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 the threshold frequency is shone on a metal surface, 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 in 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 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 metallic bond holding it in the metal

Question 10

What is the photoelectric equation?

Answer 10

hf = ϕ + Ek(max)

Question 11

What is the definition of stopping potential?

Answer 11

The p.d. needed to stop the fastest moving electrons, with Ek (max)

Question 12

What is the equation for stopping potential?

Answer 12

e Vs = Ek (max)
where e - charge on the electron
Vs - stopping potential
Ek (max)

Question 13

What is an electron volt?

Answer 13

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

Question 14

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

Answer 14

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

Question 15

How does a fluorescent tube work?

Answer 15

• High voltage applied across a mercury vapour which 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 16

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

Answer 16

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

Question 17

What is wave particle duality?

Answer 17

All particles have both particle and wave properties, waves can have particle properties

Question 18

How and why does the amount of diffraction change when the momentum is changed?

Answer 18

• If you increase the electron speed, and therefore the electron momentum, the wavelength is shorter, and the spread of lines is smaller
• If particles with a greater mass were travelling at the same speed as the electrons, the momentum would be greater, the wavelength shorter, and the spread of lines is smaller

Question 19

What had to be done before de Broglie could have his theory accepted?

Answer 19

• Evaluation by peer review
• Tested experimentally
• Theory was then accepted and validated by the scientific community