Overview
This lecture covers the nature of light, the wave and particle models, the electromagnetic spectrum, and the photoelectric effect, highlighting their relevance to quantum mechanics.
The Nature of Light and Early Quantum Theory
- Subatomic particles like electrons behave differently than macroscopic objects, showing both particle and wave characteristics.
- Classic Newtonian physics explained large objects, but not behaviors at the atomic scale.
- Quantum mechanics explains how electrons exist and behave in atoms as probability clouds rather than fixed paths.
Properties of Electromagnetic Radiation
- Light is electromagnetic radiation consisting of perpendicular oscillating electric and magnetic fields.
- All electromagnetic waves travel at the speed of light (c = 3.0 × 10⁸ m/s).
- Amplitude is the wave’s height and measures light intensity (brightness).
- Wavelength (λ) is the distance from crest to crest, determining light’s color.
- Frequency (ν) is the number of wave cycles per second, measured in hertz (Hz = 1/s).
Color, Intensity, and the Electromagnetic Spectrum
- Wavelength determines color; longer wavelengths are red, shorter are blue or violet.
- Amplitude and wavelength are independent; brightness and color are unrelated.
- White light contains all visible wavelengths, and objects appear colored by reflecting particular wavelengths.
- The electromagnetic spectrum includes, in order of increasing frequency (decreasing wavelength): radio, microwave, infrared, visible, ultraviolet, x-ray, gamma ray.
- Higher frequency waves (ultraviolet, x-rays, gamma rays) have more energy and can cause biological damage.
Wave Behaviors and Interactions
- Waves can interact (interference): constructive (amplify) or destructive (cancel).
- Waves bend around obstacles or openings (diffraction), unlike particles.
- Two slits create patterns of constructive and destructive interference, forming alternating light and dark bands.
The Photoelectric Effect and Quantum Theory
- The photoelectric effect: shining light on metal releases electrons ("photoelectrons").
- Classical theory predicted higher intensity light would release more electrons; Einstein showed frequency (not intensity) is key.
- There is a threshold frequency needed to release electrons; below this, no electrons are emitted regardless of intensity.
- Light delivers energy in packets called photons (quantum theory).
- The energy of a photon: E = (h × c) / λ, where h is Planck's constant.
- Extra photon energy above threshold becomes the kinetic energy of ejected electrons.
Key Terms & Definitions
- Electromagnetic Radiation — Energy transmitted as oscillating electric and magnetic fields.
- Amplitude — Height of a wave, measuring light intensity.
- Wavelength (λ) — Distance between wave crests, determining color.
- Frequency (ν) — Number of wave cycles per second, in hertz (Hz).
- Photon — Packet (quantum) of light energy.
- Photoelectric Effect — Emission of electrons from metal when struck by light above threshold frequency.
- Constructive Interference — Waves combine to make a larger wave.
- Destructive Interference — Waves combine to cancel each other out.
- Diffraction — Bending of waves around obstacles or openings.
- Threshold Frequency — Minimum frequency of light needed to eject electrons from a metal.
Action Items / Next Steps
- Memorize the speed of light (3.0 × 10⁸ m/s) and Planck’s constant.
- Know formulas: c = λν and E = (h × c) / λ.
- Review the electromagnetic spectrum and the relationship between wavelength, frequency, and energy.
- Prepare for homework or quizzes using these formulas and concepts.