Electromagnetic radiation and quantum phenomena

Question 1

what is the photoelectric effect

Answer 1

is where the photoelctrons are emitted from the surface of a metal after light above a certain frequency is shone on it

Question 2

what is threshold frequency

Answer 2

the minimum amount of energy needed to emit electrons from the surface if a metal

Question 3

why cant threshold frequency be explained by wave theory

Answer 3

as it suggests that any frequency of light should be able to cause photoelectric emission as the energy absorbed by each electron will gradually increase with each incoming wave

Question 4

what could threshold frequency be explained by

Answer 4

by the photon model of light

Question 5

what did the photon model of light suggest

Answer 5

• that EM waves travel in discrete packets called photons which have an energy which is directly proportional to the frequency
• each electron can be absorbed by a single photon therefore a photoelectron is only emitted if the frequency is above the threshold frequency
• if the intensity of light is increased, if the frequency is above the threshold frequency, more photoelectrons are emitted per second

Question 6

what is the work function

Answer 6

is the minimum energy required for electrons to be emitted from the surface of a metal

Question 7

what is the stopping potential

Answer 7

is the potential difference you would need to apply across the metal to stop the photoelectrons with the max kinetic energy

Question 8

what can you find by measuring the stopping potential

Answer 8

you can find the max kinetic energy

Question 9

where can electrons only exist in an atom

Answer 9

in discrete energy levels

Question 10

what is excitation

Answer 10

when an electron gains enough energy (from a collision with an electron) to move to a higher energy level

Question 11

what is ionisation

Answer 11

when an electron gain enough energy to leave the atom all together

Question 12

when does ionisation occur

Answer 12

when the energy of the free electron is greater than the ionisation energy

Question 13

what is the original energy level of an atom called

Answer 13

the ground state

Question 14

when an electron becomes excited it will quickly return to its original energy level what does it release the energy that it gained in the form of

Answer 14

a photon

Question 15

what is an example of a practical use of excitation

Answer 15

a fluorescent lamp in order to produce light

Question 16

how does a fluorescent lamp produce light

Answer 16

• a voltage is applied across it which accelerates the free electrons through the tube, which collide with the mercury atoms causing them to become ionized, releasing more free electrons
• the free electrons collide with the mercury atoms, causing them to become excited
• when they deexcite they release photons, most of which are in the UV range
  -the (phosphorous) fluorescent coating on the inside of the tube, absorbs these UV photons and therefore electrons in the atoms of the coating become excited and de-excite releasing photons of visible light

Question 17

what unit is used to describe the energy difference between energy levels

Answer 17

eV (electron volts)

Question 18

what is an electron volt

Answer 18

is defined as the energy gained by one electron when passing through a potential difference of 1 volt

Question 19

how many joules in 1 eV equal too

Answer 19

1.6x10^-19

Question 20

what do you get by passing the light from a fluorescent tube through a diffraction grating or prism

Answer 20

a line spectrum

Question 21

what will each line in the spectrum represent

Answer 21

a different wavelength of light emitted by the tube

Question 22

as this spectrum is not continuous what does it contain

Answer 22

only discrete values of wavelength

Question 23

what will correspond to the wavelengths shown on the spectrum and what does this show evidence for

Answer 23

the photon energies emitted, which show evidence that electrons in atoms can only transition between discrete energy levels

Question 24

what type of spectrum do you get by passing white light through a cooled gas

Answer 24

a line absorption spectrum

Question 25

what does a line absorption spectrum look like

Answer 25

looks like a continuous spectrum of all possible wavelengths with black lines at certain wavelengths

Question 26

what do the black lines represent on a line absorption spectrum

Answer 26

the possible differences in energy levels as the atoms in the gas can only absorb photons of an energy equal to the exact difference between two energy levels

Question 27

what is the difference between two energy levels equal to

Answer 27

the photon energy emitted by a fluorescent tube or absorbed in a line absorption spectrum

Question 28

what properties can light be shown as having

Answer 28

wave and particle properties

Question 29

what are the examples of light acting as wave

Answer 29

diffraction and interference

Question 30

what is the example of light acting as a particle

Answer 30

the photoelectric effect

Question 31

what else csan be observed as having wave and particle properties

Answer 31

an electron

Question 32

how can the wave nature of electrons be observed

Answer 32

through electron diffraction

Question 33

what was De Broglies hypothesis

Answer 33

that if light was shown to have particle properties then particles should have wave like properties

Question 34

what is De Broglies equation

Answer 34

lambda = h/(mv)

Question 35

what can you tell from using De Broglies equation

Answer 35

the amount if diffraction changes as a particles momentum changes

Question 36

if the momentum is increased what happens to the wavelength and diffraction

Answer 36

wavelength will decrease and diffraction will decrease so the concentric rings of the interference pattern become closer

Question 37

why does knowledge and understanding of any scientific concept change overtime

Answer 37

because more experimental evidence is gathered by the scientific community

Question 38

how can experimental evidence become accepted

Answer 38

after it is published and peer reviewed by the community to become validated and then it is eventually accepted