The EM Spectrum & the Atmosphere
One kind of wave over a vast range of wavelengths.
Lesson at a glanceShow the key points
- Spectrum ≈ 30 fm to 30 km; visible 380–780 nm; ν = c/λ.
- All produced by accelerated charges: LC circuits, hot bodies, atoms, decelerated electrons, nuclei.
- Troposphere, stratosphere (ozone), mesosphere, ionosphere.
- Ozone absorbs UV; IR largely absorbed; visible mostly transmitted.
Maxwell’s equations don’t pick a wavelength. Radio waves, microwaves, light and gamma rays are the same phenomenon over an enormous range of scales.
The electromagnetic spectrum
Visible light spans only about 380–780 nm. Known electromagnetic waves stretch from about 30 fm (30 × 10⁻¹⁵ m) to 30 km. All travel at c in vacuum, so frequency and wavelength are tied by ν = c/λ. The boundaries between named regions are not sharp; gamma rays and X-rays overlap a good deal, and it is only on average that gamma rays are shorter.
Sources and uses
Every electromagnetic wave comes ultimately from accelerated charges, whose changing fields make up the wave.
- Radio waves: charges oscillating in LC circuits; radio and TV broadcasting.
- Microwaves: oscillating currents in electric circuits; radar, cooking, communication.
- Infrared: emitted by atoms and molecules of hot bodies; used in physiotherapy.
- Visible light: emitted by atoms, linked to the acceleration of their electrons.
- Ultraviolet: produced the same way; the Sun emits a lot, harmful in large doses.
- X-rays: most commonly from fast electrons slowing down in a metal target; medical diagnosis, harmful to tissue.
- Gamma rays: emitted by atomic nuclei; the shortest wavelengths we usually meet.
Find the wavelength of an FM station at 100 MHz and of a microwave oven at 2.45 GHz, and the frequency of 0.10 nm X-rays.
Radiation and the atmosphere
The atmosphere reaches about 300 km up, in rough layers without sharp boundaries:
- Troposphere, up to about 12 km: almost all clouds, water vapour and ice. Air at its top is about a tenth as dense as at the ground.
- Stratosphere, 12–50 km, with the ozone layer in its upper part; air at its top is about 10⁻³ times as dense as at the ground.
- Mesosphere, about 50–80 km; above it, the ionosphere.
The Sun is the main source of radiation reaching us, and much of it is absorbed on the way down. Visible light is only weakly absorbed. Much of the infrared is absorbed and heats the air. The ozone layer absorbs most of the ultraviolet and shorter wavelengths, converting it to infrared that heats the atmosphere and ground, and so protects living things.
- 1One spectrum from ≈ 30 fm to 30 km: gamma, X-ray, UV, visible (380–780 nm), IR, microwave, radio; ν = c/λ.
- 2All are produced by accelerated charges.
- 3Atmosphere: troposphere, stratosphere (ozone), mesosphere, ionosphere.
- 4Ozone absorbs UV; IR is largely absorbed low down; visible light mostly gets through.
Check yourself
Keep exploring
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