Module E.2 IBDP HL only Track

Quantum Physics

Investigate the photoelectric mechanism, matter wave duality, and tunneling probability behaviors.

01. Observe 02. Think 03. Apply 04. Achieve Excellence

Quantum Physics

Introduction

Quantum physics is where the wave model and particle model stop being rivals and become complementary. The guide focuses on the photoelectric effect, matter waves, diffraction of particles, and Compton scattering.

Guide Focus

  • Use the photoelectric effect as evidence for photon behaviour.
  • Apply Einstein’s equation Emax = hf - Phi.
  • Use de Broglie wavelength and Compton scattering evidence.

Key Concepts

1. Photoelectric effect

Light can release electrons from a metal only if its frequency exceeds a threshold frequency. Increasing intensity below threshold does not release electrons, which supports a photon model of light.

2. Work function and maximum kinetic energy

The work function Phi is the minimum energy needed to remove an electron. Einstein’s equation is Emax = hf - Phi.

3. Matter waves

Particles can show diffraction, demonstrating wave-like behaviour. The de Broglie wavelength is lambda = h / p.

4. Compton scattering

Compton scattering provides further evidence for photon momentum. A photon scattered by an electron has increased wavelength, with shift delta lambda = h/(me c)(1 - cos theta).

Common Mistakes

  • Saying photoelectron energy increases with intensity rather than frequency.
  • Forgetting that no electrons are emitted below threshold frequency.
  • Using mass instead of momentum in lambda = h / p.

Exam Tips

  • Draw photoelectric graphs with frequency on the horizontal axis to identify threshold and Planck’s constant.
  • For diffraction of particles, larger momentum means smaller wavelength.
  • The Compton formula uses scattering angle theta, not incidence angle.

Practice Questions

Question 1 (Multiple Choice)

In the photoelectric effect, increasing the frequency of light above threshold increases the:

A. Maximum kinetic energy of photoelectrons. B. Work function of the metal. C. Threshold frequency. D. Charge on each electron.

Correct Answer: A

Solution Architecture

Emax = hf - Phi, so increasing f increases maximum kinetic energy when the metal is unchanged.

Question 2 (Structured Paper 2 Style)

Light of frequency 8.0 x 10^14 Hz falls on a metal with work function 2.0 eV.

(a) Calculate the photon energy in eV. Use h = 4.14 x 10^-15 eV s. [2 marks]

(b) Calculate the maximum kinetic energy. [1 mark]

Paper 2 Structured Problem

Markscheme Breakdown

Part (a) Solution:

E = hf = 4.14 x 10^-15 x 8.0 x 10^14 = 3.31 eV.

Part (b) Solution:

Emax = 3.31 - 2.0 = 1.31 eV.