3.10-3.11 - Electromagnetic spectrum order and speed
Visible light is only a small part of a much larger family of electromagnetic radiations. Pearson Edexcel expects you to know the names in the correct order, how wavelength and frequency change across that order, and the one speed rule that applies in free space. This lesson is about the spectrum itself, not the later uses and risks of each radiation type.
One continuous spectrum
The electromagnetic spectrum is a continuous range of electromagnetic waves. Continuous means the range does not stop and restart between named regions; scientists give names to different bands so that the spectrum is easier to discuss.
Electromagnetic spectrum
The electromagnetic spectrum is the continuous range of electromagnetic radiations, including radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays.
Visible light is part of this spectrum. It is not a different kind of wave from the rest of the list; it is the part that human eyes can detect. All electromagnetic waves are transverse, and all of them can travel through empty space.
The named regions you need for this lesson are:
| Region | What the name tells you at this point |
|---|---|
| radio | an electromagnetic radiation in the longest-wavelength part of this lesson's order |
| microwave | next after radio when wavelength decreases |
| infrared | just beyond the red end of visible light |
| visible | the small region detected by human eyes |
| ultraviolet | just beyond the violet end of visible light |
| X-ray | beyond ultraviolet towards shorter wavelength |
| gamma | the shortest-wavelength end of this lesson's order |
The boundary between neighbouring named regions is not the main exam point here. The main point is that all seven names belong to one continuous electromagnetic spectrum.
Same speed in free space
All electromagnetic radiations travel at the same speed in free space. Free space means a vacuum, with no material medium such as air, glass or water.
At International GCSE level this shared speed is usually written as about:
That is the speed of light in a vacuum. The phrase "speed of light" can sound as if it only belongs to visible light, but in free space radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays all travel at that same speed.
This is a common trap: higher frequency does not mean a higher speed for electromagnetic waves in free space. A gamma ray has a higher frequency than a radio wave, but in free space they travel at the same speed.
The spectrum order is about wavelength and frequency, not about faster and slower electromagnetic waves.
Continue this lesson.
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