Light, Waves & the Electromagnetic Spectrum

Almost everything we know about distant objects comes from analyzing their light. Light is an electromagnetic wave — coupled oscillating electric and magnetic fields propagating at c = 3 × 10⁵ km/s. The visible band (400–700 nm) contains six colors — red, orange, yellow, green, blue, violet — but the full EM spectrum spans radio, microwave, infrared, visible, ultraviolet, X-ray, and gamma-ray. Only visible, radio, and some infrared penetrate Earth's atmosphere; everything else requires space telescopes.

Any warm body emits blackbody radiation whose spectrum depends only on temperature. Wien's law (λ_max = 2.9 × 10⁶ / T, nm vs. K) gives the peak wavelength — so a star's color reveals its temperature. The Sun (~5,800 K) peaks at ~500 nm (yellow-green); hot blue stars exceed 30,000 K; cool red stars are below 3,500 K. Stefan's law (E ∝ T⁴) gives the total radiated power.

Spectroscopy breaks light into wavelengths. Kirchhoff's three laws describe the three spectrum types: (1) a dense source produces a continuous spectrum; (2) a hot rarefied gas produces a bright-line emission spectrum; (3) a cool gas in front of a continuous source produces dark absorption lines. Each element has a unique spectral fingerprint set by atomic energy levels (Bohr model). Helium was actually discovered in the Sun's spectrum before being found on Earth.

The Doppler effect shifts spectral lines depending on relative motion: light from an approaching source is blueshifted (shorter wavelength), light from a receding source is redshifted. The shift obeys Δλ/λ = v/c. This single tool reveals stellar velocities, binary orbits, galactic rotation, and ultimately the expansion of the universe.