Photoelectric Effect

Objectives

Lesson outcomes

Describe how electromagnetic radiation can emit photoelectrons from a metal surface.
Use threshold frequency, threshold wavelength, and work function to decide whether emission occurs.
Apply the photoelectric equation to maximum kinetic energy calculations.
Explain why intensity changes current but not maximum photoelectron kinetic energy.

Syllabus

CIE 9702 syllabus points

10 linked

22.1.1 understand that electromagnetic radiation has a particulate nature
22.1.2 understand that a photon is a quantum of electromagnetic energy
22.1.3 recall and use E = hf
22.1.4 use the electronvolt (eV) as a unit of energy
22.1.5 understand that a photon has momentum and that the momentum is given by p = E / c
22.2.1 understand that photoelectrons may be emitted from a metal surface when it is illuminated by electromagnetic radiation
22.2.2 understand and use the terms threshold frequency and threshold wavelength
22.2.3 explain photoelectric emission in terms of photon energy and work function energy 1
22.2.4 recall and use hf = Φ + 2 mvmax2
22.2.5 explain why the maximum kinetic energy of photoelectrons is independent of intensity, whereas the photoelectric current is proportional to intensity

Lesson Notes

Student guidance and lesson notes

Overview

This lesson uses the photoelectric effect as evidence that electromagnetic radiation transfers energy in photons. You will connect threshold behaviour, work function, maximum kinetic energy, and intensity to the photon model of light.

What You Need to Know

Photoelectrons may be emitted when electromagnetic radiation is incident on a metal surface.
Emission only occurs if each photon has enough energy to overcome the work function of the metal.
Threshold frequency is the minimum frequency needed for emission. Threshold wavelength is the corresponding maximum wavelength.
The photoelectric equation links photon energy, work function, and maximum kinetic energy.
Increasing intensity increases the rate of photon arrival, so it can increase photoelectric current.
Increasing frequency above threshold increases maximum photoelectron kinetic energy.

How to Work Through It

Start by predicting what happens when frequency and intensity are changed separately.
Define threshold frequency, threshold wavelength, and work function.
Use photon energy and the photoelectric equation in worked calculations.
Explain the observations that cannot be explained by a purely wave model of light.

Check Your Understanding

Why is there no emission below the threshold frequency, no matter how intense the light is?
What happens to maximum kinetic energy when the frequency is increased above threshold?
What happens to photoelectric current when the intensity is increased at fixed frequency?
How does the work function determine the threshold frequency of a metal?

Common Mistakes

Saying intensity gives each electron more energy. In the photon model, intensity changes the number of photons per second.
Treating threshold wavelength like threshold frequency. Longer wavelength means lower photon energy.
Forgetting that the photoelectric equation uses maximum kinetic energy.
Mixing up work function energy with the kinetic energy left after emission.

Next Steps

Practise explaining the photoelectric effect in words as well as using the equation.
Bring photon energy ideas into line spectra and wave-particle duality.