Lesson 2: The Photoelectric Effect & Work Function
Demonstrating understanding of quantum phenomena, atomic structures, nuclear reactions, and special relativity.
🎯 Ngā Whāinga Akoranga | Learning Intentions
Einstein's photoelectric equation E_k(max) = hf - Φ, threshold frequency f_0 = Φ/h, stopping voltage V_s, and evidence for photon particle nature.
Graph maximum kinetic energy vs frequency, calculate work function Φ and threshold wavelength λ_0, and explain why wave theory fails.
🎥 Media Anchor & Pedagogical Scaffold
Einstein's Photoelectric Effect & Work Function
Video Clip: Photoelectric effect — photons, work function, and Einstein's explanation | Khan Academy (Runtime: 10m 24s).
🧠 1. Before Viewing (Activate & Predict)
Why does bright red light fail to eject a single electron from a metal plate while dim ultraviolet light ejects them instantly?
👁️ 2. During Viewing (Watch With a Job)
- Define the photoelectric effect: when does it occur, and why does increasing light intensity (but not frequency) fail to eject electrons?
- Work function: what is the work function (φ) of a metal, and what does it represent physically? Write Einstein's equation for photoelectric emission.
- Calculate: a metal has a work function of 3.0 eV. What is the minimum frequency of light needed to eject electrons? (h = 6.63 × 10⁻³⁴ J·s, 1 eV = 1.6 × 10⁻¹⁹ J)
🗣️ 3. After Viewing & Kaiako Move (Process & Apply)
Kaiako Move: Use interactive PhET Photoelectric Effect simulation to model current vs voltage under variable wavelengths.
Immediate Task: Complete Section 2 of your Physics Portfolio: Photoelectric Effect Graph Analysis & Work Function Calculations.
⚡ Whakaoho | Do Now: Modern Physics Recall (10 mins)
The experiment that broke the wave theory. Shine intense red light on a metal: no electrons, no matter how bright or how long you wait. Shine faint blue light: electrons immediately.
Two minutes: if light were purely a wave, brighter should mean more energy delivered, and waiting longer should let energy accumulate. Neither works. State what property of light is actually controlling whether an electron escapes — and what that forces you to conclude about how light arrives.
📖 Activity 1: Physics Concept Exploration & Problem Solving (25 mins)
Plot and extract (15 min). Using the data provided, graph maximum kinetic energy against frequency. Your line has a gradient and two intercepts. Identify what each one physically is — gradient, x-intercept, y-intercept — and extract Planck's constant and the work function from your own graph.
Predict three metals (10 min). Given three work functions, calculate each threshold frequency and decide which metals emit electrons under the given light source. For any that do not, state exactly what would have to change — and confirm that increasing intensity is not the answer.
📝 Activity 2: Level 3 Physics Portfolio Task & Merit/Excellence Scaffolding (20 mins)
Portfolio — Section 2. Submit: (1) your E_k vs f graph with gradient and intercepts identified and h and Φ extracted; (2) three threshold-frequency calculations with verdicts; (3) a paragraph stating precisely which observations the wave model cannot explain and why the photon model can.
🏫 Kaiako Planning & Pedagogy Notes
NCEA Level 3 Alignment: Direct preparation for Level 3 Physics (Demonstrate understanding of application of modern physics). Emphasise clear physical explanations and multi-step mathematical working for Merit/Excellence grades.