Electromagnetic Radiation and Quantum Phenomena

Question 1

What happens when you shine a light on a metal?

Answer 1

• If you shine a light of a high enough frequency onto the surface of a metal, the metal will emitt electrons
• For most metals, the frequency falls within the UV range

Question 2

Why does the metal emit electrons?

Answer 2

1. Free electrons of the surface of the metal absorb energy from the light
2. If an electron absorbs enough energy, the bonds holding it to the metal break and the electron is released
3. This is called the photoelectric effect, and the electrons emitted are called photoelectrons

Question 3

What was the first main conclusion from experiments?

Answer 3

For a given metal, no photoelectrons are emitted if the radiation has a frequency below a certain value, called the threshold frequency

Question 4

What was the second main conclusion from the experiments?

Answer 4

• The photoelectrons are emitted with a variety of kinetic energies ranging from zero to some maximum value
• This value of maximum kinetic energy increases with the frequency of the radiation, and is unaffected by the intensity of the radiation
• Intensity is the power (the energy transferred per second) hitting a given area of the metal

Question 5

What was the third main conclusion from the experiments?

Answer 5

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

Question 6

What was wave theory?

Answer 6

• According to wave theory:
  1. For a particular frequency of light the energy carried is proportional to the intensity of the beam
  2. The energy carried by the light would be spread evenly over the wavefront
  3. Each free electron on the surface of the metal would gain a bit of energy from each incoming wave
  4. Gradually, each electron would gain enough energy to leave the metal

Question 7

Why could the photoelectric effect not be explained using wave theory?

Answer 7

• Therefore the higher the intensity of the waves the more energy is should transfer to each electron, the kinetic energy should increase with intensity
• There is noe explanation for the kinetic energy depending on the frequency and there is also no explanation for the threshold frequency as according to wave theory the electrons should be emitted eventually, no matter what the frequency is

Question 8

How was the photoelectric effect explained?

Answer 8

• Einsteins photon model of light
• Einstein suggested that EM waves (and the energy they carry) exist in discrete packets - called photons)
• Einstein saw these photons of light as having one on one, particle like interaction with an electron in a metal surface
• A photon would transfer all its energy to one specific electron

Question 9

How is the energy carried by one if these photons defined?

Answer 9

E=hf=hc/lamda

Question 10

What does the photon model suggest?

Answer 10

1. When light hits a surface, the metal is bombarded by photons
2. If one of these photon collides with a free electron, the electron will gain energy equal to hf

Question 11

What needs to happen for an electron to leave the surface of a metal?

Answer 11

• It needs enough energy to break the bonds holding it there
• This energy is called the work function (which has the symbol phi) and its value depends on the metal

Question 12

How does the photon model explain threshold frequency?

Answer 12

1. If the energy gained by an electron (on the surface of a metal) from a photon is GREATER than the work function, the electron is emitted
2. If it isn’t, the metal will heat up, but no electrons will be emitted

Question 13

What is needed for electron to be related?

Answer 13

phi

Question 14

How does the photon model explain maximum kinetic energy?

Answer 14

1. The energy transferred to an electron is hf
2. The kinetic energy the electron will be carrying when it leaves the metal is hf MINUS any energy it’s LOST on the way out
   - Electrons deeper down in the metal lose more energy that the electrons on the surface, which explains the range of energies

Question 15

What is the minimum amount of energy electrons can lose?

Answer 15

-The work function and so the maximum kinetic energy of a photoelectron is Ek max is given by the photoelectric equation:
hf = phi + Ekmax
-The kinetic energy of the electrons is independent of intensity (the number of photons per second on an area), as they can only absorb one photon at a time
-Increasing the intensity just means more photons per second on an area - each photon has the same energy as before

Question 16

What does the stopping potential give?

Answer 16

• The stopping potential gives the maximum kinetic energy
  1. The maximum kinetic energy can be measured using the idea of stopping potential
  2. The emitted electrons are made to lose their energy by doing work against an applied potential difference
  3. The stopping potential Vs, is the pd needed to stop the fastest moving electrons with Emax
  4. The work done by the pd in stopping the fastest electrons is equal the energy they were carrying

Question 17

How do electrons exist?

Answer 17

• In discrete energy levels
• Electrons in an atom can only exist in certain well defined energy levels and each level is given a number with n=1 representing ground state

Question 18

Can electrons move down energy levels?

Answer 18

1. Electrons can move down energy levels by emitting a photon
2. Since these transitions are between definite energy levels, the energy of each photon emitted can only take a certain allowed value
3. The energies involve are so tiny that it makes sense to use a more appropriate unit than the joule such as an electron volt

Question 19

What is an electron volt (eV)?

Answer 19

• The kinetic energy carried by an electron after it has been accelerated through a potential difference of 1 volt
• Energy gained by electron (eV) = accelerating voltage (V)
• 1 eV = 1.6 x 10^-19 J

Question 20

What does the energy carried by each photon show?

Answer 20

1. The energy carried by each photon is equal to the difference in energies between the two levels

Question 21

Can electrons move up energy levels?

Answer 21

1. Electrons can move up energy levels if they absorb a photon with the exact energy difference between the two levels. The movement of an electron to a higher energy level is called excitation

Question 22

When is an atom ionised?

Answer 22

1. If an electron is removed from an atom, we say that the atom is ionised
2. The energy of each energy level within an atom gives the amount of energy needed to move an electron in that level from the atom
3. The ionisation energy of an atom is the atom of energy needed to completely remove an electron from the atom from the ground state (n=1)

Question 23

What happens when the initial high voltage is applied in a fluorescent light?

