9.4.1 The Electromagnetic Spectrum

Having gone through your O-Level education, you are probably comfortable with the knowledge that light is a wave. But light is not an ordinary wave. Has it occurred to you that there is something truly unique about light?

Firstly, light is not a mechanical wave. It does not have any oscillating particles. Instead, it has oscillating fields. Two fields in fact, an electric field and a magnetic field (hence the name electromagnetic waves). And the two fields oscillate at right angle to each other.

And since there is no oscillating particles, electromagnetic waves do not require any medium to propagate. The next time you marvel at the beauty of a sunrise, remember that the light managed to reach you, despite the vast vacuum between the Sun and the Earth.

Furthermore, electromagnetic waves propagate at an incredible speed. In vacuum the speed of light is \displaystyle \displaystyle c=3.00\times {{10}^{8}}\text{ m }{{\text{s}}^{{\text{-1}}}}. In glass, it is about one-third slower. But still incredibly fast.

Unlike mechanical waves, the intensity I of an EM wave is only dependent on the amplitude A of its oscillating electric or magnetic field (\displaystyle \displaystyle \mathrm{I}\propto {{A}^{2}} to be specific) and independent of its frequency f (\displaystyle \displaystyle \mathrm{I}\propto {{A}^{2}}{{f}^{2}} for mechanical waves).

The range of wavelengths for electromagnetic waves is truly mind boggling, from picometers (gamma rays) to kilometers (radio waves). In comparison, the human eye is only sensitive to light of wavelength between 400 nm (violet) to 700 nm (red).

The syllabus DOES require you to “memorize” the wavelengths of the different type of EM waves. You should find the following table useful.

Common NameWavelength
Radio>m
Micro~cm
IR> 750 nm
Visible (ROYGBIV)400 nm to 750 nm
UV< 400 nm
X-ray~10-10 m
g-ray<10-12 m

Animation

E and M Oscillations

Song

The Electromagnetic Spectrum Song

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