7: Quantum Behaviour

Question 1

What is a quantum?

Answer 1

A single discrete packet of EM radiation

Question 2

What did Max Planck suggest about how EM waves could be released?

Answer 2

Said they can only be released discrete packets

Question 3

What did Einstein suggest the EM waves could exist as?

Answer 3

E M waves, and the energy they carry, can only exist in discrete packets. These wave packets are called photons

Question 4

How did Einstein believe that a photon acts?

Answer 4

As a particle. And will either transfer all or none of its energy when interacting with another particle

Question 5

Do photons have charge?

Answer 5

No

Question 6

What is an electronvolt defined as?

Answer 6

The kinetic energy gained by an electron when it is accelerated through a potential difference of 1 Volt

Question 7

What is threshold voltage? (LEDs)

Answer 7

The minimum voltage at which current will pass through an LED

Question 8

Explain the threshold voltage, for LEDs

Answer 8

This is the voltage needed to give the electrons the same energy as a photon emitted by the LED. All of the electron’s kinetic energy after it is accelerated over this potential difference is transferred into a photon

Question 9

Describe the set up for the experiment to find the Planck constant using LEDs

Answer 9

Connect an LED, of known wavelength, to the electrical circuit.
Connect an ammeter in series with the LED and a resistor with your power source. Put a voltmeter in parallel with your LED.
Close any blackout blinds and place a shaded tube over the LED to look through. The room should be as dark as possible so you can see when the LED first begins to make light

Question 10

Describe the experiment to find the Planck constant using LEDs

Answer 10

Start off with no current flowing through the circuit, then adjust the variable power source until the current just begins to flow through the circuit and the LED lights up. Record the voltage across the LED.
Repeat this experiment with a number of LEDs of different colours that emit light at different wavelengths. Plot a graph of threshold voltage against frequency. You should get a straight line graph with the gradient of h/e. Find h. Repeat the experiment to find an average value of h

Question 11

What is the photoelectric effect?

Answer 11

When a light with a high enough frequency is shone onto the surface of the metal, and causes electrons to be emitted. For most metals, this frequency falls in the UV range

Question 12

What is the photoelectric effect evidence for?

Answer 12

The theory that light is quantised. It can’t be explained with wave theory

Question 13

Describe how the photoelectric effect works

Answer 13

Free electrons on the surface of the metal absorb energy from the light, making them vibrate
If an electron absorbs enough energy, the bonds holding it to the metal break and the electron is released
The electrons emitted are called photoelectrons

Question 14

What are the conclusions of the photoelectric effect?

Answer 14

1) For a given metal, no photoelectrons are emitted if the radiation has a frequency below the threshold frequency
2) The photoelectrons are emitted with a variety of kinetic energies ranging from 0 to a max. value. This value of maximum kinetic energy increase with the frequency of the radiation, and is unaffected by the intensity of the radiation

Question 15

What is the relationship between photoelectrons emitted and intensity of the radiation?

Answer 15

The number of photoelectrons emitted per second is proportional to the intensity of the radiation

Question 16

Describe wave theory (Energy, intensity) and how it describes what would happen to electrons on the surface of metals

Answer 16

For a particular frequency of light, the energy carried is proportional to the intensity of the beam
The energy carried by the light would be spread evenly over the wavefront
Each electron on the surface of the metal would gain a bit of energy from each incoming wave
Gradually, each electron would gain enough energy to leave the metal

Question 17

Why can wave theory not explain the photoelectric effect?

Answer 17

If the light had a lower frequency it would take longer for the electrons to get enough energy – but it would happen eventually. There is no explanation for the threshold frequency
The higher the intensity of the wave, the more energy it should transfer to each electron – kinetic energy increase with intensity. There is no explanation for the kinetic energy depending only on the frequency

Question 18

Explain the photoelectric effect in terms of the photon model - why does the frequency affect the kinetic energy of the electrons, not intensity of the light

Answer 18

When light hits the surface, the matter was bombarded by photons
If one of these photos is absorbed by a free electron, the electron will gain energy equal to hf. So higher frequency will result in a higher kinetic energy
Each electron only absorbs one photon at a time, so all the energy it needs to gain before it can be released must come from that one photon
So an increase in the intensity of the light won’t affect the kinetic energy of the electrons – only the frequency will

Question 19

What is the work function energy?

Answer 19

Before an electron can leave the surface of the metal, it needs enough energy to break the bonds holding it there. This is the work function energy and its value depends on the metal

Question 20

How can threshold frequency be explained by the photon model?

Answer 20

If the energy gained by an electron from a photon is greater than the work function, the electron is emitted. If it isn’t, the metal will heat up, but no electrons will be emitted. Since, for electrons to be released, hf ≥ Φ, the threshold frequency must be f = Φ/h

Question 21

Electrons in atoms exist in well-defined [ ]

Answer 21

Discrete energy levels

Question 22

Which energy level is n=1?

Answer 22

The ground state

Question 23

How can electrons move down energy levels?

Answer 23

By emitting a photon

Question 24

Why is the energy of a photon, emitted by electron when it moves down an energy level, always one of some certain values?

Answer 24

The transitions are between definite energy levels

Question 25

What is the energy of a photon equal to, when an electron moves down an energy level?

Answer 25

The difference in energy is between the two levels

Question 26

[ ] gases produce line emission spectra

Answer 26

Hot

Question 27

What is excitation?

Answer 27

When the atoms in a gas becoming excited. If you heat a gas to a high temperature, many of its electrons move to higher energy levels

Question 28

Why do hot gases produce line emission spectra?

Answer 28

The electrons move to higher energy levels. As they fall back to the ground state, these electrons emit energy as photons. If you split the light from the hot gas with a prism or diffraction grating, you get a line spectrum.

