Electromagnetic Waves

What they are — oscillating electric and magnetic fields at right angles to each other and to the direction of travel. They are transverse, and they require no medium, so they travel through a vacuum.

Speed in a vacuum — all electromagnetic waves travel at the same speed in a vacuum:

c = 3.00 × 10⁸ m s⁻¹

Since c = fλ, a higher frequency always means a shorter wavelength.

The spectrum, in order of increasing frequency — radio, microwave, infrared, visible, ultraviolet, X-rays, gamma rays. Wavelength decreases along the same order, and photon energy increases.

Approximate wavelengths — radio above about 10⁻¹ m; microwave 10⁻³ to 10⁻¹ m; infrared 7 × 10⁻⁷ to 10⁻³ m; visible 4 × 10⁻⁷ to 7 × 10⁻⁷ m; ultraviolet 10⁻⁸ to 4 × 10⁻⁷ m; X-rays 10⁻¹³ to 10⁻⁸ m; gamma below about 10⁻¹⁰ m.

Visible light — roughly 400 nm (violet) to 700 nm (red). Red has the longest wavelength and lowest frequency of the visible range, violet the shortest and highest.

X-rays and gamma rays overlap in wavelength, and are distinguished by origin rather than by where they sit on the scale: X-rays are produced by decelerating electrons, gamma rays by changes within an atomic nucleus. A 10⁻¹² m photon is an X-ray or a gamma ray depending on where it came from.

Example: this is worth remembering precisely because it breaks the pattern. Every other boundary in the spectrum is a matter of wavelength; this one is a matter of source.

Why higher frequency tends to be more dangerous — photon energy is proportional to frequency, so ultraviolet, X-ray and gamma photons carry enough energy to ionise atoms, damaging molecules including DNA. Radio and microwave photons individually carry far too little energy to ionise, whatever the intensity.