Lecture 3 Wk 1

All images sourced from: Blackman, Bottle, Schmid, Mocerino, and Wille, Chemistry, 2012 (John Wiley & Sons) ISBN: 9780470810866

Overview of Lecture Topics:
- Light
- Atomic structure
- Elements
- Electronic structure

Learning Outcomes:
- Understand the relationship between frequency, wavelength, and energy.
- Recognize the wave and particle natures of light.
- Understand that light is a form of electromagnetic radiation.

Light as a Source of Energy:
- Absorption and emission of light by atoms/molecules critical for understanding their structure.
- Light carries energy.

Two Theories of Light:
1. Light as a wave.
2. Light as a beam of particles.
- Historical perspective:
  - Newton (1687) supported particle theory.
  - Huygens (1690) supported wave theory.

Thomas Young’s Double-Slit Experiment:
- Early 19th-century evidence favored wave theory.
- Light produced a diffraction pattern typical of waves.
- Inquiry into what oscillates in a light wave.

Electromagnetic Waves:
- James Clerk Maxwell (1873) defined light as an electromagnetic wave.
- Oscillating electric and magnetic fields propagate as light.
- Verified by Heinrich Hertz (1887).
- Speed of electromagnetic wave: 3 x 10^8 m/s.

Electromagnetic Radiation Spectrum:
- Wavelength (nm) distribution:
  - Gamma rays
  - X-rays
  - Ultraviolet
  - Visible
  - Infrared
  - Microwaves
  - Radio frequency
- Visible light occupies a narrow band in this spectrum: 400-750 nm.

The Wave Nature of Light:
- Wavelength (λ): Distance between identical points on a wave.
- Frequency (ν): Number of wave crests passing a point per unit time.
- Amplitude (A): Height of the wave; related to intensity/brightness.

Properties of Electromagnetic Waves:
- All travel at the same speed (speed of light in vacuum: c = 2.998 × 10^8 m/s).
- Relationship: c = λν (wavelength and frequency correlation).

Origins of Quantum Theory:
- Early 20th century brought a shift in understanding energy and light.
- Blackbody Radiation phenomenon exhibited discrepancies with classical theories (e.g., ultraviolet catastrophe).
- Max Planck (1900) introduced energy quantization: E = hν
  - (ν = frequency of oscillation, h = Planck’s constant).

Further Developments in Quantum Theory:
- Despite quantization, light was still viewed as a wave by most physicists, including Planck.
- Photoelectric Effect: Light can eject electrons from metals only if frequency is above a specific threshold.
  - Classical view: energy proportional to amplitude (wave theory) vs. light's energy proportional to frequency (particle view).
- Einstein (1905) proposed light has both wave and particle natures (photons).

The Particle Nature of Light:
- All electromagnetic radiation travels at the same speed, but energy varies with frequency.
- Higher frequency = faster arriving photons = higher energy.
- Formula: E = hν (Planck's relation).
- Constant: h = 6.626 x 10−34 Js.

Wave-Particle Duality:
- Light exhibits both wave behaviors (as shown in Young's double-slit experiment) and particle behaviors (photoelectric effect).
- Chemists utilize the model (wave or particle) that best fits experimental data.

Final Learning Outcomes:
- Understand the relationship c = λν and E = hν.
- Evidence for both wave and particle behavior demonstrated by experiments.
- Recognize oscillation of electric and magnetic fields in wave model of light.

Preparation for Next Lecture:
- Question: Comparing red and blue light in terms of frequency, wavelength, speed, and energy.
- (a) Which has higher frequency? (b) Longer wavelength? (c) Faster speed? (d) Greater energy?