Theme E — Nuclear and Quantum Physics
E.2.1 The Photoelectric Effect — IB Physics HL Revision Notes & Topical Questions
5 min read 10 parts Lesson 11 of 45 1 quiz
Lesson summary
E.2.1 The Photoelectric Effect — IB PHYSICS HL notes: Describe the photoelectric effect, its observations, the threshold frequency and work function, and why the wave model fails.Definition — electrons are emitted from a metal surface when it absorbs electromagnetic radiationThe …
- Property of the metal
- Energy well
- One photon, one electron
- Tightly bound electrons
- Link to threshold
- Alkali metals
Part 1 of 8The effect
Definition
- Definition electrons are emitted from a metal surface when it absorbs electromagnetic radiation
- The emitted electrons are photoelectrons
Discovery
- Discovery by Hertz in 1887 and explained by Einstein in 1905 using photons
Evidence
- Evidence it shows that light is quantised in discrete packets, since each electron absorbs a single photon
Apparatus
- Apparatus an evacuated tube with an emitter plate, a collector, a variable supply and an ammeter
- A clean metal surface is needed
Part 2 of 8Threshold frequency
Definition
- Definition the minimum frequency of incident radiation required to remove a photoelectron from the surface of a metal
Property of the metal
- Property of the metal threshold frequency and wavelength vary from metal to metal
Examples
- Examples sodium 4.40 × 1014 Hz, zinc 1.02 × 1015 Hz and copper 1.13 × 1015 Hz
- (values are given in questions)
Relation
- Relation the threshold wavelength is λ0 = c/f0
Part 3 of 8Work function
Work function
Definition
- Definition the minimum energy needed to release a photoelectron from the surface of a metal
Energy well
- Energy well the electrons are trapped in a well whose depth from the surface to the top is Φ
One photon, one electron
- One photon, one electron an electron escapes only if it absorbs a photon of energy at least Φ
Tightly bound electrons
- Tightly bound electrons need more energy to escape
Link to threshold
- Link to threshold hf0 = Φ
Alkali metals
- Alkali metals weak attraction between surface electrons and ions, so thresholds are in the visible region
Transition metals
- Transition metals stronger attraction, so thresholds are in the ultraviolet
Part 4 of 8Observations
Threshold
- Threshold below f0 no electrons are emitted however intense the light or long the exposure
No delay
- No delay emission starts almost instantly (about 10−9 s) even for dim light above the threshold
Photocurrent
- Photocurrent proportional to the intensity at a given frequency above threshold
Maximum kinetic energy
- Maximum kinetic energy it does not depend on intensity but increases linearly with frequency
Part 5 of 8Why wave theory fails
Continuous energy
- Continuous energy waves would give energy gradually, so any frequency should eject electrons after a delay
Intensity
- Intensity wave theory predicts the kinetic energy would depend on intensity
Photon theory
- Photon theory light comes in quanta hf and each photon gives all its energy to one electron
Low frequency
- Low frequency a photon with hf below Φ frees no electron whatever the number of photons
Wording
- Wording a low-frequency light fails because each photon has too little energy, not because it is too weak
Part 6 of 8Kinetic energy and intensity
Independent of intensity
- Independent of intensity each electron absorbs only one photon
More photons
- More photons a higher intensity gives more photoelectrons and not faster ones
Why a maximum
- Why a maximum surface electrons lose only Φ while deeper electrons lose extra energy getting out
Photoelectric current
- Photoelectric current the number of electrons per second multiplied by the electron charge
Part 7 of 8Stopping potential
Method
- Method a reverse p.d. applied between emitter and collector increases until no electrons cross
Equation
- Ek,max = eVs
Graph
- Graph current is saturated at positive voltages, falls to zero at −Vs
Higher intensity
- Higher intensity a larger saturation current but the same Vs
Higher frequency
- Higher frequency a larger magnitude of Vs
Part 8 of 8More from the specification
More from the specification
Photoelectric Effect and Photon Model
- The photoelectric effect demonstrates the particulate nature of light: incident photons of frequency f transfer discrete quanta of energy E = h·f to surface electrons, with stopping potential measuring maximum kinetic energy.
Compton Scattering and Photon Momentum
- In Compton scattering, a high-energy X-ray photon collides elastically with a stationary electron;
- the scattered photon exhibits a wavelength change that increases with scattering angle θ (Δλ = (h / me c)(1 - cos θ)).
De Broglie Wavelength and Matter Waves
- Wave-particle duality extends to material particles: moving electrons possess a de Broglie wavelength λ = h / p, exhibiting wave diffraction when scattered through crystalline lattices.
Diffraction
- Spreading of waves through a gap or around an edge
Constant
- Not changing.
Scattering
- Deflection of particles from their original path.
Stopping potential
- The p.d. that stops the most energetic photoelectrons
Scattering angle
- The angle θ through which the photon is deflected.
Wave-particle duality
- The property that radiation and matter show both wave and particle behaviour.
Compton scattering
- Scattering of a photon by an outer electron, with transfer of part of the photon's energy.
Key terms
| Term | Meaning |
|---|---|
| Photoelectric effect | The emission of electrons from a metal surface when radiation of sufficient frequency falls on it. |
| Photoelectron | An electron emitted from a surface by the photoelectric effect. |
| Threshold frequency | The minimum frequency of radiation that causes photoemission from a given metal. |
| Work function | The minimum energy needed to release a photoelectron from the surface of a metal. |
| Stopping potential | The potential difference that just stops photoelectron emission. |
Avoid these mistakes
1
Saying a brighter light gives the electrons more energy — it gives more electrons.
2
Thinking electrons store energy from several photons — each absorbs one photon.
3
Expecting a time delay for dim light — emission is immediate.
4
Confusing saturation current (rate) with stopping potential (maximum energy).
5
Saying low-frequency light fails because it is too weak — each photon has too little energy.
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