The spectrum
All electromagnetic waves travel at c = 3 × 10⁸ m/s in vacuum and obey c = fλ. They differ only in frequency and wavelength. In order of increasing frequency (decreasing wavelength): radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. The bands overlap at their edges; the limits below are approximate.
Radio waves (λ > 0.1 m)
Produced by accelerated charges in conducting wires (antennas). Used in radio and TV broadcasting and mobile phones. An FM signal of 100 MHz has λ = c/f = 3 m.
Microwaves (0.1 m to 1 mm)
Produced by special vacuum tubes such as klystrons, magnetrons and Gunn diodes. Used in radar for aircraft navigation and in microwave ovens, where the frequency is chosen so that water molecules in food absorb the energy and heat up. A 10 GHz wave has λ = 3 cm.
Infrared (1 mm to 700 nm)
Emitted by hot bodies and molecules; often called heat waves. Used in remote controls of TVs, physiotherapy and night-vision devices; infrared trapped by the atmosphere keeps the Earth warm (greenhouse effect).
Visible light (700 nm to 400 nm)
The only band our eyes detect, from red (longest) to violet (shortest). Light of 600 nm has f = c/λ = 5 × 10¹⁴ Hz.
Ultraviolet (400 nm to about 1 nm)
Produced by very hot bodies such as the Sun and by special lamps. Most of the Sun's UV is absorbed by the ozone layer. Used to kill germs in water purifiers and in LASIK eye surgery; large doses harm skin and eyes.
X-rays (about 1 nm to 10⁻³ nm)
Produced when high-energy electrons strike a metal target. Used in medical diagnosis and some cancer treatment; overexposure damages living tissue.
Gamma rays (below about 10⁻³ nm)
Emitted in nuclear reactions and by radioactive nuclei. Used in medicine to destroy cancer cells.
Key points
- Increasing frequency: radio → microwave → IR → visible → UV → X → gamma.
- All travel at c in vacuum; use c = fλ.
- Radar and microwave ovens use microwaves.
- Ozone absorbs UV; infrared is felt as heat.
Exam tips
- Remember the order with the phrase "Raging Martians Invaded Venus Using X-ray Guns".
- Convert units carefully: 1 MHz = 10⁶ Hz, 1 GHz = 10⁹ Hz, 1 nm = 10⁻⁹ m.