13.1 The Photon Model

Question 1

What is a photon

Answer 1

Tiny packets of energy

Question 2

What is the equation of photon energy and what does this mean

Answer 2

E = h x f
Energy of the photon = Plancks constant x frequency of the electromagnetic radiation
Each photon is directly proportional to it’s frequency

Question 3

Define plancks constant

Answer 3

6.63 x 10^-34 Js

Question 4

Give the equation for the energy of a photon in terms of its wavelength

Answer 4

E = h x c / lambda
Energy of a photon = plancks constant x speed of light through a vacuum / wavelength

Question 5

If KEₘₐₓ of photoelectrons is plotted on the y-axis against the frequency, f, of incident light for a particular metal, what will the y-intercept and gradient represent?

Answer 5

the y-intercept of the line of best will be –Φ, the gradient of the line of best fit will be h

Question 6

What is the name for a quantum of light energy?

Answer 6

A photon

Question 7

Define the work function of a metal

Answer 7

The minimum energy an electron needs to escape from the metal surface

Question 8

How does the photon model explain why no electrons are emitted if the frequency of the incident light is below the threshold frequency

Answer 8

An electron in the metal can only absorb energy from a single photon, gaining energy ( hf ), the electron is emitted if this energy is greater than the metals work function

Question 9

Define threshold frequency of a metal

Answer 9

The minimum frequency of photons needed for the photoelectric emission of electrons from the metal

Question 10

No electrons are emitted if the frequency of the incident light is below the threshold frequency. Why can’t the wave model of light explain this?

Answer 10

The energy of light is proportional to intensity, so electrons should eventually gain enough to leave metal; there is no reason why there should be a threshold frequency

Question 11

Describe the photon model of light

Answer 11

The photon model of light treats electromagnetic radiation as ‘packets’ of energy with the enegry, E = h x f

Question 12

State the photoelectric effect equation

Answer 12

hf = Φ + KEₘₐₓ

Question 13

Describe how the photoelectric effect can be demonstrated with a gold leaf electroscope

Answer 13

• The gold leave is given an excess negative charge causing the leaf to repel from the plate.
• Shining the UV light on the zinc plate causes photoelectrons to be emitted and so the leaf drops.
• Shining the torch on the zinc disc does not cause photoemission, as the frequency of visible light is below the threshold frequency for the zinc so the leaf remains in position.

Question 14

What is the name given to electrons emitted due to the photoelectric effect

Answer 14

Photoelectrons

Question 15

Define the electron volt

Answer 15

The energy transferred to/from an electron when it passes through a potential difference of 1 volt

Question 16

What is a gold-leaf electroscope

Answer 16

A device with a metal stem and a gold leaf that is used to identify and measure electric charge - historically used as a voltmeter to measure large voltages

Question 17

Define threshold voltage ( diode )

Answer 17

The minimum potential difference at which a diode begins to conduct

Question 18

1eV in joules

Answer 18

1eV = 1.6 x 10^-19 J

Question 19

How does the photon model explain why KE max of the emitted electrons does not change with intensity but increases with frequency

Answer 19

• increasing intensity = increasing the number of photons arriving per second
• each photon has the same energy and can only interact with one electron
  However
• if frequency increases beyond threshold frequency = excess energy gained by photo electron goes to kinetic energy store
• increasing KE max

Question 20

How can electron diffraction be observed

Answer 20

By firing a beam of electrons at a thin slice of polycrystalline graphite in a vacuum

Question 21

State the de broglie equation

Answer 21

Lambda = h ÷ m x v