest research has shown electrons are behaving as both individual particles and waves at the same time. They are showing wave particle duality.

Question 41

Tell me about electron microscopy

Answer 41

One of the most important applications of the wave nature of electrons is it’s use to study objects at a very small scale. An electron beam can work like a beam of light, but with some different properties that can make the electron beam much more useful for microscopy.

The main advantage is that the wavelength of an electron beam can be controlled by altering the voltage applied to accelerate the electrons. The wavelength can be adjusted to same size of atom so that even atoms can be imaged. A rough rule of thumb is the minimum sized object that can be imaged is the minimum wavelength.

Question 42

What are energy levels

Answer 42

The energy values that the electrons could have are limited to a small number of exact values, often called energy levels
- small energy bands

Question 43

What’s the ground state

Answer 43

This is the lowest energy level of an electron, with a quantum number (or level) of n = 1.

Question 44

What is excitation

Answer 44

In order for an electron to move up energy levels, the electron must take in some energy. This is excitation. Electrons can become excited if the atom collides with another particle. Alternatively if the electron absorbs a photon that has exactly the correct amount of energy, the electron can jump to a higher energy level.

A photon with exactly a specific energy difference, could be absorbed by an electron in the ground state which would lift the electron up to the energy level above the ground state, the photon would no longer exist as a result.

Question 45

What happens when an incident photon does not have energy exactly equivalent to a jump between the current position of the electron and one of the higher levels.

Answer 45

The photon will not be absorbed - the photon and electron will not interact at all.

If gas atoms are illuminated by a range of frequencies (colours), those with the correct frequency values will be absorbed, so there will be some colours missing from the light after it passes through the gas.

Question 46

What is de-excitation

Answer 46

If an electron is already excited, after a random amount of time it will de-excite. This may include dropping straight down to the ground state, or it may drop to an intermediate level if there is one. This energy is emitted as a photon, with exaclty the same energy difference between the levels. The frequency of the emitted photon can be calculated from e = hf.

Question 47

What’s ionisation energy

Answer 47

The energy required to ionise an atom in its ground state - to liberate the electron from the atom so it’s free.

Question 48

What’s a line spectrum / spectra

Answer 48

Light made up of multiple wavelengths can be split up to show which colours are present. This could be done using a diffraction grating in which the amount of diffraction is dependant on the wavelength, and so various colours will spread differnt amounts. The resulting spectrum will often be a series of individual lines, if the original light contained only a select few wavelengths. Such a line spectrum is the typical result of exciting the atoms of a gas, perhaps by heating a gas.

Each coloured line is a wavelength of light given off as a result of an electron dropping between two energy levels. The different energy gaps cause the difference in wavelengths emitted.

Question 49

How can we investigate gas discharge spectra

Answer 49

A high voltage will cause an electric current to pass through a gas in a discharge tube. The electrical energy excites the electrons in each atom of the gas, and then they drop energy levels at random times, giving off a photon for each energy level transition. As there are so many atoms, it appears as if the gas is continuously emitting light, and of all the possible colours - all the possible transitions within its energy level ladder. Using a diffraction grating, we can analyse the light emitted from the tube to detect the separate colours, or wavelengths, being emitted.we observe a line spectrum.

Question 50

What is the intensity of radiation / light

Answer 50

When a lamp emits light, we could measure its intensity. This is the amount of energy it carries per unit area and per unit time. Power is the rate of energy transfer, so this becomes:

Intensity (W m^-2) = power (W) / area (m^2)

I = P/A

Question 51

Define ground state

Answer 51

This is the lowest energy level for a system. For example, when all the electrons in an atom are in the lowest energy levels they can occupy, the atom is said to be in its ground state.

Question 52

Define excitation

Answer 52

Excitation is an energy state for a system that is higher energy than the ground state. For example, in an atom, if an electron is in a higher energy level than the ground state, that atom is said to be excited.

Question 53

Define ionisation energy

Answer 53

It’s the minimum energy required by an electron in an atoms ground state in order to remove the electron completely from the atom.

Question 54

Define a line spectrum

Answer 54

It’s a series of individual lines of colour showing the frequencies present in a light source.