Section 2 - Electromagnetic Radiation and Quantum Phenomena

Question 1

Are electromagnetic waves transverse or longitudinal?

Answer 1

Transverse

Question 2

What phenomenon can be used to show that light behaves as a particle?

Answer 2

The photoelectric effect

Question 3

Describe the photoelectric effect

Answer 3

When light above a particluar frequency is shone on metal, electrons are released - these released electrons are photoelectrons

Question 4

What is the threshold frequency?

Answer 4

The minimum frequency of light required for an electron to be emitted

Question 5

What equation is used to determine the energy of a photon?

Answer 5

E = hf = hc/λ

Question 6

Why does a photon need to have a minimum frequency on order to liberate an electron?

Answer 6

• The energy of the photon is detmernied by its frequency
• The photon’s energy must be greater than the work function (energy needed to break bonds holding the electron) in order for an electron to be emitted

Question 7

If a photon has a frequency higher than the threshold frequency, what would occur?

Answer 7

The electron will be liberated and the remaining energy is the kinetic energy of the electron

Question 8

If light is incident on a metal and photoelectric emission does NOT occur what is the effect of increasing light intensity?

Answer 8

• If it is more intense then there would be more photons incident on the metal each second
• However each photon still carries the same amount of energy as before
• Therefore it still does not contain enough energy to liberate an electron
• No effect

Question 9

What is the definition of the work function?

Answer 9

The energy required by an electron to overcome the metalic bond holding it in the metal

Question 10

What is an electron volt?

Answer 10

The kinetic energy of an electron that has been accelerated from the rest through a potential difference of 1V

Question 11

How do you convert electron volts to joules (and vice versa)?

Answer 11

Electron volt - Joules x1.6x10^-19
Joules - Electron volts /1.6x10^-19

Question 12

How does a fluorescent tube work?

Answer 12

• High voltage applied across a mercury vapour accelerates fast moving free electrons which collide with the mercury atoms
• Mercury electrons are excited and then return to the ground state, releasing a UV photon
• The tube’s phosphorus coating absorbs the UV photons and its electrons are excited, they cascade down the energy levels and emit visible light photons

Question 13

What can be used as evidence for the discrete energy levels in atoms?

Answer 13

Line emission and absorption spectra as the lines appear at discrete points which show where a light photon of specific frequency and wavelength has been absorbed or emitted
This shows electrons can only absorb an exact amount of energy to be excited to the next discrete energy level

Question 14

What is wave particle duality?

Answer 14

All particles have both particle and wave properties, waves can have particle properties e.g. light acts as a particle in the photoelectric efect and as a wave when it is diffracted