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The electromagnetic spectrum

Waves, sound and light · level 6, lesson 2 of 6

A question to start

Rovers talk to Earth by radio, the Sun warms Mars with infrared, and Mars's surface is hit by ultraviolet that never reaches Earth's ground. Are these all the same kind of thing?

The idea

Yes. Radio, infrared, visible light, ultraviolet, X-rays and gamma rays are all electromagnetic waves. They differ only in wavelength (and so frequency). All of them travel through empty space at the same speed, \(c = 3.00\times10^8\) m/s.

Type Wavelength, about Used for
Radio longer than about 30 cm radio, TV
Microwave 1 mm to 30 cm talking to rovers, radar, phone signals, cooking
Infrared 750 nm to 1 mm heat, remote controls
Visible 380 to 750 nm seeing
Ultraviolet 10 to 380 nm sunburn, killing germs
X-rays 0.01 to 10 nm medical images
Gamma rays less than 0.01 nm from radioactive atoms
radiomicroinfraredUVX-raysgamma visiblelong wavelengthshort wavelength

Shorter wavelengths carry more energy in each little packet of light. That is why ultraviolet and X-rays can damage living cells, while radio waves cannot.

Worked examples

1. A radio wave. At 8.4 GHz (\(8.4\times10^9\) Hz), \(\lambda = 3.00\times10^8/8.4\times10^9 = 0.036\) m, 3.6 cm.

2. A microwave oven at 2.45 GHz: \(\lambda = 3.00\times10^8/2.45\times10^9 = 0.12\) m.

3. Ultraviolet on Mars. Earth's ozone layer blocks most ultraviolet; Mars has almost no ozone, so much more reaches its ground.

Try it

  1. What do all electromagnetic waves have in common?
  2. Put these in order of wavelength, longest first: X-rays, radio, visible, infrared.
  3. Find the frequency of an infrared wave with a wavelength of 10 µm (\(10^{-5}\) m).
  4. Why are ultraviolet and X-rays more harmful than radio waves?
Answers
  1. They are all electromagnetic waves and travel at \(c\) in empty space.
  2. Radio, infrared, visible, X-rays.
  3. \(3.00\times10^8/10^{-5} = 3.0\times10^{13}\) Hz.
  4. Their short wavelengths mean each packet of light carries much more energy.

Common mistake

"Radio waves are sound waves." Radio is an electromagnetic wave, like light, and can cross empty space. A radio set turns it into sound for you to hear.

Where it leads