Caption: The electromagnetic spectrum.

Features:

1. The continuum of electromagnetic radiation (EMR) is called the electromagnetic spectrum.

2. We divide the electromagnetic spectrum into wavelength bands (or frequency bands) with SHARP BOUNDARIES for convenience in analysis and often because these wavelength bands have different emission and absorption processes on average.

3. There are, however, NO real SHARP BOUNDARIES in electromagnetic spectrum---the electromagnetic spectrum forms a continuum as far as we can tell.

   Some relatively sharp boundaries do occur in particular emission and absorption processes: e.g., absorption edges.

4. The well-known wave specification formulae for EMR in vacuum are, respectively,

       fλ = c ,     λ=c/f ,     f = c/λ ,

   where f is frequency, λ is wavelength, f is frequency and c is the vacuum light speed c = 2.99792458*10**8 m/s (exact by definition) ≅ 3*10**8 m/s = 3*10**5 km/s ≅ 1 ft/ns.

   From the formulae, we see that frequency and wavelength are inversely related and contain the same information in different forms.

5. Frequency bands: gamma ray (old convention 0--0.1 Å), X-ray (old convention 0.1 Å -- 0.01 μm), ultraviolet = UV (0.01--0.4 μm) ( extreme ultraviolet = EUV (0.01--0.121 μm), far ultraviolet = FUV (0.121--0.2 μm), middle ultraviolet = MUV (0.2--0.3 μm), near ultraviolet = NUV (0.3--0.4 μm)), visible (fiducial range 0.4--0.7 μm), infrared (0.7 μm -- 0.1 cm), microwave (0.1--100 cm), radio (0.1 cm -- 10**5 km).