Order these by increasing frequency and identify each band: (a) wavelength 500 nm, (b) frequency 100 MHz, (c) wavelength 1 cm, (d) photon energy 10 keV. Also find the frequency of (a). Increasing frequency means decreasing wavelength. Compute frequencies: (a) f = c/λ = 3×10⁸/500×10⁻⁹ = 6×10¹⁴ Hz — visible light (green). (b) 100 MHz = 10⁸ Hz — radio (FM band). (c) f = 3×10⁸/0.01 = 3×10¹⁰ Hz = 30 GHz — microwaves. (d) E = hf gives f = E/h = (10×10³ × 1.6×10⁻¹⁹)/6.63×10⁻³⁴ ≈ 2.4×10¹⁸ Hz — X-rays. Order of increasing frequency: (b) radio 10⁸ Hz < (c) microwave 3×10¹⁰ Hz < (a) visible 6×10¹⁴ Hz < (d) X-ray 2.4×10¹⁸ Hz. Notice the enormous range — the spectrum spans over 20 orders of magnitude, yet c = fλ holds for every one.