The electromagnetic spectrum
One family of waves
A radio signal, the heat from a stove, sunlight and a hospital X-ray look nothing alike. Yet all of them are electromagnetic waves: oscillating E and B fields that cross a vacuum at the same speed, c = 3 × 10⁸ metres per second.
They differ only in frequency and wavelength, linked by c = νλ. Arranged in order of frequency, they form the electromagnetic spectrum.
The range is enormous: from radio waves hundreds of metres long to gamma rays far smaller than an atom. So we draw the spectrum on a scale where each step is ten times the last.
The regions have names, but there are no sharp boundaries between them; neighbouring regions overlap. In order of increasing frequency: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays.
Radio waves have wavelengths longer than about 0.1 metre. They are produced by the accelerated motion of charges in conducting wires, the aerials of transmitters, and picked up by the aerials of receivers.
The AM band runs from 530 to 1710 kilohertz. Television uses 54 to 890 megahertz, FM radio 88 to 108 megahertz, and mobile phones the ultra high frequency band.
Take an FM station at 100 megahertz. λ equals 3 × 10⁸ over 10⁸, which is 3 metres. An AM station at 1000 kilohertz sends waves 300 metres long.
Microwaves are short radio waves, from 0.1 metre down to about 1 millimetre. They are produced by special vacuum tubes, klystrons and magnetrons, and by Gunn diodes. Radar uses them to guide aircraft, and speed guns use them to time fast balls and cars.
In a microwave oven, the frequency is chosen to match the natural frequency of water molecules. The water in the food absorbs the energy, its molecules move faster, and the food heats up.
A typical oven works at 2.45 gigahertz. λ equals 3 × 10⁸ over 2.45 × 10⁹, about 0.122 metre, or 12.2 centimetres.
From infrared to gamma rays
Infrared runs from about 1 millimetre down to 700 nanometres. It is given out by hot bodies and molecules. It is called heat waves, because water molecules in most materials absorb it readily, move faster, and the material warms up.
Infrared lamps are used in physical therapy. The greenhouse effect, infrared trapped by the atmosphere, keeps the Earth warm. Satellites carry infrared detectors, and the remote control of your TV sends infrared from a small LED.
Visible light is the narrow band our eyes detect: from about 700 nanometres, red, to 400 nanometres, violet. In frequency, that is about 4 × 10¹⁴ to 7 × 10¹⁴ hertz. Check red: 3 × 10⁸ over 7 × 10⁻⁷ is 4.3 × 10¹⁴ hertz.
Other animals see differently. Snakes can detect infrared, and many insects can see well into the ultraviolet.
Ultraviolet runs from 400 nanometres down to about 0.6 nanometre. The Sun is an important source. Most of its UV is absorbed by the ozone layer high in the atmosphere, which protects us.
Too much UV is harmful. Welders wear special glass goggles to protect their eyes from the strong UV of the welding arc. Because of its short wavelength, UV can be focused into very narrow beams, as in LASIK eye surgery. UV lamps also kill germs in water purifiers.
X-rays come next, from about 10 nanometres down to 10⁻⁴ nanometre. They are produced by firing high-energy electrons at a metal target. Doctors use them to look inside the body and to treat some cancers, but too much exposure damages living tissue.
Gamma rays have the highest frequencies and the shortest wavelengths. They come from nuclear reactions and radioactive nuclei. In medicine they are used to destroy cancer cells.
X-rays and gamma rays overlap in wavelength. The name tells us how the radiation was made: from electrons in an X-ray tube, or from a nucleus.
So, from radio to gamma: the frequency rises and the wavelength falls, but every one of them is the same kind of wave, travelling at c. Each region needs its own way to produce it and its own detector.
Try it: the spectrum ruler
- Drag the marker along the ruler. Each tick is a factor of 10. Watch ν = c/λ rise as λ falls.
- Try the example buttons: FM radio, the microwave oven, the TV remote, red light, X-rays.
- Find the visible band. It is a tiny sliver of the whole spectrum; the zoom strip below shows its colours.
- For each region, read how it is produced, detected and used.
Inside the oven
A microwave oven works at 2.45 GHz. What is the wavelength of its waves?
- 12.2 cm
- 1.22 m
- 12.2 m
- 0.122 mm
Answer
A. λ = c/ν = 3 × 10⁸ / 2.45 × 10⁹ = 0.122 m = 12.2 cm, well inside the microwave region (1 mm to 0.1 m). 1.22 m comes from treating giga as 10⁸; 0.122 mm writes the answer in millimetres instead of metres.
Goggles for welders
Welders wear special glass goggles. Which radiation from the welding arc are these mainly meant to block?
- Infrared
- Ultraviolet
- X-rays
- Microwaves
Answer
B. A welding arc gives out a large amount of ultraviolet, which harms the eyes. Ordinary glass absorbs much of it, which is why welders use special glass goggles or face masks with glass windows. The arc does give out infrared (heat) too, but the eye danger from the arc is mainly UV. An arc does not produce X-rays or microwaves.
Summary
- All EM waves in vacuum:
c = νλ = 3 × 10⁸ m/s - Visible light (ν ≈ 4–7 × 10¹⁴ Hz):
λ ≈ 400–700 nm
- In order of increasing frequency (decreasing wavelength): radio, microwaves, infrared, visible, ultraviolet, X-rays, gamma rays.
- All are the same kind of wave; they differ in frequency, how they are produced and how they are detected. Regions overlap; boundaries are not sharp.
- Radio: aerials (AM, FM, TV, mobiles). Microwaves: klystron, magnetron; radar, ovens. Infrared: hot bodies; heat, remotes, greenhouse effect.
- Ultraviolet: the Sun, absorbed by ozone; germ-killing lamps, LASIK. X-rays: electrons hitting a metal target; medical imaging.
- Gamma rays: nuclear reactions and radioactive nuclei; used to destroy cancer cells.