Lesson 1: Photoelectric Effect | Modern Physics

Lesson at a Glance | He Tirohanga Whakamua

Do NowWhy bright red light cannot eject electrons while dim UV light does10 min

Einstein's Photon ModelLight quanta

E = h f

& Work Function

Φ = h f_{0}

15 min

Photoelectric EquationSolving

E_{k} = h f - Φ = e V_{s}

and

E_{k}

f

graphs15 min

Portfolio EntryWrite Section 1: Photoelectric Effect & Photon Calculation Map10 min

Exit DrillCalculate stopping voltage for zinc (

Φ = 4.3 eV

) exposed to UV light (

λ = 200 nm

)5 min

Ngā Whāinga Ako | Learning Intentions

Students will know

Why classical wave theory fails to explain the photoelectric effect (intensity vs frequency mismatch, zero time-delay emission).
Einstein's Photon Quanta Model: Light travels as localized energy packets where $E = h f = \frac{h c}{λ}$ .
Einstein's Photoelectric Equation:
• $E_{k, m a x} = h f - Φ$
• Threshold Frequency: $f_{0} = \frac{Φ}{h}$
• Stopping Voltage: $e V_{s} = E_{k, m a x}$

Students will demonstrate

By plotting an $E_{k, m a x}$ vs frequency graph to determine Planck's constant ( $h$ ) from gradient and work function ( $Φ$ ) from y-intercept.
By completing Section 1 of their Level 3 Modern Physics Mastery Portfolio.

Do Now | Tīmatanga Whakaaro (10 min)

Classical Physics Breakdown Prompt:

"If light were a continuous classical wave, shining an extremely bright red floodlight on a metal plate should eventually heat up and eject electrons, while a dim UV lamp shouldn't. Why does dim UV light eject electrons instantly, while intense red light ejections never occur?"

Unpack: Because light is quantized into photons ( $E = h f$ )! A single red photon lacks the individual energy required to overcome the metal's binding energy (Work Function $Φ$ ), no matter how many red photons hit the plate. A UV photon has frequency $f & g t; f_{0}$ , providing instant 1-to-1 energy transfer to eject an electron!

Einstein's Photoelectric Equation & Graph (15 min)

1. Energy Balance Equation

$h f = Φ + E_{k, m a x}$
$h f$ : Incident photon energy
$Φ$ : Work function (minimum energy to free electron)
$E_{k, m a x}$ : Maximum kinetic energy of emitted photoelectron.

2. $E_{k}$ vs Frequency Graph Interpretation

Gradient = Planck's constant ( $h = 6.63 \times 10^{- 34} J s$ ).
x-intercept = Threshold frequency ( $f_{0}$ ).
y-intercept = $- Φ$ (Negative work function).

📁 Physics Modern Portfolio — Section 1: Photoelectric Effect

Students open their Level 3 Physics Portfolio and complete Section 1:

Section 1 Requirements:

1. Photoelectric Experiment Diagram: Draw a vacuum phototube circuit with incident light, emitter plate, collector plate, variable power supply, and ammeter.

2. Multi-Step Photon Solver: Calculate photon energy ( $E = h f$ ), threshold frequency ( $f_{0}$ ), $E_{k, m a x}$ , and stopping voltage ( $V_{s}$ ) for potassium ( $Φ = 2.30 eV$ ) illuminated by $400 nm$ violet light.

3. Excellence Classical vs Quantum Analysis: 1-paragraph critique explaining why classical wave theory cannot account for zero emission time-lag or stopping voltage independence from light intensity.

Exit Verification | Ka Mutu Hoki (5 min)

Exit Check:

"My Section 1 calculates photon energy E = 3.10 eV for 400nm light, yielding photoelectron Ek = 0.80 eV and stopping voltage Vs = 0.80 V for potassium."

Teacher Planning & NCEA Alignment

NCEA Level 3 Physics Alignment (3 Credits Internal):

Photoelectric Effect: Demonstrate understanding of photon quanta, work function, threshold frequency, stopping voltage, and $E_{k}$ vs $f$ graphs.
Quantum Physics Applications: Contrast classical wave theory with quantum particle models.

Vocabulary: Photoelectric effect, photon ( $E = h f$ ), Planck's constant ( $h$ ), work function ( $Φ$ ), threshold frequency ( $f_{0}$ ), photoelectron, stopping voltage ( $V_{s}$ ), electron-volt ( $eV$ ).

Other teaching approach: Guided Viewing & Problem Practice →

Lesson 1: The Quantum Revolution & Photoelectric Effect