Light Beyond Seeing
The light our eyes detect — red through violet — is a tiny sliver of the electromagnetic spectrum. The universe radiates across all wavelengths, and every band tells a different story.
The full spectrum
All electromagnetic radiation is the same phenomenon: oscillating electric and magnetic fields propagating at the speed of light, $c \approx 3 \times 10^8$ m/s. What changes is the wavelength $\lambda$ (or equivalently, the frequency $f = c/\lambda$ and the photon energy $E = hf$).
| Band | Wavelength | Sees | Telescope |
|---|---|---|---|
| Radio | > 1 mm | Gas clouds, pulsars, cosmic microwave background | Arecibo, ALMA, SKA |
| Infrared | 700 nm – 1 mm | Dust, cool stars, distant galaxies (redshifted) | JWST, Spitzer |
| Visible | 400 – 700 nm | Stars, galaxies (as our eyes see them) | Hubble, ground telescopes |
| Ultraviolet | 10 – 400 nm | Hot stars, active galactic nuclei | Hubble (UV mode) |
| X-ray | 0.01 – 10 nm | Black hole accretion, supernova remnants, hot gas | Chandra, XMM-Newton |
| Gamma ray | < 0.01 nm | Gamma-ray bursts, cosmic rays, most energetic events | Fermi |
Wien’s law and blackbody radiation
Hot objects radiate across the spectrum, but the peak wavelength depends on temperature:
$$\lambda_{\text{peak}} = \frac{2.898 \times 10^{-3}}{T} \text{ m}$$
The Sun (5,778 K) peaks at ~500 nm (green-yellow visible light). A red dwarf (3,000 K) peaks in the infrared. A neutron star (1,000,000 K) peaks in X-rays. Temperature determines colour — which is why the Hertzsprung-Russell diagram’s x-axis is temperature.
Atmospheric windows
Earth’s atmosphere blocks most of the spectrum. Only radio and visible/near-infrared pass through easily — these are the “atmospheric windows.” Everything else (UV, X-ray, gamma, most infrared) requires space telescopes.
This is why JWST had to go to space (infrared), why Chandra is in orbit (X-rays), and why ground-based radio telescopes can be enormous (the atmosphere is transparent at those wavelengths).
Multi-wavelength astronomy
The deepest understanding comes from observing the same object across multiple wavelengths. The Crab Nebula in radio shows synchrotron emission from electrons spiralling in magnetic fields. In visible light, you see the expanding gas shell. In X-rays, the central pulsar blazes. Same object, completely different physics revealed at each wavelength.