quantum physics

Question 1

what is the photoelectric effect?

Answer 1

process in which electrons are emitted from a metal surface when electromagnetic radiation of sufficiently high frequency is incident on the surface

Question 2

what is a photoelectron?

Answer 2

electron emitted from the surface of a material due to the incident electromagnetic radiation

Question 3

what happens when EM radiation is incident on emitter such that the collector is sufficiently positive?

Answer 3

• every single photoelectron is attracted to C
• rate of emission of photoelectrons = rate at which photoelectrons reach C
• ammeter reads a constant saturated/max photocurrent

Question 4

what happens when EM radiation is incident on emitter such that the collector is sufficiently negative?

Answer 4

• most energetic photoelectrons do not have sufficient energy to reach C
• photocurrent is zero

Question 5

define stopping potential

Answer 5

magnitude of the negative potential of collector wrt emitter which prevents the most energetic photoelectrons from reaching the collector, results in zero photocurrent

Question 6

what are the observations of photoelectric effect?

Answer 6

1. saturated photocurrent is proportional to the intensity of the monochromatic EM radiation at a constant frequency
2. existence of the threshold frequency (below which no emission of photoelectrons occurs irrespective of the intensity of the EM radiation
3. max KE independent of intensity (energies emitted increases linearly with frequency)
4. insignificant time lag (almost immediate emissions)

Question 7

formula for the quantum theory of light

Answer 7

E = hf = hc/lumda

where
E = energy carried in EACH quantum emitted
h = 6.33 * 10^-34
f = frequency
c = 3.0*10^8

Question 8

define a photon

Answer 8

quantum of electromagnetic energy

Question 9

define the work function energy of a metal

Answer 9

the minimum amount of energy necessary for an electron to escape from the surface of a material

Question 10

what is einstein’s photoelectric equation?

Answer 10

photon energy = work function energy + maximum KE of a photoelectron

Question 11

relate max KE with stopping potential

Answer 11

EK max = eVs

Question 12

what are the formulas for the particulate nature of light?

Answer 12

lumda = h/p
p = hf/c

Question 13

how to prove light behaves as a wave?

Answer 13

interference / diffraction of light

Question 14

how to prove light behaves as particles?

Answer 14

photoelectric effect

Question 15

electrons behave as particles

Answer 15

electrons undergo collision, have mass and charge

Question 16

electrons behave as a wave

Answer 16

electron diffraction

Question 17

key features of the energy level diagram

Answer 17

• ground state = lowest energy level (n=1)
• higher energy levels –> excited state
• n = infinity (0 eV) –> electron escaped from atom
• lower energy –> more negative energy value = more stable states
• ionisation energy from n = 1 –> n = infinity

Question 18

what is ionisation

Answer 18

process of creating charged particles

Question 19

what is excitation

Answer 19

process whereby atoms absorb energy without ionisation

Question 20

what are the two main sources of energy that can cause the electron to be ionised or excited to a higher energy state

Answer 20

1. particle collision (high speed particle collides and imparts energy to an atom –> excited electron at a higher energy level **energy of electrons need not match)
2. photons (for an orbital electron to gain energy, photon energy must be exactyl equal to energy difference btwn 2 levels in the atom)

Question 21

explain emission line spectrum

Answer 21

gases (H,Ne) in a discharge tube at low pressure with high voltage applied btwn ends of a tube, gas glows, light emitted from tube.
light examined through a diffraction grating with a spectrometer –> distinct lines observed –> emission line spectrum

gas heated/bombarded by electrons –> electrons in gas excited to higher energy levels
de-excitation –> photons emitted with energy corresponding to energy difference btwn 2 energy levels

energy levels & differences are discrete –> corresponds to series of lines on spectrum

discrete bright lines of definite wavelength on a dark backrground