13.1 +13.2 The Photon Model

Question 1

What are photons?

Answer 1

discrete pakets (quantumns) of EM energy

Question 2

how did the ideas of photons come about?

Answer 2

the wave theory of light was not enough to explain the photo eletric effect
* established by einstein
* Mack Planck used these ideas to understand block body radiation (calculations worked when quantising )

Question 3

what was the ultraviolet distaster?

Answer 3

1. Lord Rayleigh was trying to find a function (mathematical relationship) between the frequencies emmited by EM radiation emmited by a black body as a afunction of it’s temperature
2. math was coming out wrong and not matchng the experimented values
3. his stuff was saying that blackbodies emmite infinte intensities of EM radiation at UV frequencies
4. this violated law of conservation of energy
5. instead Planck came up with a better equation by assuming that the vibration of energy particles was quantised

Question 4

what is the equation for the energy of a photon?

Answer 4

E= h f

Question 5

What is Plancks constant ?

Answer 5

Question 6

what is a laser beam?

Answer 6

a device which emmits photons of the same frequnecy

Question 7

how would you calculate the power of a beam (laser)?

Answer 7

Question 8

Answer 8

Question 9

what is the electron-volt eV ?

Answer 9

• another unit of energy
• the work done on/ by an electron in accelerating it between a potential difference of 1 volt
• E= QV
• E = ev

voltage times elementary charge (e-)

Question 10

how do you convert from eV to Joules ?

Answer 10

conversion factor of

Question 11

What is the photo-electric effect?

Answer 11

when UV light is shone on a metal, it emits eletrcons from the surface

Question 12

How is the photot eletric effect demonstrated by the gold leaf electronscope?

Answer 12

Question 13

what is the definition of the photoelectric effect?

Answer 13

the emission of electrons from the surface of a metal when it is illuminated by electromagnetic radiation (above a certain threshold frequency)

Question 14

what are the conditions required for the photo eletric effect to take place?

Answer 14

• incident frequnecy of EM radiation is above a certain threshold frequency

regardless of intensity

Question 15

What happens if you increase the intensity of the incident radiation ?

photo eletric effect

Answer 15

number of electrons emitted per second increases
1. this is becuase with greater intensity there are more photons per unit of time
1. increased number of photon electron interactions
1. increasing number of emitted electrons
1.

not their kinetic energy

Question 16

how can the kinetic energy of the emitted elecetrons be increases?

photo electric effect

Answer 16

increasing the frequency of incident radiation

increasing energy given

Question 17

what is the work function Φ?

Answer 17

the minimum amount of energy needed by an electron to escape a metal surface

Question 18

Explain the photo electric effect in full depth

Answer 18

1. a single electron on a metal absorbs the energy carried by a single photon
2. if the energy carried by a single photon exceeds the work function Φ of the metal, the electron is able to escape
3. any energy in excess of the work function absorbed by the elctron becomes its kinetic energy

Question 19

Why will very few electrons ever achieve maximum kinetic energy?

Answer 19

• only electrons near the surface of the metal will achieve maximum kinetic energy as the value of the work function Φ works most accuratly at their position
• most electrons deeper in will actually require more energy than Φ in order to leave as they have to overcome more attractive forces from ions deeper in the metal
• there there is less of the energy left for kineti energy

Question 20

How do LEDs work?

Answer 20

Excitation and de-excitation but energy given via electricity

Question 21

how can LEDs be used to estimate planck’s constant?

Answer 21

Question 22

What is the assumption made in this practical?

Answer 22

at the threshold p.d, the energy of a single electron is transferred completely to the energy of a single photon

Question 23

How could the practical be made more reliable ?

Answer 23

Doing it again on different coloured LEDs (different wavelengths)