4.2. Travelling Waves

Some of these key points are summarised on the graphic below. It's worth having a general knowledge of some of the rough wavelengths (in particular those for visible light, approximately 400 - 700 nm).

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A few pointers that can help remember some of the approximate wavelengths:

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Radio waves ≈ metres or km's (as radio waves can diffract round tall buildings and hills for communication, so the wavelengths must be a similar order of magnitude as these objects)

Microwaves ≈ mm's or cm's (as these are the gaps between local hot and cold bits in food heated in the microwave oven)

Infrared ≈ 10ˉ⁴ m - 10ˉ⁶ m (wavelengths between red visible and microwaves)

Red visible light ≈ 666 nm (666 being the number of the red devil).

Green visible (≈ 555 nm) and blue visible (≈ 444 nm) (easy to remember once you know red's wavelength)
UV ≈ 10ˉ⁷ m - 10ˉ⁹ m (wavelengths between violet and X-rays)

X-rays ≈ 10ˉ⁹ - 10ˉ¹² m, (X-ray diffraction is used to image atomic structures, because the wavelength is similar to atomic diameters)

Gamma Rays ≈ <10ˉ¹² m (the most energetic waves have the shortest wavelengths)

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All parts of the electromagnetic spectrum travel at 3 x 10⁸ msˉ¹ in a vacuum (though may slow down by different amounts when travelling through different media). At this stage, make sure you are confident calculating wavelengths and frequencies using your wave equation.