Chapter 3 - Quantum Phenomena

Question 1

What is threshold frequency?

Answer 1

The minimum frequency to overcome work function

Question 2

Why is the photoelectric effect not observed below the threshold frequency?

Answer 2

If the frequency is below the threshold frequency, it doesn’t supply the electrons on the surface of the metal enough energy to break their bonds and emit a photon

Question 3

What is meant by the wave-particle duality of electromagnetic radiation

Answer 3

The electromagnetic spectrum can be waves or particles, shown by the way they can refract but also b radiation

Question 4

Which aspect of the wave-particle duality of electromagnetic radiation is shown by the photoelectric effect?-

Answer 4

Particle

Question 5

During the photoelectric effect, if the frequency is maintained but the intensity is doubled, what happens to the number of electrons released per second and maximum kinetic energy?

Answer 5

Electrons released per second = doubles
Maximum kinetic energy = stays the same

Question 6

Wave particle duality

Answer 6

• Sometimes matter will behave as a wave and sometimes as a particle
• We can calculate the warelength of any particle using the de Broglie wavelength
  = h/mv
  v = velocity
  m = mass
  h = Planck’s constant

Question 7

Examples of wave properties in wave particle duality

Answer 7

Refraction
Reflection
Diffraction
Interference

Question 8

Examples of particle properties in wave particle duality

Answer 8

Photoelectric effect
Momentum
Mass
Energy levels (absorption, emission)
Alpha, beta radiation

Question 9

What is the photoelectric effect?

Answer 9

The photoelectric effect is the emission of electrons from a metal surface when light is incident on the surface. Emission only takes place if the frequency of the incident light is greater than a minimum value, which is known as the threshold frequency.
Light is composed in packers of energy called photons. Einstein explained this by assuming a single electron in the metal, or near its surface, absorbs a photon and gains sufficient energy to escape from the metal.
Energy of a photon = hf
There is a minimum amount of energy, the work function, of the metal needed by an electron to escape from the metal surface

Question 10

Equation for the maximum kinetic energy of a photoelectron

Answer 10

Maximum kinetic energy = (planck’s constant x frequency) - work function

Question 11

What happens during photoelectric emission?

Answer 11

The photon arrives at the metal surface
The photon is absorbed
The electron gains energy
The electron uses energy to reach the surface
The electron does work to leave the metal surface

Question 12

Ionisation

Answer 12

An ion is a charged atom. This means the numbers of electrons and protons is not equal
A positively charged ion is created when electrons are removed and the opposite for a negatively charged ion
Any process creating ions is called ionisation

For example:
- Alpha, beta, and gamma radiation create ions when they pass through substances and collide with the atoms of the substance
- Electrons passing through a fluorescent tube create ions when they collide with the atoms of the gas or vapour in the table
- Absorption of electromagnetic radiation
- Heating them

Question 13

Excitation

Answer 13

Gas atoms can absorb energy from colliding electrons without being ionised. This process is called excitation and happens a certain energies relating to the atoms of the gas.
When excitation occurs, the colliding electron makes an electron inside the atom move from an inner shell to an outer shell. Energy is needed for this process (moving an atomic electron to a higher energy level around the atom). The excitation energy is always less than the ionisation energy of the atom, because the atomic electron is not removed completely from the atom when excitation occurs
An electron will only move to an outer electron shell if it absorbs the exact energy
Energy is needed to move the electron to an outer shell because it is moving away from the nucleus

Can happen by:
- absorbing some or all of the kinetic energy of another electron (collision)
- absorbing all the energy from an incident photon

Question 14

Work function

Answer 14

Minimum energy of the incident photon to cause a photoelectron to be released from the metal surface

Question 15

Threshold frequency

Answer 15

Minimum frequency of the incident photon to cause a photoelectron to be released from the metal surface

Question 16

Photoelectric effect - conservation of energy

Answer 16

One photon can release one photoelectron
The energy from the incident photon will be used to release the photoelectron from the surface, any extra energy will become the kinetic energy of the photoelectron

Question 17

Kinetic energy equation

Answer 17

Kinetic energy (max) = 1/2 x mv^2

Question 18

hf = work function + kinetic energy

Answer 18

Energy of incident photon = energy needed to release an electron from the metal + kinetic energy of the released electron

Question 19

More about photoelectric effect

Answer 19

Increasing the intensity (brightness)
- increases number of photons emitted by the source each second
- increases the number of photoelectrons emitted from the surface (if above threshold frequency)

Kinetic energy is proportional to the frequency of the incident radiation
- above threshold frequency
- conservation of energy

Stopping potential
- positive potential difference (voltage) of the surface, at which photoelectrons are no longer released
- depends on the surface and frequency of incident radiation

Question 20

What are the differences in the photoelectric effect depending on if the incident radiation is a wave or particle?

Answer 20

Wave:
- photo electrons released at all frequencies
- minimum intensity needed

Particle:
- photoelectrons released when the frequency is above a minimum value
- number of photoelectrons released is proportional to the intensity

Question 21

Energy levels in atoms

Answer 21

The lowest energy level is called the ground state, and is labelled n = 1
All atoms of the same element have the same energy level diagram

Question 22

De-excitation

Answer 22

• An excited atom is unstable
• there is now a vacancy in a lower energy electron shell
• this will be “filled” by an electron moving down from an outer shell
• a photon with energy exactly matching the energy difference between levels will be emitted

Question 23

Fluoresence

Answer 23

• contains mercury vapour
• mercury coms are excited by collisions with electrons
  
  • absorb some of their kinetic energy
  • equal to energy difference between levels
• energy level spacing leads to emission of ultraviolet photons as mercury de excites
• these uv photons are absorbed by the phosphor coating
• this excites the phosphor atoms
• phosphor emits visible light photons as it de excites