Question 29

What does the line emission spectrum look like for a hot gas? What do the lines mean?

Answer 29

Bright lines against a black background. Each line on the spectrum corresponds to a particular wavelength of the light emitted by the source. Since only certain photon energies are allowed, you only see the corresponding wavelengths

Question 30

Describe the absorption spectrum, when white light is shone through a cool gas

Answer 30

The spectrum of white light is continuous. It contains all possible wavelengths

Question 31

Where do hot things emit a continuous spectrum?

Answer 31

In the visible and infrared parts

Question 32

[ ] gases remove certain wavelengths from the continuous spectrum

Answer 32

Cool

Question 33

When do you get line absorption spectra?

Answer 33

When light from a continuous spectrum of energy passes through a cool gas

Question 34

In which state will most of the electrons, in a cool gas, be?

Answer 34

Ground state

Question 35

Explain how absorption spectra work

Answer 35

Most electrons in the ground states. Photons of the correct wavelength are absorbed by the electrons to excite them to higher energy levels. These wavelength are then missing from the continuous spectrum when it comes out the other side of the gas.

Question 36

What does an absorption spectrum look like?

Answer 36

Continuous spectrum, with black lines in it corresponding to the absorbed wavelength

Question 37

What happens, if you compare the absorption and emission spectra of a particular gas?

Answer 37

The black lines in the absorption spectra match up to the bright lines in the emission spectrum

Question 38

What did Feynman suggest about the path of a photon?

Answer 38

Instead of just taking one route to the detector, the photon will take all the possible parts to the detector in one go.

Question 39

How can you work out the frequency of the phaser of a photon?

Answer 39

f = E/h

Question 40

How can you find the probability that a quantum will arrive at a point?

Answer 40

Square the resultant phasor amplitude

Question 41

Photon: describe the relationship between brightness and probability

Answer 41

The more probable it is that a photon will arrive at a point, the brighter it will appear

Question 42

Photons: which path is the quickest route?

Answer 42

The path that gives the highest probability. A straight line

Question 43

Which phasor will contribute the most to the resultant amplitude and probability of a quantum arriving at the end of the quickest path?

Answer 43

The final phasor

Question 44

Explain refraction in terms of quantum behaviour and phasors

Answer 44

When light travels in water, it slows down, but its frequency stays the same. This means the photons still have the same energy, and a photon’s phasor will still have the same amplitude and frequency of rotation whatever the material its travelling through.

Question 45

Refraction: Which path is responsible for the photon that will get to your eye?

Answer 45

If you add up all the phasors for all the possible paths, it’s the path that takes the shortest time that contributes the most to the resultant amplitude (and so gives the highest probability that the photon will get to your eye)

Question 46

Describe the paths of a focused photon (or any other quanta)

Answer 46

Make sure all the straight line paths from the source to the focus point take the same amount of time - so the final phasors for every path will be in the same direction

Question 47

Describe the paths of photons passing through a convex lens

Answer 47

The paths towards the edges of the lens are longer than those that go through the middle. You make the time taken for each path the same by increasing the amount of glass in the middle part of the lens to increase the time it takes to travel along the shorter paths between the source and detector

Question 48

What did de Broglie suggest about electrons?

Answer 48

If ‘wave-like’ light showed particle properties, ‘particles’ like electrons should be expected to show wave-like properties

Question 49

What can be used to find the probability of finding an electron at a particular point?

Answer 49

The de Broglie wave of a particle - which can be interpreted as a ‘probability wave’

Question 50

Explain how the slit experiments work with electrons instead of photons

Answer 50

You can show interference and superposition patterns using a fluorescent screen. As an electron hits the screen, it causes a photon to be released, so you can see the location of the electron

Question 51

Explain the bright/dark fringes in the electron slit experiment

Answer 51

Bright fringes in an electron interference pattern show where the probability of an electron arriving is high
Dark fringes show where the probability of an electron hitting the screen is low

Question 52

What is evidence for electrons being quantum objects?

Answer 52

Electron diffraction

Question 53

When are (electron) diffraction patterns observed?

Answer 53

When accelerated electrons in a vacuum tube interact with the space in a graphite crystal

Question 54

What happens to the diffraction pattern when you increase the accelerating voltage of the electrons? Link this to the de Broglie equation

Answer 54

You increase the electron speed. The diffraction pattern circles all squashed together towards the middle. This fits in with the de Broglie equation - if the velocity is higher, the wavelength is shorter and the spread of the lines is smaller

Question 55

Why don’t electrons show quantum behaviour (diffraction) all the time?

Answer 55

You only get the fraction of a particle interacts with an object of about the same size as its De Broglie wavelength

Question 56

What is the relationship between wavelength and diffraction effects?

Answer 56

A shorter wavelength gives less diffraction effects

Question 57

Describe how the relationship between wavelength and diffraction effect is used in electron microscopes

Answer 57

Diffraction effects blur detail on the image. If you want to resolve tiny detail in an image, you need a shorter wavelength. Light blurs out detail more than electrons do, so an electron microscope can resolve finer detail than a light microscope. They can let you look at smaller things

Question 58

Why does particle theory not support interference patterns?

Answer 58

To have interference patterns, you need superposition. Particle theory says the particles are physical objects which cannot superpose with other particles
You also need at least two slits to create an interference pattern – classic particles would either go through one slit or the other, not both. However, interference patterns can be seen when only a single electron at a time it sent through narrow slits

Question 59

Which theory explains: diffraction, superposition and interference, refraction, and the photoelectric effect?

Answer 59

Diffraction: wave theory
Superposition and interference: Wave theory
Refraction: wave theory
Photoelectric effect: particle theory