Electromagnetic Waves
The electromagnetic spectrum
Place radiation in the spectrum and find photon energies.
All electromagnetic waves are the same kind of wave and travel at the same speed c in vacuum. They differ only in frequency and wavelength. The full range is the electromagnetic spectrum:
| Type | Wavelength (roughly) | Made by | Used for |
|---|---|---|---|
| Radio | more than 0.1 m | charges oscillating in aerials | radio, TV, mobile signals |
| Microwaves | 1 mm to 0.1 m | special valves (klystrons, magnetrons) | radar, microwave ovens |
| Infrared | 700 nm to 1 mm | hot bodies, molecules | remote controls, heat therapy, night vision |
| Visible light | 400 nm to 700 nm | electrons in atoms | seeing |
| Ultraviolet | 1 nm to 400 nm | very hot bodies (the Sun) | sterilising water, killing germs |
| X-rays | 10⁻³ nm to 10 nm | fast electrons hitting metal | medical images, cancer therapy |
| Gamma rays | below 10⁻³ nm | atomic nuclei | cancer treatment, nuclear studies |
Remember the order from long to short wavelength (low to high frequency): radio, micro, infrared, visible, UV, X, gamma.
Visible light is a tiny window: violet near 400 nm, red near 700 nm. Our eyes evolved for the light the Sun gives most of.
Each wave also comes in packets of energy called photons:
E = hν = hc / λ
A handy shortcut: E in eV ≈ 1240 / (λ in nm). Red light of 620 nm has photons of 2 eV.
Shorter wavelength means more energetic photons. That is why UV burns skin and X-rays and gamma rays can damage cells, while radio waves pass through us harmlessly.
The photon energy matches the energy changes in the source: about an eV for electrons jumping in atoms (light), keV for inner electrons (X-rays), MeV for nuclei (gamma rays).
Earth's atmosphere blocks most UV (the ozone layer absorbs it), and greenhouse gases trap infrared, keeping the Earth warm.
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