1. Unit 2 Particles and Waves Photoelectric Effect
CfE Higher Physics
Unit 2
Particles and Waves
Photoelectric Effect

2. Learning Intentions
State that photoelectric emission from a surface occurs only if the frequency of the incident radiation is greater than some threshold frequency, fo, which depends on the nature of the surface

3. Photoelectric Emission
When electromagnetic radiation above a certain frequency strikes a surface electrons can be emitted.
emitted electron
incident radiation -
surface
Photoelectric emission is the principle used in the operation of photodiodes, solar cells and LDRs.

4. Photoelectric Effect
The photoelectric effect can be demonstrated using a negatively charged electroscope.
u.v. radiation
zinc plate
leaf falls slowly

5. Photoelectric Effect Observations
Photoelectric emission only takes place when the incident radiation is above a certain threshold frequency, f0.
If the frequency of the incident radiation is greater than the threshold frequency, increasing the irradiance increases the rate of photoelectric emission.
If the frequency of the incident radiation is smaller than the threshold frequency, increasing the irradiance does not cause photoelectric emission.

6. Photoelectric Effect Conclusion
Electromagnetic radiation is not a wave, but a particle.
This theory is known as wave-particle duality.
Electrons will only be emitted if the incident frequency is greater than the threshold frequency, f0.
Electrons will not be emitted if the incident frequency is less than the threshold frequency – no matter how bright the radiation!

7. Unit 2 Particles and Waves Wave particle duality
CfE Higher Physics
Unit 2
Particles and Waves
Wave particle duality

8. Learning Intentions
State that a beam of radiation can be regarded as a stream of individual energy bundles called photons, each having an energy E = hf, where h is Planck’s constant and f is the frequency of the radiation.
Carry out calculations involving the relationship E=hf
Explain that if N photons per second are incident per unit area on a surface, the irradiance at the surface I = Nhf.

9. Wave-particle duality
Electromagnetic radiation is not a continuous train of waves but consists of wave packets, or quanta, called photons.
photon
Photons have an associated frequency and wavelength. They also have an associated energy, which depends on their frequency

10. Energy of a Photon The energy of a photon is given by the formula:
E = hf
E = energy of photon (J)
h = Planck’s constant = 6.63 x Js
f = frequency of photon (Hz)

11. Irradiance of Photons
Irradiance is the power per unit area. If N photons per second are incident per unit area on a surface the irradiance at the surface is given by the formula:
I = Nhf
Not on formula sheet
I = irradiance (Wm-2)
N = number of photons per second per square metre
h = Planck’s constant = 6.63 x Js
f = frequency photons (Hz)

12. Learning Intentions
State that photoelectrons are ejected with a maximum kinetic energy, Ek, which is given by the difference between the energy of the incident photon hf and the work function hfo of the surface: Ek = hf – hfo.
State that for frequencies smaller than the threshold value, an increase in the irradiance of the radiation at the surface will not cause photoelectric emission.
State that for frequencies greater than the threshold value, the photoelectric current produced by monochromatic radiation is directly proportional to the irradiance of the radiation at the surface.

13. Threshold Frequency and Work Function
The minimum frequency of electromagnetic radiation required to eject an electron from a surface is called the threshold frequency, f0.
The minimum energy required to eject an electron from a surface is called the work function, E0.
E0 = hf0

14. Threshold Frequency and Work Function (continued)
If the energy of the incident photon is greater than the work function then the extra energy will appear as kinetic energy of the electron.
Ek = E - E0 or
Ek = hf - hf0

15. Worked Example
The work function of a metal is 4.9 x J. Calculate the kinetic energy of the electrons emitted when radiation with a frequency of 8.10 x1014 Hz is incident on the metal.
Ek = hf - E0
= (6.63 x x 8.10 x 1014) x 10-19
= 4.70 x J