4.5 - Quantum Physics

Question 1

What are photons?

Answer 1

Photons are particles of light.
They are sometimes referred to as a discrete packet of energy of EM radiation

Question 2

True or false: The energy of a photon is proportional to the wavelength of the light.

Answer 2

False.
It’s proportional to the frequency: E = hf
Energy is inversely proportional to the wavelength.

Question 3

What is ‘h’ in the equation E = hf? Give units.

Answer 3

h is the Planck constant, measured in Js.

Question 4

What quantity can be measured in electron volts (eV)?

Answer 4

Energy

Question 5

Describe an experiment which can be used to estimate the value of the Planck constant.

Answer 5

• An LED will only allow current to pass after a minimum voltage has been put across it.
• At this voltage all the electrons will have the same energy as a photon emitted by the LED (which you will know the frequency of).
• Finding the threshold voltage by seeing when current flows in the circuit can then be used to find h from: h = E/f.

Question 6

Describe how to improve the accuracy of the estimate of this experiment

Answer 6

• To improve the accuracy of this estimate, the experiment can be repeated with a variety of different coloured LEDs, which each emit different wavelengths of light.
• The values of wavelength and threshold p.d. for each can be recorded, and a graph drawn of V against 1/λ.
• The gradient of this graph will be equal to hc /e.
• As the speed of light and the electron charge are known constants, we can calculate the value of h from this.

Question 7

What is the photoelectric effect?

Answer 7

The photoelectric effect is a phenomenon where shining light with enough energy onto a metal releases electrons (and can cause a current to flow).
The electrons emitted are called photoelectrons.

Question 8

Which features of the photoelectric effect can’t be explained if light is a wave?

Answer 8

If light was a wave, then the energy of the electrons released would increase with increasing intensity of the light - but this isn’t the case. Instead the energy of the electrons depends on frequency (and no electrons are released below a certain threshold value, no matter how intense the light is).

Question 9

How many photons does each photoelectron absorb prior to emission?

Answer 9

Only 1.
If it doesn’t contain enough energy the electron will re-emit the energy rather than being released.

Question 10

How does the photon model of light explain the threshold frequency seen in the photoelectric effect?

Answer 10

Each electron absorbs a single photon. This single photon must have enough energy for the electron to be released, if it doesn’t the energy is re-emitted. The electron can’t build up energy as it could if light was a wave.

Question 11

What is the name given to the minimum amount of energy an electron requires to leave the surface of a metal?

Answer 11

The work function, 𝜙

Question 12

Write a word equation relating the energy of an incident photon to the work function and the kinetic energy of released electrons.

Answer 12

Photon energy = work function + kinetic energy

Question 13

True or false: The rate of emission of photoelectrons is proportional to intensity (provided the light is above threshold frequency).

Answer 13

True.
Higher intensity means more photons, this means more electrons can absorb energy and be released.

Question 14

Does the maximum kinetic energy of a released electron depend on the intensity of light hitting the surface?

Answer 14

No.
Energy transferred is due to a one-to-one interaction, and so depends on frequency, not intensity

Question 15

What experimental evidence appears to show particles behaving as waves?

Answer 15

• Electron diffraction.
• Electrons will diffract is passed through the spaces between atoms in graphite (like a tiny diffraction grating).
• This wouldn’t happen if electrons were behaving as particles only.

Question 16

Which equation relates the wave and particle properties of electrons?

Answer 16

λ = h/p

The de Broglie equation

Where λ = wavelength (wave-property), h = Planck’s constant, and p = momentum (particle-property)