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Electromagnetic spectrum facts.

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Introduction

Electromagnetic spectrum facts

  • Waves carry vibrations through a medium.
  • They transfer the energy locked up in the vibrations.
  • Waves have a measurable speed, wavelength and frequency.
  • Waves meeting a boundary between mediums may be reflected, refracted or absorbed - often a mixture of all three.
  • Waves passing through a gap may be diffracted (spread) - the spreading is only noticeable if the gap is similar to the wavelength.
  • Electromagnetic waves carry transverse vibrations in electrical and magnetic fields, not vibrating particles.
  • E-m waves don't need matter to travel through - they can travel through empty space (a vacuum).
  • In a vacuum, all e-m waves travel at (approximately) 300 million metres per second (3 x 108m/s) - the fastest speed in the universe.
  • When e-m waves travel through matter (for example, light through air or glass), they travel a bit slower than this but rarely less than half as fast as in vacuum.
  • Waves of different frequencies travel at different speeds in transparent matter - so a mixture of waves can be separated out by diffraction. For example, white light is split up into a mixture of colours when it goes through a prism.
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Middle

Check your syllabus to see if there are particular examples or applications you need to know about in detail. This summary should give you some useful background ideas and information.

Why a spectrum?

At first sight, these waves seem a pretty mixed bunch. However:

  • they can all travel in a vacuum without a material medium (stuff) to carry the vibrations
  • they all have the same enormous speed in a vacuum; (3 x 108m/s)
  • they all have a similarity in their sources - all involve electrical charges accelerating or vibrating (not obvious, not easy, and not normally examined in GCSE - but very important!)

Notice that as the waves get shorter the dangers of over-exposure get more drastic, even for quite weak sources. The shorter the waves, the more drastic the effect on atoms that absorb them.

  • Long wave - particles (molecules) wobble, matter gets hotter
  • Short waves - particles split up and/or ionise (form charged particles), heating and chemical changes

Hence the greater precautions taken with short waves:

  • ultraviolet - skin blockers, sunglasses
  • X-rays,gamma-rays - lead shields, minimal exposure, keeping away

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Conclusion

The frequency (f) is the number of complete waves passing a point each second. It's a 'number per second' so it's measured in /s or s-1; usually called hertz (Hz) after a German physicist.


1 kilohertz = 1 kHz = 1000 Hz
1 megahertz = 1 MHz = 1,000,000 Hz
For example:
100 complete sound waves enter your ear in a second (you'd hear a deep hum).
f = 100 per second
= 100 /s = 100 s
-1
= 100Hz

The speed of a wave (v) is just what it says. It's the speed at which the vibrations in the wave move from one point to the next. Wave speed is measured in metres per second (m/s, ms-1).


For example:
speed of sound in air = 330 m/s (approximate)
speed of light in space = 300,000,000 m/s

Print out or copy this page if you want a basic revision sheet. You may want to check out the next section first.

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