Scientists have found that many types of wave can be arranged together like the notes on a piano keyboard, to form a scale.
The 'low notes' have a low frequency and a long wavelength.
The 'high notes' have a high frequency and a short wavelength.
When we say "wave", you might think of a wave on the sea. There, it's nice and obvious what's going on - the surface of the sea is vibrating up and down.
With a sound wave, it's the air particles that are vibrating.
So what's vibrating when an electromagnetic wave passes by?
That's not so easy.
Electromagnetic waves are vibrations of magnetic and electric fields.
So they don't need air in order to travel. They don't need anything to be there at all.
http://www.youtube.com/watch?v=kfS5Qn0wn2o
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