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5.2.4 Groups of the electromagnetic spectrum

The electromagnetic spectrum

Definition

Electromagnetic spectrum

The continuous range of electromagnetic waves, arranged in order of wavelength or frequency.

  1. The electromagnetic spectrum is a continuous range of waves that is divided into seven main groups.
  2. In order of decreasing wavelength, the groups are radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays.
  3. All seven groups are the same kind of wave, so they are all transverse and they all travel at 3.0×108 m/s3.0\times10^{8}\ \text{m/s}3.0×108 m/s in a vacuum.
  4. The groups differ only in their wavelength and frequency, and these differences give each group its own properties.
  5. Because the spectrum is continuous, neighbouring groups merge into one another with no sharp boundary between them.
  6. The X-ray and gamma-ray groups even overlap, and a wave in the overlap is named by how it was produced rather than by its wavelength.

Long to short wavelength

  1. Moving along the spectrum from radio waves towards gamma rays, each group has a shorter wavelength than the one before it.
  2. Radio waves have the longest wavelengths, typically from about 1 m1\ \text{m}1 m to more than 1 km1\ \text{km}1 km.
  3. Microwaves have wavelengths of about a millimetre up to a few tens of centimetres, so a typical value is a few centimetres.
  4. Infrared has a typical wavelength of about 1×10−5 m1\times10^{-5}\ \text{m}1×10−5 m, which is much thinner than a human hair.
  5. Visible light has wavelengths of a few hundred nanometres, where 1 nm=1×10−9 m1\ \text{nm}=1\times10^{-9}\ \text{m}1 nm=1×10−9 m.
  6. Ultraviolet has a typical wavelength of about 1×10−8 m1\times10^{-8}\ \text{m}1×10−8 m.
  7. X-rays have a typical wavelength of about 1×10−10 m1\times10^{-10}\ \text{m}1×10−10 m, which is roughly the size of an atom.
  8. Gamma rays have the shortest wavelengths, typically 1×10−12 m1\times10^{-12}\ \text{m}1×10−12 m or less, which is smaller than an atom.

Low to high frequency

  1. All the groups travel at the same speed, and v=fλv=f\lambdav=fλ, so a shorter wavelength always goes with a higher frequency.
  2. The order of increasing frequency is therefore the same order as the order of decreasing wavelength.
  3. Radio waves have the lowest frequencies, and gamma rays have the highest frequencies.
  4. Frequencies range from about 1×105 Hz1\times10^{5}\ \text{Hz}1×105 Hz for long radio waves to more than 1×1020 Hz1\times10^{20}\ \text{Hz}1×1020 Hz for gamma rays.
  5. Any group nearer the gamma-ray end has a higher frequency and a shorter wavelength than a group nearer the radio end.

The colours of visible light

  1. Visible light is a narrow band of the spectrum that your eyes see as different colours.
  2. In order of decreasing wavelength, the colours are red, orange, yellow, green, blue, indigo and violet.
  3. Red light has the longest wavelength and lowest frequency in the visible band.
  4. Violet light has the shortest wavelength and highest frequency in the visible band.
  5. So the visible band runs from red, next to infrared, to violet, next to ultraviolet.
Example

Identifying a group from its wavelength

  • A wave has a wavelength of 3.0 cm3.0\ \text{cm}3.0 cm.
  • Convert to metres: 3.0 cm=0.030 m3.0\ \text{cm}=0.030\ \text{m}3.0 cm=0.030 m, which is a few centimetres, so the wave is a microwave.
  • Its frequency comes from the wave equation:
f=vλ=3.0×1080.030 f=\frac{v}{\lambda}=\frac{3.0\times10^{8}}{0.030} f=λv​=0.0303.0×108​ f=1.0×1010 Hz f=1.0\times10^{10}\ \text{Hz} f=1.0×1010 Hz
  • A shorter wavelength would move the wave towards infrared, and a longer one would move it towards radio waves.
Note

Remembering the order

The sentence Raging Martians Invaded Venus Using X-ray Guns gives the first letters of radio, microwave, infrared, visible, ultraviolet, X-ray and gamma in order of decreasing wavelength.

Common Mistake
  • Do not swap infrared and ultraviolet, or X-rays and gamma rays, because the order must run radio, microwave, infrared, visible, ultraviolet, X-ray, gamma.
  • Do not say gamma rays have the longest wavelength because they have the highest frequency, because a high frequency always goes with a short wavelength.
  • Do not say the groups travel at different speeds in a vacuum, because they differ only in wavelength and frequency.
Exam technique

Ordering and comparing groups

  • When you complete a diagram of the spectrum, check whether the arrow is labelled increasing wavelength or increasing frequency before you fill in the gaps.
  • To compare two groups, find them in the order: the one nearer radio waves has the longer wavelength and the lower frequency.
  • Learn the two neighbours of visible light, infrared on the long-wavelength side and ultraviolet on the short-wavelength side, because they are often tested.
Self review
  • List the seven main groups of the electromagnetic spectrum in order of decreasing wavelength.
  • Which group has the highest frequency?
  • Which has the longer wavelength, microwaves or infrared?
  • Which colour of visible light has the shortest wavelength?
  • Why is the order of increasing frequency the same as the order of decreasing wavelength?
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Revision notes for OCR GCSE Physics 5.2.4 Groups of the electromagnetic spectrum: explanations and worked examples.

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