4.5. Quantum Physics

Page 1: Introduction to Quantum Physics

• PMT resources for OCR A Physics A-level, Topic 4.5

• Focus on significant concepts and applications related to quantum physics

Page 2: Photons and Electromagnetic Radiation

Photon Model

• Electromagnetic radiation behaves as a continuous wave

  • Evidence: Diffraction and interference

• Interaction with matter occurs in discrete packets called photons

• Energy of a photon:

  • Formula: ( E = hf = \frac{hc}{\lambda} )

  • Where:

    • ( h ): Planck constant (6.63 x 10^-34 Js)

    • ( c ): speed of light; ( \lambda ): wavelength

• Electronvolt (eV):

  • Preferred unit for photon energy due to smaller scale; 1 eV = 1.60 x 10^-19 joules

  • Formula for work done: ( W = VQ )

Using LEDs to Estimate Planck's Constant

• Experiment setup with LEDs that emit light at a specific color

• Measure threshold potential difference to turn on LEDs

• Energy calculation: Equate electron energy with photon energy using ( eV = \frac{hc}{\lambda} )

• Create a graph of threshold p.d.

  • Gradient of graph = ( \frac{hc}{e} )

Page 3: The Photoelectric Effect

Overview

• Electromagnetic radiation causes electron release from a metal surface (photoelectric effect)

• Demonstration: Gold leaf electroscope with zinc plate

  • UV light results in observable electron release, while visible light does not

Key Observations

• Intensity of visible light does not influence electron release

• UV light, even at low intensity, causes instantaneous electron release

Energy Conservation in Photon-Electron Interaction

• Work Function (( \phi )): Minimum energy required for electron release

• Relationship with photon frequency:

  • Each photon must meet or exceed ( \phi ) to liberate an electron

  • Creates the concept of threshold frequency

Einstein's Photoelectric Equation

• Formula: ( hf = \phi + KE_{max} )

  • ( KE_{max} ): Maximum kinetic energy of ejected electrons

  • Energy must be in consistent units

• Observation: No electron emission if frequency is below threshold, regardless of intensity

Page 4: Factors Influencing the Photoelectric Effect

Intensity and Frequency

• Higher frequency radiation increases kinetic energy of emitted electrons

• Increasing intensity raises the number of emitted electrons but not their energy

Wave-Particle Duality

• De Broglie Equation: ( \lambda = \frac{h}{p} = \frac{h}{mv} )

  • Relates particle wavelength inversely to momentum

• Higher mass results in lower wavelengths, difficult to detect wave properties

Evidence of Wave-Particle Duality

• Electrons exhibit particle characteristics (mass, charge) and wave properties (diffraction)

• Experiment: Electron diffraction through polycrystalline graphite produces a diffraction pattern

• Electrons can diffract, affirming wave behavior in addition to particle characteristics.