Electromagnetic Spectrum
All electromagnetic radiation — from radio waves to visible light to X-rays to gamma rays — is the same physical phenomenon: oscillating electric and magnetic fields propagating through space at the speed of light (c ≈ 3×10⁸ m/s). What differs is frequency (and equivalently, wavelength and energy per photon). The electromagnetic spectrum is the full range of these frequencies, arranged from lowest to highest.
Visible light occupies a tiny slice of this spectrum — roughly wavelengths from 380 nm (violet) to 700 nm (red). Our eyes evolved to be sensitive to this narrow band because it corresponds to the peak of solar output. The universe is broadcasting across all frequencies simultaneously; we are picking up a very small channel.
The Spectrum
From lowest to highest frequency (longest to shortest wavelength):
- Radio waves — broadcast communications, telescopes observing distant galaxies
- Microwaves — radar, cellular communications, the cosmic microwave background (relic of the Big Bang)
- Infrared — heat radiation; objects at room temperature emit in this range
- Visible light — the narrow band our eyes detect
- Ultraviolet — causes sunburn; ionizes some molecules
- X-rays — penetrate soft tissue; used in medical imaging
- Gamma rays — emitted by nuclear reactions and some astronomical events; highly ionizing
Light as Both Wave and Particle
Classically, electromagnetic radiation is described as a continuous wave. Quantum mechanics requires a different picture: the energy comes in discrete packets called photons. The energy of a single photon is proportional to its frequency: E = hf, where h is Planck's constant. Visible light photons carry enough energy to trigger the chemical reaction in a retinal cell. Radio photons carry so little energy that it takes vast numbers to have any biological effect. Gamma photons carry enough energy to ionize atoms and break DNA strands.
The wave and particle descriptions are complementary, not contradictory — they apply in different experimental contexts. This is the first hint of wave-particle-duality.
Connections
- four-fundamental-forces — electromagnetism is one of the four; light is how the EM force propagates energy
- wave-particle-duality — visible light was the original context where wave-particle tension became apparent
- quantum-probabilism — photons obey the same quantum probability rules as electrons