1. Photoelectric Effect
Topic 14.2

2. Outcomes
You will describe the photoelectric effect in terms of the intensity and wavelength or frequency of the incident light and surface material
You will describe, quantitatively, photoelectric emission, using concepts related to the conservation of energy
You will describe the photoelectric effect as a phenomenon that supports the notion of the wave-particle duality of EMR

3. The Photoelectric Effect
A metal surface emitted electrons when illuminated by light
Each metal has a particular frequency of light that must hit it in order to emit electrons (threshold frequency)
If this frequency is not met, then no electrons will be released
As soon as this frequency is met (NO MATTER THE INTENSITY) electrons will be released
The intensity of the light dictates the number of the electrons released (more photons), while the frequency predicts the energy of electrons (higher energy photons)

5. Calculating the Energy of the photon
As we’ve seen previously, the energy of a photon can be found using either:

6. Calculating Energy of the Photoelectron
The energy of the emitted electron can be found using Conservation of Energy:
Since, the energy of the photon striking the plate must equal the energy of the freed electron AND the work done to free it (work function)
This leads to:
Ei = Ef
Ephoton = Ek-electron + Work done

7. Calculating Energy of the Photoelectron
The energy of the emitted electron can also be calculated using:
The stopping voltage is measured by reversing the polarity of the photoelectric cell and adjusting the voltage until no electron jumps the gap

8. Calculating Work Function
This is really a special form of:
When the kinetic energy of the released electron is zero, then the work function and the energy of the absorbed photon are equal

9. Sample Problem
Blue light shines on a metal surface and causes photoemission of electrons. If a stopping potential of 2.6 V is required to completely prevent electrons from reaching the collector, determine the maximum kinetic energy of the electrons. Express your answer in units of joules and electron volts.

10. Sample Problem
Experiments show that the work function for cesium metal is 2.10 eV. Determine the threshold frequency and wavelength for photons capable of producing photoemission from cesium.

11. Sample Problem
Using Table 14.1, determine the maximum speed of electrons emitted from an aluminium surface if the surface is illuminated with 125-nm ultraviolet (UV) light.

12. Homework
Concept Qs: p. 720 #1-7, 11
Calculation Qs: p. 125 #1-17 odd