Overview
This lecture covers the properties of light, particularly photons, and explains how to calculate their energy and frequency using key equations. It also reviews the electromagnetic spectrum, especially the visible light range.
Properties of Light & Photons
- Light consists of massless, chargeless particles called photons, which have both wave and particle properties.
- Photon properties include energy (E), frequency (ν), wavelength (λ), and speed (c); amplitude relates to intensity, not covered in detail.
- Photons are a type of electromagnetic radiation, not to be confused with radioactive emissions.
Key Equations & Constants
- Energy and frequency: E = hν
- Energy and wavelength: E = hc/λ
- Speed of light: c = λν
- Planck's constant (h): 6.626 × 10⁻³⁴ J·s
- Speed of light (c): 3.00 × 10⁸ m/s
Units & Conversions
- Energy is measured in joules (J).
- Frequency is measured in Hertz (Hz) or s⁻¹.
- Wavelength is usually in meters (m); convert nanometers (nm) to meters for calculations (1 m = 10⁹ nm).
- Always match units with constants when performing calculations.
Example Calculations (Violet & Red Photons)
- Violet photon (405 nm):
- Convert to meters: 4.05 × 10⁻⁷ m.
- Frequency: ν = c/λ = 7.41 × 10¹⁴ s⁻¹.
- Energy: E = hc/λ = 4.91 × 10⁻¹⁹ J.
- Red photon (650 nm):
- Convert to meters: 6.50 × 10⁻⁷ m.
- Frequency: ν = 4.62 × 10¹⁴ s⁻¹.
- Energy: E = 3.06 × 10⁻¹⁹ J.
Relationships Between Photon Properties
- Higher frequency means higher energy; energy and frequency are directly proportional.
- Longer wavelength means lower energy; energy and wavelength are inversely proportional.
- Wavelength and frequency are inversely related: as one increases, the other decreases.
- All photons travel at the speed of light, regardless of energy or wavelength.
Electromagnetic Spectrum Overview
- Ranges from low energy (radio waves) to high energy (gamma rays).
- Visible light occupies a small range (about 400–800 nm) within the spectrum: violet (~400 nm) to red (~800 nm).
- UV, x-rays, and gamma rays are higher energy; UVB and UVC are ionizing and can be harmful.
Typical Values for Visible Light
- Wavelength: 400–800 nm (visible range).
- Frequency: on the order of 10¹⁴ Hz.
- Energy: on the order of 10⁻¹⁹ J per photon.
Key Terms & Definitions
- Photon — A massless, chargeless particle of light with wave and particle characteristics.
- Wavelength (λ) — The distance between successive wave peaks; measured in meters.
- Frequency (ν) — Number of wave cycles per second; measured in Hertz (Hz or s⁻¹).
- Amplitude — Height of the wave, related to light intensity (not discussed in detail).
- Electromagnetic spectrum — Range of all possible photon energies/wavelengths, from radio waves to gamma rays.
- Planck’s constant (h) — Fundamental constant relating photon energy and frequency.
Action Items / Next Steps
- Practice converting between nanometers and meters for wavelength.
- Memorize the three main equations: E = hν, E = hc/λ, c = λν.
- Review the order and range of the electromagnetic spectrum, focusing on visible light.