Answer 23

1. Fluorescent tubes contain mercury vapour, across which an initial high voltage is applied
2. This high voltage accelerates fast moving free electrons that ionise some of the mercury atoms, producing more free electrons

Question 24

What happens to the free electrons in a fluorescent lamp?

Answer 24

1. When this flow of free electrons collides with electrons in other mercury atoms, the electrons in the mercury atoms are excited to higher energy levels
2. When these excited electrons return to their ground states, they emit photons in the UV range

Question 25

Why is there a phosphorus coating in fluorescent light?

Answer 25

1. A phosphorus coding oin the inside of the tube absorbed these photons in the UV ranges citing its electrons to much higher orbits
2. These electrons then cascade down the energy levels, emitting many lower energy photons in the form of visible light

Question 26

What type of emission do fluorescent tubes produce?

Answer 26

1. If you split the light from a fluorescent tube with a prism or a diffraction grating you get a line spectrum
2. A line spectrum is seem as a series of bright lines against a black background
3. Each line corresponds to a particular wavelength of light emitted by the source
4. Since only certain photons energies are allowed, you only see the wavelength corresponding to these energies

Question 27

What does shining white light through a cool gas give?

Answer 27

an absorption spectrum

Question 28

What does continuous spectra contain?

Answer 28

• All possible wavelengths
  1. The spectrum of white light is continues
  2. if you split the light up with a prism, the colours all merge into each other and there aren’t any gaps in the spectrum
  3. HOT objects emit a continuous spectrum in the visible infrared
  4. All the wavelengths are allowed because the electrons are not confined to energy levels in the object producing the continuous spectrum. The electrons are not bound to atoms and are free

Question 29

What do cool gases remove?

Answer 29

Cool gases remove certain wavelengths from the continuous spectrum

Question 30

Why do cool gases remove certain wavelengths from the continuous spectrum?

Answer 30

1. You get a line absorption spectrum when light with a continuous spectrum of energy (white light) passes through a cool gas
2. At low temperatures, most of the electrons in the gas atoms will be in their ground states
3. The electrons can only absorb photons with energies equal to the difference between two energy levels
4. Photons if the corresponding wavelengths are absorbed by the electrons to excite them to higher energy levels
5. These wavelengths are then missing from the continuous spectrum when it comes out of the other sides of the gas
6. You see a continuous spectrum with black lines in it corresponding to the absorbed wavelengths
7. If you compare the absorption and emission spectra of a particular gas, the black lines in absorption spectrum match up the the bright lines in the emission spectrum

Question 31

What shows light as a wave?

Answer 31

1. Light produces interference and diffraction patterns - alternating bands of dark and light
2. These can only be explained using waves interfering constructively (when two waves overlap in phase) or interfering destructively (when two waves are out of phase)

Question 32

What shows light as a particle?

Answer 32

1. Einstein explained the results of the photoelectricity experiments by thinking of the beam of light as a series of particle-like photons
2. If a photon of light is a discrete bundle of energy, then it can interact with an electron in a one-to-one way
3. All th energy in thephtoon is given to one electron

Question 33

Who came up with wave particle duality and what did they say?

Answer 33

1. Louis de Broglie suggested that: ‘ if ‘wave-like’ light showed particle properties (photons), ‘particles’ like electrons should be expected to show wave-like properties’
2. The de Broglie equation relates. wave property (wavelength) to a moving particle property (momentum, mv)
3. The de Broglie wave of a particle can be interpreted as a ‘probability wave’
4. Many physicists at the time were not very impressed - his ideas were just speculation. But later experiments confirmed the wave nature of electrons

Question 34

How does electron diffraction show the wave nature of electrons?

Answer 34

1. Diffraction patterns are observed when accelerated electrons in a vacuum tube interact with the spaces in a graphite crystal
2. This confirms that electrons show wave-like properties

Question 35

What does wave theory suggest about electron diffraction?

Answer 35

• According to wave theory, the spread of the line sin the diffraction increases if the wavelength is greater
• In electron diffraction experiments, a smaller accelerating voltage, i.e.i slower electrons, gives more widely-spaced rings
• Increase the electron speed (and therefore the electron momentum) and the diffraction pattern circles squash towards the middle. This fits in with the de Broglie equation above - if the momentum is greater, the wavelength is shorter and the spread of the lines is smaller

Question 36

What is wavelength for electrons like?

Answer 36

• In general wavelength for electrons accelerated in a vacuum tube is about the same size as electromagnetic waves in the X-ray part of the spectrum
• If particles with a greater mass (e.g.) neutrons were travelling at the same speed as the electrons, they would show a more tightly packed diffraction pattern
• That’s because a neutron’s mass (and therefore its momentum) is much greater than an electron’s, and so a neutron has a shorter de Broglie length

Question 37

Why do particles not show wave-like properties all the time?

Answer 37

• You only get diffraction if a particle interacts with an object of about the same size as its de Broglie wavelength
• A tennis ball, for example, with mass 0.058kg and speed 100ms-1 has a de Broglie wavelength of 10^-34m and that is 10^19 times smaller than the nucleus of an atoms so there is nothing that small for it to interact with

Question 38

When was wave-particle duality accepted?

Answer 38

1. De Broglie first hypothesised wave-particle duality to explain observations of light acting as both a particle and a wave
2. But his theory was not accepted straight away and the scientists had to evaluate de Broglie’s theory (by process know as peer review) before he published it, and then it was tested with experiments
3. Once enough evidence was found to back it up, the theory was validated by the scientific community
4. Scientists’s understanding of the nature of matter has changed over time through this process of hypothesis and validation
5. De Broglie’s theory is accepted to be true - that is until any new conflicting evidence comes along